-
Notifications
You must be signed in to change notification settings - Fork 53.3k
/
Debugging390.txt
2535 lines (2095 loc) · 95 KB
/
Debugging390.txt
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
Debugging on Linux for s/390 & z/Architecture
by
Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com)
Copyright (C) 2000-2001 IBM Deutschland Entwicklung GmbH, IBM Corporation
Best viewed with fixed width fonts
Overview of Document:
=====================
This document is intended to give a good overview of how to debug
Linux for s/390 & z/Architecture. It isn't intended as a complete reference & not a
tutorial on the fundamentals of C & assembly. It doesn't go into
390 IO in any detail. It is intended to complement the documents in the
reference section below & any other worthwhile references you get.
It is intended like the Enterprise Systems Architecture/390 Reference Summary
to be printed out & used as a quick cheat sheet self help style reference when
problems occur.
Contents
========
Register Set
Address Spaces on Intel Linux
Address Spaces on Linux for s/390 & z/Architecture
The Linux for s/390 & z/Architecture Kernel Task Structure
Register Usage & Stackframes on Linux for s/390 & z/Architecture
A sample program with comments
Compiling programs for debugging on Linux for s/390 & z/Architecture
Figuring out gcc compile errors
Debugging Tools
objdump
strace
Performance Debugging
Debugging under VM
s/390 & z/Architecture IO Overview
Debugging IO on s/390 & z/Architecture under VM
GDB on s/390 & z/Architecture
Stack chaining in gdb by hand
Examining core dumps
ldd
Debugging modules
The proc file system
Starting points for debugging scripting languages etc.
Dumptool & Lcrash
SysRq
References
Special Thanks
Register Set
============
The current architectures have the following registers.
16 General propose registers, 32 bit on s/390 64 bit on z/Architecture, r0-r15 or gpr0-gpr15 used for arithmetic & addressing.
16 Control registers, 32 bit on s/390 64 bit on z/Architecture, ( cr0-cr15 kernel usage only ) used for memory management,
interrupt control,debugging control etc.
16 Access registers ( ar0-ar15 ) 32 bit on s/390 & z/Architecture
not used by normal programs but potentially could
be used as temporary storage. Their main purpose is their 1 to 1
association with general purpose registers and are used in
the kernel for copying data between kernel & user address spaces.
Access register 0 ( & access register 1 on z/Architecture ( needs 64 bit
pointer ) ) is currently used by the pthread library as a pointer to
the current running threads private area.
16 64 bit floating point registers (fp0-fp15 ) IEEE & HFP floating
point format compliant on G5 upwards & a Floating point control reg (FPC)
4 64 bit registers (fp0,fp2,fp4 & fp6) HFP only on older machines.
Note:
Linux (currently) always uses IEEE & emulates G5 IEEE format on older machines,
( provided the kernel is configured for this ).
The PSW is the most important register on the machine it
is 64 bit on s/390 & 128 bit on z/Architecture & serves the roles of
a program counter (pc), condition code register,memory space designator.
In IBM standard notation I am counting bit 0 as the MSB.
It has several advantages over a normal program counter
in that you can change address translation & program counter
in a single instruction. To change address translation,
e.g. switching address translation off requires that you
have a logical=physical mapping for the address you are
currently running at.
Bit Value
s/390 z/Architecture
0 0 Reserved ( must be 0 ) otherwise specification exception occurs.
1 1 Program Event Recording 1 PER enabled,
PER is used to facilitate debugging e.g. single stepping.
2-4 2-4 Reserved ( must be 0 ).
5 5 Dynamic address translation 1=DAT on.
6 6 Input/Output interrupt Mask
7 7 External interrupt Mask used primarily for interprocessor signalling &
clock interrupts.
8-11 8-11 PSW Key used for complex memory protection mechanism not used under linux
12 12 1 on s/390 0 on z/Architecture
13 13 Machine Check Mask 1=enable machine check interrupts
14 14 Wait State set this to 1 to stop the processor except for interrupts & give
time to other LPARS used in CPU idle in the kernel to increase overall
usage of processor resources.
15 15 Problem state ( if set to 1 certain instructions are disabled )
all linux user programs run with this bit 1
( useful info for debugging under VM ).
16-17 16-17 Address Space Control
00 Primary Space Mode when DAT on
The linux kernel currently runs in this mode, CR1 is affiliated with
this mode & points to the primary segment table origin etc.
01 Access register mode this mode is used in functions to
copy data between kernel & user space.
10 Secondary space mode not used in linux however CR7 the
register affiliated with this mode is & this & normally
CR13=CR7 to allow us to copy data between kernel & user space.
