cmd/dist: go_bootstrap is broken on arm

[davecheney]

cmd/go
./make.bash: line 153:  2242 Segmentation fault      (core dumped) "$GOTOOLDIR"/go_bootstrap clean -i std
Build complete, duration 4m48.860849034s. Result: error: exit status 139

The arm build broke somewhere around revision 2ec2931 but was obscured by #9734

[davecheney]

I believe this is an alignment issue

rogram received signal SIGSEGV, Segmentation fault.
0x0009fcec in runtime.argv_index (argv=0xbefff7c9, i=525, ~r2=0x7061 <error: Cannot access memory at address 0x7061>) at /home/dfc/go/src/runtime/runtime1.go:45
45              return *(**byte)(add(unsafe.Pointer(argv), uintptr(i)*ptrSize))
(gdb) disassemble 
Dump of assembler code for function runtime.argv_index:
   0x0009fcd4 <+0>:     mov     r0, #0
   0x0009fcd8 <+4>:     ldr     r3, [sp, #4]
   0x0009fcdc <+8>:     ldr     r0, [sp, #8]
   0x0009fce0 <+12>:    lsl     r2, r0, #2
   0x0009fce4 <+16>:    mov     r0, #0
   0x0009fce8 <+20>:    add     r0, r3, r2
=> 0x0009fcec <+24>:    ldr     r1, [r0]
   0x0009fcf0 <+28>:    str     r1, [sp, #12]
   0x0009fcf4 <+32>:    add     pc, lr, #0
End of assembler dump.
(gdb) info registers 
r0             0xbefffffd       3204448253    <<< boom, not word aligned and we're using LDR
r1             0x7061   28769

[randall77]

The argv pointer is not aligned. It gets set by runtime1.go:args(), and that gets called from asm_arm.s:rt0_go. So either that bad argv is from something getting clobbered, or the main argv started out as bad. We should probably add an assert to args() to be sure.

[davecheney]

This code has not changed betwen g01.4 and tip. What I think is an issue is 5a is miscompiling MOVM.P.W [R0,R1],0(R13)

Compare the output from 5a compiling rt0_linux_arm.s

anda(~/go/src) % ../../go1.4/pkg/tool/linux_arm/5a -I cmd/ld -S runtime/rt0_linux_arm.s | grep -A5 ^_rt0_arm_linux1                                                      
_rt0_arm_linux1 t=1 nosplit size=16 value=0 args=0xffffffff80000000 locals=0x0 leaf
        0x0000 00000 (runtime/rt0_linux_arm.s:13)       TEXT    _rt0_arm_linux1+0(SB),4,$-4--2147483648
        0x0000 00000 (runtime/rt0_linux_arm.s:24)       MOVM.P.W        [R0,R1],0(R13)
        0x0004 00004 (runtime/rt0_linux_arm.s:45)       MOVW    $20,R7
        0x0008 00008 (runtime/rt0_linux_arm.s:46)       SWI     ,$0
        0x000c 00012 (runtime/rt0_linux_arm.s:56)       B       ,runtime.rt0_go(SB)

and tip

panda(~/go/src) % ../../go/pkg/tool/linux_arm/5a -I cmd/ld -S runtime/rt0_linux_arm.s | grep -A5 ^_rt0_arm_linux1
_rt0_arm_linux1 t=1 nosplit size=16 value=0 args=0xffffffff80000000 locals=0x0 leaf
        0x0000 00000 (runtime/rt0_linux_arm.s:13)       TEXT    _rt0_arm_linux1+0(SB),4,$-4--2147483648
        0x0000 00000 (runtime/rt0_linux_arm.s:24)       MOVM.P.W        ],0(R13)
        0x0004 00004 (runtime/rt0_linux_arm.s:45)       MOVW    $20,R7
        0x0008 00008 (runtime/rt0_linux_arm.s:46)       SWI     ,$0
        0x000c 00012 (runtime/rt0_linux_arm.s:56)       B       ,runtime.rt0_go(SB)

gdb also shows the same for a go1.4 binary

(gdb) disassemble/r _rt0_arm_linux1
Dump of assembler code for function _rt0_arm_linux1:
   0x000728b8 <+0>:     03 00 2d e9     push    {r0, r1}
   0x000728bc <+4>:     14 70 a0 e3     mov     r7, #20
   0x000728c0 <+8>:     00 00 00 ef     svc     0x00000000
   0x000728c4 <+12>:    04 d0 4d e2     sub     sp, sp, #4
   0x000728c8 <+16>:    52 62 ff eb     bl      0x4b218 <runtime.setup_auxv>
   0x000728cc <+20>:    04 d0 8d e2     add     sp, sp, #4
   0x000728d0 <+24>:    39 f5 ff ea     b       0x6fdbc <runtime.rt0_go>

and compiled with tip's 5a

(gdb) disassemble /r _rt0_arm_linux1
Dump of assembler code for function _rt0_arm_linux1:
   0x000c62dc <+0>:     00 00 2d e9     push    {}      ; <UNPREDICTABLE>
   0x000c62e0 <+4>:     14 70 a0 e3     mov     r7, #20
   0x000c62e4 <+8>:     00 00 00 ef     svc     0x00000000
   0x000c62e8 <+12>:    80 f3 ff ea     b       0xc30f0 <runtime.rt0_go>

note the push instruction is malformed so R1 which holds argv is not saved correctly.

I believe that the other MOVM encodings are simliarly affected

[minux]

which revision are you testing for tip? I tried the latest master branch, cross compiling on x86_64 produces the correct result. 0006d6d8 <_rt0_arm_linux1>: 6d6d8: e92d0003 push {r0, r1} 6d6dc: e3a07014 mov r7, #20 6d6e0: ef000000 svc 0x00000000 6d6e4: eafff384 b 6a4fc <runtime.rt0_go> The only difference between a real ARM and x86_64 besides word size is that char is unsigned on ARM, so I tried -funsigned-char, but make.bash still produced the correct go command (at least _rt0_arm_linux1 is correctly assembled.)

[davecheney]

I'm at 5ab3823c and I see

(gdb) disassemble _rt0_arm_linux1
Dump of assembler code for function _rt0_arm_linux1:
0x000c6db4 <+0>: push {} ;
0x000c6db8 <+4>: mov r7, #20
0x000c6dbc <+8>: svc 0x00000000
0x000c6dc0 <+12>: b 0xc3bc8 <runtime.rt0_go>
End of assembler dump.

[ianlancetaylor]

What's the status of this issue? The ARM Linux builders seem to be working on the dashboard.

[rsc]

Running

GOOS=linux GOARCH=arm go tool asm -S rt0_linux_arm.s

I get:

_rt0_arm_linux1 t=1 nosplit size=16 value=0 args=0xffffffff80000000 locals=0x0 leaf
    0x0000 00000 (rt0_linux_arm.s:65)   TEXT    _rt0_arm_linux1(SB), 4, $-4
    0x0000 00000 (rt0_linux_arm.s:76)   MOVM.P.W    [R0,R1], (R13)
    0x0004 00004 (rt0_linux_arm.s:97)   MOVW    $20, R7
    0x0008 00008 (rt0_linux_arm.s:98)   SWI $0
    0x000c 00012 (rt0_linux_arm.s:108)  JMP runtime.rt0_go(SB)
    0x0000 03 00 2d e9 14 70 a0 e3 00 00 00