We do this as follows:
We set ar2 to 0 to designate its
affiliated gpr ( gpr2 )to point to primary=kernel space.
We set ar4 to 1 to designate its
affiliated gpr ( gpr4 ) to point to secondary=home=user space
& then essentially do a memcopy(gpr2,gpr4,size) to
copy data between the address spaces, the reason we use home space for the
kernel & don't keep secondary space free is that code will not run in
secondary space.
11 Home Space Mode all user programs run in this mode.
it is affiliated with CR13.
18-19 18-19 Condition codes (CC)
20 20 Fixed point overflow mask if 1=FPU exceptions for this event
occur ( normally 0 )
21 21 Decimal overflow mask if 1=FPU exceptions for this event occur
( normally 0 )
22 22 Exponent underflow mask if 1=FPU exceptions for this event occur
( normally 0 )
23 23 Significance Mask if 1=FPU exceptions for this event occur
( normally 0 )
24-31 24-30 Reserved Must be 0.
31 Extended Addressing Mode
32 Basic Addressing Mode
Used to set addressing mode
PSW 31 PSW 32
0 0 24 bit
0 1 31 bit
1 1 64 bit
32 1=31 bit addressing mode 0=24 bit addressing mode (for backward
compatibility), linux always runs with this bit set to 1
33-64 Instruction address.
33-63 Reserved must be 0
64-127 Address
In 24 bits mode bits 64-103=0 bits 104-127 Address
In 31 bits mode bits 64-96=0 bits 97-127 Address
Note: unlike 31 bit mode on s/390 bit 96 must be zero
when loading the address with LPSWE otherwise a
specification exception occurs, LPSW is fully backward
compatible.
Prefix Page(s)
--------------
This per cpu memory area is too intimately tied to the processor not to mention.
It exists between the real addresses 0-4096 on s/390 & 0-8192 z/Architecture & is exchanged
with a 1 page on s/390 or 2 pages on z/Architecture in absolute storage by the set
prefix instruction in linux'es startup.
This page is mapped to a different prefix for each processor in an SMP configuration
( assuming the os designer is sane of course :-) ).
Bytes 0-512 ( 200 hex ) on s/390 & 0-512,4096-4544,4604-5119 currently on z/Architecture
are used by the processor itself for holding such information as exception indications &
entry points for exceptions.
Bytes after 0xc00 hex are used by linux for per processor globals on s/390 & z/Architecture
( there is a gap on z/Architecture too currently between 0xc00 & 1000 which linux uses ).
The closest thing to this on traditional architectures is the interrupt
vector table. This is a good thing & does simplify some of the kernel coding
however it means that we now cannot catch stray NULL pointers in the
kernel without hard coded checks.
Address Spaces on Intel Linux
=============================
The traditional Intel Linux is approximately mapped as follows forgive
the ascii art.
0xFFFFFFFF 4GB Himem *****************
* *
* Kernel Space *
* *
***************** ****************
User Space Himem (typically 0xC0000000 3GB )* User Stack * * *
***************** * *
* Shared Libs * * Next Process *
***************** * to *
* * <== * Run * <==
* User Program * * *
* Data BSS * * *
* Text * * *
* Sections * * *
0x00000000 ***************** ****************
Now it is easy to see that on Intel it is quite easy to recognise a kernel address
as being one greater than user space himem ( in this case 0xC0000000).
& addresses of less than this are the ones in the current running program on this
processor ( if an smp box ).
If using the virtual machine ( VM ) as a debugger it is quite difficult to
know which user process is running as the address space you are looking at
could be from any process in the run queue.
The limitation of Intels addressing technique is that the linux
kernel uses a very simple real address to virtual addressing technique
of Real Address=Virtual Address-User Space Himem.
This means that on Intel the kernel linux can typically only address
Himem=0xFFFFFFFF-0xC0000000=1GB & this is all the RAM these machines
can typically use.
They can lower User Himem to 2GB or lower & thus be
able to use 2GB of RAM however this shrinks the maximum size
of User Space from 3GB to 2GB they have a no win limit of 4GB unless
they go to 64 Bit.
On 390 our limitations & strengths make us slightly different.
For backward compatibility we are only allowed use 31 bits (2GB)
of our 32 bit addresses, however, we use entirely separate address
spaces for the user & kernel.
This means we can support 2GB of non Extended RAM on s/390, & more
with the Extended memory management swap device &
currently 4TB of physical memory currently on z/Architecture.
Address Spaces on Linux for s/390 & z/Architecture
==================================================
Our addressing scheme is as follows
Himem 0x7fffffff 2GB on s/390 ***************** ****************
currently 0x3ffffffffff (2^42)-1 * User Stack * * *
on z/Architecture. ***************** * *
* Shared Libs * * *
***************** * *
* * * Kernel *
* User Program * * *
* Data BSS * * *
* Text * * *
* Sections * * *
0x00000000 ***************** ****************
This also means that we need to look at the PSW problem state bit
or the addressing mode to decide whether we are looking at
user or kernel space.
Virtual Addresses on s/390 & z/Architecture
===========================================
A virtual address on s/390 is made up of 3 parts
The SX ( segment index, roughly corresponding to the PGD & PMD in linux terminology )
being bits 1-11.
The PX ( page index, corresponding to the page table entry (pte) in linux terminology )
being bits 12-19.
The remaining bits BX (the byte index are the offset in the page )
i.e. bits 20 to 31.
On z/Architecture in linux we currently make up an address from 4 parts.
The region index bits (RX) 0-32 we currently use bits 22-32
The segment index (SX) being bits 33-43
The page index (PX) being bits 44-51
The byte index (BX) being bits 52-63
Notes:
1) s/390 has no PMD so the PMD is really the PGD also.
A lot of this stuff is defined in pgtable.h.
2) Also seeing as s/390's page indexes are only 1k in size
(bits 12-19 x 4 bytes per pte ) we use 1 ( page 4k )
to make the best use of memory by updating 4 segment indices
entries each time we mess with a PMD & use offsets
0,1024,2048 & 3072 in this page as for our segment indexes.
On z/Architecture our page indexes are now 2k in size
( bits 12-19 x 8 bytes per pte ) we do a similar trick
but only mess with 2 segment indices each time we mess with
a PMD.
3) As z/Architecture supports up to a massive 5-level page table lookup we
can only use 3 currently on Linux ( as this is all the generic kernel
currently supports ) however this may change in future
this allows us to access ( according to my sums )
4TB of virtual storage per process i.e.
4096*512(PTES)*1024(PMDS)*2048(PGD) = 4398046511104 bytes,
enough for another 2 or 3 of years I think :-).
to do this we use a region-third-table designation type in
our address space control registers.
The Linux for s/390 & z/Architecture Kernel Task Structure
==========================================================
Each process/thread under Linux for S390 has its own kernel task_struct
defined in linux/include/linux/sched.h
The S390 on initialisation & resuming of a process on a cpu sets
the __LC_KERNEL_STACK variable in the spare prefix area for this cpu
(which we use for per-processor globals).
The kernel stack pointer is intimately tied with the task structure for
each processor as follows.
s/390
************************
* 1 page kernel stack *
* ( 4K ) *
************************
* 1 page task_struct *
* ( 4K ) *
8K aligned ************************
z/Architecture
************************
* 2 page kernel stack *
* ( 8K ) *
************************
* 2 page task_struct *
* ( 8K ) *
16K aligned ************************
What this means is that we don't need to dedicate any register or global variable
to point to the current running process & can retrieve it with the following
very simple construct for s/390 & one very similar for z/Architecture.
static inline struct task_struct * get_current(void)
{
struct task_struct *current;
__asm__("lhi %0,-8192\n\t"
"nr %0,15"
: "=r" (current) );
return current;
}
i.e. just anding the current kernel stack pointer with the mask -8192.
Thankfully because Linux doesn't have support for nested IO interrupts
& our devices have large buffers can survive interrupts being shut for
short amounts of time we don't need a separate stack for interrupts.
Register Usage & Stackframes on Linux for s/390 & z/Architecture
=================================================================
Overview:
---------
This is the code that gcc produces at the top & the bottom of
each function. It usually is fairly consistent & similar from
function to function & if you know its layout you can probably
make some headway in finding the ultimate cause of a problem
after a crash without a source level debugger.
Note: To follow stackframes requires a knowledge of C or Pascal &
limited knowledge of one assembly language.
It should be noted that there are some differences between the
s/390 & z/Architecture stack layouts as the z/Architecture stack layout didn't have
to maintain compatibility with older linkage formats.
Glossary:
---------
alloca:
This is a built in compiler function for runtime allocation
of extra space on the callers stack which is obviously freed
up on function exit ( e.g. the caller may choose to allocate nothing
of a buffer of 4k if required for temporary purposes ), it generates
very efficient code ( a few cycles ) when compared to alternatives
like malloc.
automatics: These are local variables on the stack,
i.e they aren't in registers & they aren't static.
back-chain:
This is a pointer to the stack pointer before entering a
framed functions ( see frameless function ) prologue got by
dereferencing the address of the current stack pointer,
i.e. got by accessing the 32 bit value at the stack pointers
current location.
base-pointer:
This is a pointer to the back of the literal pool which
is an area just behind each procedure used to store constants
in each function.
call-clobbered: The caller probably needs to save these registers if there
is something of value in them, on the stack or elsewhere before making a
call to another procedure so that it can restore it later.
epilogue:
The code generated by the compiler to return to the caller.
frameless-function
A frameless function in Linux for s390 & z/Architecture is one which doesn't
need more than the register save area ( 96 bytes on s/390, 160 on z/Architecture )
given to it by the caller.
A frameless function never:
1) Sets up a back chain.
2) Calls alloca.
3) Calls other normal functions
4) Has automatics.
GOT-pointer:
This is a pointer to the global-offset-table in ELF
( Executable Linkable Format, Linux'es most common executable format ),
all globals & shared library objects are found using this pointer.
lazy-binding
ELF shared libraries are typically only loaded when routines in the shared
library are actually first called at runtime. This is lazy binding.
procedure-linkage-table
This is a table found from the GOT which contains pointers to routines
in other shared libraries which can't be called to by easier means.
prologue:
The code generated by the compiler to set up the stack frame.
outgoing-args:
This is extra area allocated on the stack of the calling function if the
parameters for the callee's cannot all be put in registers, the same
area can be reused by each function the caller calls.
routine-descriptor:
A COFF executable format based concept of a procedure reference
actually being 8 bytes or more as opposed to a simple pointer to the routine.
This is typically defined as follows
Routine Descriptor offset 0=Pointer to Function
Routine Descriptor offset 4=Pointer to Table of Contents
The table of contents/TOC is roughly equivalent to a GOT pointer.
& it means that shared libraries etc. can be shared between several
environments each with their own TOC.
static-chain: This is used in nested functions a concept adopted from pascal
by gcc not used in ansi C or C++ ( although quite useful ), basically it
is a pointer used to reference local variables of enclosing functions.
You might come across this stuff once or twice in your lifetime.
e.g.
The function below should return 11 though gcc may get upset & toss warnings
about unused variables.
int FunctionA(int a)
{
int b;
FunctionC(int c)
{
b=c+1;
}
FunctionC(10);
return(b);
}
s/390 & z/Architecture Register usage
=====================================
r0 used by syscalls/assembly call-clobbered
r1 used by syscalls/assembly call-clobbered
r2 argument 0 / return value 0 call-clobbered
r3 argument 1 / return value 1 (if long long) call-clobbered
r4 argument 2 call-clobbered
r5 argument 3 call-clobbered
r6 argument 4 saved
r7 pointer-to arguments 5 to ... saved
r8 this & that saved
r9 this & that saved
r10 static-chain ( if nested function ) saved
r11 frame-pointer ( if function used alloca ) saved
r12 got-pointer saved
r13 base-pointer saved
r14 return-address saved
r15 stack-pointer saved
f0 argument 0 / return value ( float/double ) call-clobbered
f2 argument 1 call-clobbered
f4 z/Architecture argument 2 saved
f6 z/Architecture argument 3 saved
The remaining floating points
f1,f3,f5 f7-f15 are call-clobbered.
Notes:
------
1) The only requirement is that registers which are used
by the callee are saved, e.g. the compiler is perfectly
capable of using r11 for purposes other than a frame a
frame pointer if a frame pointer is not needed.
2) In functions with variable arguments e.g. printf the calling procedure
is identical to one without variable arguments & the same number of
parameters. However, the prologue of this function is somewhat more
hairy owing to it having to move these parameters to the stack to
get va_start, va_arg & va_end to work.
3) Access registers are currently unused by gcc but are used in
the kernel. Possibilities exist to use them at the moment for
temporary storage but it isn't recommended.
4) Only 4 of the floating point registers are used for
parameter passing as older machines such as G3 only have only 4
& it keeps the stack frame compatible with other compilers.
However with IEEE floating point emulation under linux on the
older machines you are free to use the other 12.
5) A long long or double parameter cannot be have the
first 4 bytes in a register & the second four bytes in the
outgoing args area. It must be purely in the outgoing args
area if crossing this boundary.
6) Floating point parameters are mixed with outgoing args
on the outgoing args area in the order the are passed in as parameters.
7) Floating point arguments 2 & 3 are saved in the outgoing args area for
z/Architecture
Stack Frame Layout
------------------
s/390 z/Architecture
0 0 back chain ( a 0 here signifies end of back chain )
4 8 eos ( end of stack, not used on Linux for S390 used in other linkage formats )
8 16 glue used in other s/390 linkage formats for saved routine descriptors etc.
12 24 glue used in other s/390 linkage formats for saved routine descriptors etc.
16 32 scratch area
20 40 scratch area
24 48 saved r6 of caller function
28 56 saved r7 of caller function
32 64 saved r8 of caller function
36 72 saved r9 of caller function
40 80 saved r10 of caller function
44 88 saved r11 of caller function
48 96 saved r12 of caller function
52 104 saved r13 of caller function
56 112 saved r14 of caller function
60 120 saved r15 of caller function
64 128 saved f4 of caller function
72 132 saved f6 of caller function
80 undefined
96 160 outgoing args passed from caller to callee
96+x 160+x possible stack alignment ( 8 bytes desirable )
96+x+y 160+x+y alloca space of caller ( if used )
96+x+y+z 160+x+y+z automatics of caller ( if used )
0 back-chain
A sample program with comments.
===============================
Comments on the function test
-----------------------------
1) It didn't need to set up a pointer to the constant pool gpr13 as it isn't used
( :-( ).
2) This is a frameless function & no stack is bought.
3) The compiler was clever enough to recognise that it could return the
value in r2 as well as use it for the passed in parameter ( :-) ).
4) The basr ( branch relative & save ) trick works as follows the instruction
has a special case with r0,r0 with some instruction operands is understood as
the literal value 0, some risc architectures also do this ). So now
we are branching to the next address & the address new program counter is
in r13,so now we subtract the size of the function prologue we have executed
+ the size of the literal pool to get to the top of the literal pool
0040037c int test(int b)
{ # Function prologue below
40037c: 90 de f0 34 stm %r13,%r14,52(%r15) # Save registers r13 & r14
400380: 0d d0 basr %r13,%r0 # Set up pointer to constant pool using
400382: a7 da ff fa ahi %r13,-6 # basr trick
return(5+b);
# Huge main program
400386: a7 2a 00 05 ahi %r2,5 # add 5 to r2
# Function epilogue below
40038a: 98 de f0 34 lm %r13,%r14,52(%r15) # restore registers r13 & 14
40038e: 07 fe br %r14 # return
}
Comments on the function main
-----------------------------
1) The compiler did this function optimally ( 8-) )
Literal pool for main.
400390: ff ff ff ec .long 0xffffffec
main(int argc,char *argv[])
{ # Function prologue below
400394: 90 bf f0 2c stm %r11,%r15,44(%r15) # Save necessary registers
400398: 18 0f lr %r0,%r15 # copy stack pointer to r0
40039a: a7 fa ff a0 ahi %r15,-96 # Make area for callee saving
40039e: 0d d0 basr %r13,%r0 # Set up r13 to point to
4003a0: a7 da ff f0 ahi %r13,-16 # literal pool
4003a4: 50 00 f0 00 st %r0,0(%r15) # Save backchain
return(test(5)); # Main Program Below
4003a8: 58 e0 d0 00 l %r14,0(%r13) # load relative address of test from
# literal pool
4003ac: a7 28 00 05 lhi %r2,5 # Set first parameter to 5
4003b0: 4d ee d0 00 bas %r14,0(%r14,%r13) # jump to test setting r14 as return
# address using branch & save instruction.
# Function Epilogue below
4003b4: 98 bf f0 8c lm %r11,%r15,140(%r15)# Restore necessary registers.
4003b8: 07 fe br %r14 # return to do program exit
}
Compiler updates
----------------
main(int argc,char *argv[])
{
4004fc: 90 7f f0 1c stm %r7,%r15,28(%r15)
400500: a7 d5 00 04 bras %r13,400508 <main+0xc>
400504: 00 40 04 f4 .long 0x004004f4
# compiler now puts constant pool in code to so it saves an instruction
400508: 18 0f lr %r0,%r15
40050a: a7 fa ff a0 ahi %r15,-96
40050e: 50 00 f0 00 st %r0,0(%r15)
return(test(5));
400512: 58 10 d0 00 l %r1,0(%r13)
400516: a7 28 00 05 lhi %r2,5
40051a: 0d e1 basr %r14,%r1
# compiler adds 1 extra instruction to epilogue this is done to
# avoid processor pipeline stalls owing to data dependencies on g5 &
# above as register 14 in the old code was needed directly after being loaded
# by the lm %r11,%r15,140(%r15) for the br %14.
40051c: 58 40 f0 98 l %r4,152(%r15)
400520: 98 7f f0 7c lm %r7,%r15,124(%r15)
400524: 07 f4 br %r4
}
Hartmut ( our compiler developer ) also has been threatening to take out the
stack backchain in optimised code as this also causes pipeline stalls, you
have been warned.
64 bit z/Architecture code disassembly
--------------------------------------
If you understand the stuff above you'll understand the stuff
below too so I'll avoid repeating myself & just say that
some of the instructions have g's on the end of them to indicate
they are 64 bit & the stack offsets are a bigger,
the only other difference you'll find between 32 & 64 bit is that
we now use f4 & f6 for floating point arguments on 64 bit.
00000000800005b0 <test>:
int test(int b)
{
return(5+b);
800005b0: a7 2a 00 05 ahi %r2,5
800005b4: b9 14 00 22 lgfr %r2,%r2 # downcast to integer
800005b8: 07 fe br %r14
800005ba: 07 07 bcr 0,%r7
}
00000000800005bc <main>:
main(int argc,char *argv[])
{
800005bc: eb bf f0 58 00 24 stmg %r11,%r15,88(%r15)
800005c2: b9 04 00 1f lgr %r1,%r15
800005c6: a7 fb ff 60 aghi %r15,-160
800005ca: e3 10 f0 00 00 24 stg %r1,0(%r15)
return(test(5));
800005d0: a7 29 00 05 lghi %r2,5
# brasl allows jumps > 64k & is overkill here bras would do fune
800005d4: c0 e5 ff ff ff ee brasl %r14,800005b0 <test>
800005da: e3 40 f1 10 00 04 lg %r4,272(%r15)
800005e0: eb bf f0 f8 00 04 lmg %r11,%r15,248(%r15)
800005e6: 07 f4 br %r4
}
Compiling programs for debugging on Linux for s/390 & z/Architecture
====================================================================
-gdwarf-2 now works it should be considered the default debugging
format for s/390 & z/Architecture as it is more reliable for debugging
shared libraries, normal -g debugging works much better now
Thanks to the IBM java compiler developers bug reports.
This is typically done adding/appending the flags -g or -gdwarf-2 to the
CFLAGS & LDFLAGS variables Makefile of the program concerned.
If using gdb & you would like accurate displays of registers &
stack traces compile without optimisation i.e make sure
that there is no -O2 or similar on the CFLAGS line of the Makefile &
the emitted gcc commands, obviously this will produce worse code
( not advisable for shipment ) but it is an aid to the debugging process.
This aids debugging because the compiler will copy parameters passed in
in registers onto the stack so backtracing & looking at passed in
parameters will work, however some larger programs which use inline functions
will not compile without optimisation.
Debugging with optimisation has since much improved after fixing
some bugs, please make sure you are using gdb-5.0 or later developed
after Nov'2000.
Figuring out gcc compile errors
===============================
If you are getting a lot of syntax errors compiling a program & the problem
isn't blatantly obvious from the source.
It often helps to just preprocess the file, this is done with the -E
option in gcc.
What this does is that it runs through the very first phase of compilation
( compilation in gcc is done in several stages & gcc calls many programs to
achieve its end result ) with the -E option gcc just calls the gcc preprocessor (cpp).
The c preprocessor does the following, it joins all the files #included together
recursively ( #include files can #include other files ) & also the c file you wish to compile.
It puts a fully qualified path of the #included files in a comment & it
does macro expansion.
This is useful for debugging because
1) You can double check whether the files you expect to be included are the ones
that are being included ( e.g. double check that you aren't going to the i386 asm directory ).
2) Check that macro definitions aren't clashing with typedefs,
3) Check that definitions aren't being used before they are being included.
4) Helps put the line emitting the error under the microscope if it contains macros.
For convenience the Linux kernel's makefile will do preprocessing automatically for you
by suffixing the file you want built with .i ( instead of .o )
e.g.
from the linux directory type
make arch/s390/kernel/signal.i
this will build
s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
-fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce -E arch/s390/kernel/signal.c
> arch/s390/kernel/signal.i
Now look at signal.i you should see something like.
# 1 "/home1/barrow/linux/include/asm/types.h" 1
typedef unsigned short umode_t;
typedef __signed__ char __s8;
typedef unsigned char __u8;
typedef __signed__ short __s16;
typedef unsigned short __u16;
If instead you are getting errors further down e.g.
unknown instruction:2515 "move.l" or better still unknown instruction:2515
"Fixme not implemented yet, call Martin" you are probably are attempting to compile some code
meant for another architecture or code that is simply not implemented, with a fixme statement
stuck into the inline assembly code so that the author of the file now knows he has work to do.
To look at the assembly emitted by gcc just before it is about to call gas ( the gnu assembler )
use the -S option.
Again for your convenience the Linux kernel's Makefile will hold your hand &
do all this donkey work for you also by building the file with the .s suffix.
e.g.
from the Linux directory type
make arch/s390/kernel/signal.s
s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
-fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce -S arch/s390/kernel/signal.c
-o arch/s390/kernel/signal.s
This will output something like, ( please note the constant pool & the useful comments
in the prologue to give you a hand at interpreting it ).
.LC54:
.string "misaligned (__u16 *) in __xchg\n"
.LC57:
.string "misaligned (__u32 *) in __xchg\n"
.L$PG1: # Pool sys_sigsuspend
.LC192:
.long -262401
.LC193:
.long -1
.LC194:
.long schedule-.L$PG1
.LC195:
.long do_signal-.L$PG1
.align 4
.globl sys_sigsuspend
.type sys_sigsuspend,@function
sys_sigsuspend:
# leaf function 0
# automatics 16
# outgoing args 0
# need frame pointer 0
# call alloca 0
# has varargs 0
# incoming args (stack) 0
# function length 168
STM 8,15,32(15)
LR 0,15
AHI 15,-112
BASR 13,0
.L$CO1: AHI 13,.L$PG1-.L$CO1
ST 0,0(15)
LR 8,2
N 5,.LC192-.L$PG1(13)
Adding -g to the above output makes the output even more useful
e.g. typing
make CC:="s390-gcc -g" kernel/sched.s
which compiles.
s390-gcc -g -D__KERNEL__ -I/home/barrow/linux-2.3/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -fno-strict-aliasing -pipe -fno-strength-reduce -S kernel/sched.c -o kernel/sched.s
also outputs stabs ( debugger ) info, from this info you can find out the
offsets & sizes of various elements in structures.
e.g. the stab for the structure
struct rlimit {
unsigned long rlim_cur;
unsigned long rlim_max;
};
is
.stabs "rlimit:T(151,2)=s8rlim_cur:(0,5),0,32;rlim_max:(0,5),32,32;;",128,0,0,0
from this stab you can see that
rlimit_cur starts at bit offset 0 & is 32 bits in size
rlimit_max starts at bit offset 32 & is 32 bits in size.
Debugging Tools:
================
objdump
=======
This is a tool with many options the most useful being ( if compiled with -g).
objdump --source <victim program or object file> > <victims debug listing >
The whole kernel can be compiled like this ( Doing this will make a 17MB kernel
& a 200 MB listing ) however you have to strip it before building the image
using the strip command to make it a more reasonable size to boot it.
A source/assembly mixed dump of the kernel can be done with the line
objdump --source vmlinux > vmlinux.lst
Also, if the file isn't compiled -g, this will output as much debugging information
as it can (e.g. function names). This is very slow as it spends lots
of time searching for debugging info. The following self explanatory line should be used
instead if the code isn't compiled -g, as it is much faster:
objdump --disassemble-all --syms vmlinux > vmlinux.lst
As hard drive space is valuable most of us use the following approach.
1) Look at the emitted psw on the console to find the crash address in the kernel.
2) Look at the file System.map ( in the linux directory ) produced when building
the kernel to find the closest address less than the current PSW to find the
offending function.
3) use grep or similar to search the source tree looking for the source file
with this function if you don't know where it is.
4) rebuild this object file with -g on, as an example suppose the file was
( /arch/s390/kernel/signal.o )
5) Assuming the file with the erroneous function is signal.c Move to the base of the
Linux source tree.
6) rm /arch/s390/kernel/signal.o
7) make /arch/s390/kernel/signal.o
8) watch the gcc command line emitted
9) type it in again or alternatively cut & paste it on the console adding the -g option.
10) objdump --source arch/s390/kernel/signal.o > signal.lst
This will output the source & the assembly intermixed, as the snippet below shows
This will unfortunately output addresses which aren't the same
as the kernel ones you should be able to get around the mental arithmetic
by playing with the --adjust-vma parameter to objdump.
static inline void spin_lock(spinlock_t *lp)
{
a0: 18 34 lr %r3,%r4
a2: a7 3a 03 bc ahi %r3,956
__asm__ __volatile(" lhi 1,-1\n"
a6: a7 18 ff ff lhi %r1,-1
aa: 1f 00 slr %r0,%r0
ac: ba 01 30 00 cs %r0,%r1,0(%r3)
b0: a7 44 ff fd jm aa <sys_sigsuspend+0x2e>
saveset = current->blocked;
b4: d2 07 f0 68 mvc 104(8,%r15),972(%r4)
b8: 43 cc
return (set->sig[0] & mask) != 0;
}
6) If debugging under VM go down to that section in the document for more info.
I now have a tool which takes the pain out of --adjust-vma
& you are able to do something like
make /arch/s390/kernel/traps.lst
& it automatically generates the correctly relocated entries for
the text segment in traps.lst.
This tool is now standard in linux distro's in scripts/makelst
strace:
-------
Q. What is it ?
A. It is a tool for intercepting calls to the kernel & logging them
to a file & on the screen.
Q. What use is it ?
A. You can use it to find out what files a particular program opens.
Example 1
---------
If you wanted to know does ping work but didn't have the source
strace ping -c 1 127.0.0.1
& then look at the man pages for each of the syscalls below,
( In fact this is sometimes easier than looking at some spaghetti
source which conditionally compiles for several architectures ).
Not everything that it throws out needs to make sense immediately.
Just looking quickly you can see that it is making up a RAW socket
for the ICMP protocol.
Doing an alarm(10) for a 10 second timeout
& doing a gettimeofday call before & after each read to see
how long the replies took, & writing some text to stdout so the user
has an idea what is going on.
socket(PF_INET, SOCK_RAW, IPPROTO_ICMP) = 3
getuid() = 0
setuid(0) = 0
stat("/usr/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory)
stat("/usr/share/locale/libc/C", 0xbffff134) = -1 ENOENT (No such file or directory)
stat("/usr/local/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory)
getpid() = 353
setsockopt(3, SOL_SOCKET, SO_BROADCAST, [1], 4) = 0
setsockopt(3, SOL_SOCKET, SO_RCVBUF, [49152], 4) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(3, 1), ...}) = 0
mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40008000
ioctl(1, TCGETS, {B9600 opost isig icanon echo ...}) = 0
write(1, "PING 127.0.0.1 (127.0.0.1): 56 d"..., 42PING 127.0.0.1 (127.0.0.1): 56 data bytes
) = 42
sigaction(SIGINT, {0x8049ba0, [], SA_RESTART}, {SIG_DFL}) = 0
sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {SIG_DFL}) = 0
gettimeofday({948904719, 138951}, NULL) = 0
sendto(3, "\10\0D\201a\1\0\0\17#\2178\307\36"..., 64, 0, {sin_family=AF_INET,
sin_port=htons(0), sin_addr=inet_addr("127.0.0.1")}, 16) = 64
sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0
sigaction(SIGALRM, {0x8049ba0, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0
alarm(10) = 0
recvfrom(3, "E\0\0T\0005\0\0@\1|r\177\0\0\1\177"..., 192, 0,
{sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84
gettimeofday({948904719, 160224}, NULL) = 0
recvfrom(3, "E\0\0T\0006\0\0\377\1\275p\177\0"..., 192, 0,
{sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84
gettimeofday({948904719, 166952}, NULL) = 0
write(1, "64 bytes from 127.0.0.1: icmp_se"...,
5764 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=28.0 ms
Example 2
---------
strace passwd 2>&1 | grep open
produces the following output
open("/etc/ld.so.cache", O_RDONLY) = 3
open("/opt/kde/lib/libc.so.5", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/lib/libc.so.5", O_RDONLY) = 3
open("/dev", O_RDONLY) = 3
open("/var/run/utmp", O_RDONLY) = 3
open("/etc/passwd", O_RDONLY) = 3
open("/etc/shadow", O_RDONLY) = 3
open("/etc/login.defs", O_RDONLY) = 4
open("/dev/tty", O_RDONLY) = 4
The 2>&1 is done to redirect stderr to stdout & grep is then filtering this input
through the pipe for each line containing the string open.
Example 3
---------
Getting sophisticated
telnetd crashes & I don't know why
Steps
-----
1) Replace the following line in /etc/inetd.conf
telnet stream tcp nowait root /usr/sbin/in.telnetd -h
with
telnet stream tcp nowait root /blah
2) Create the file /blah with the following contents to start tracing telnetd
#!/bin/bash
/usr/bin/strace -o/t1 -f /usr/sbin/in.telnetd -h
3) chmod 700 /blah to make it executable only to root
4)
killall -HUP inetd
or ps aux | grep inetd
get inetd's process id
& kill -HUP inetd to restart it.
Important options
-----------------
-o is used to tell strace to output to a file in our case t1 in the root directory
-f is to follow children i.e.
e.g in our case above telnetd will start the login process & subsequently a shell like bash.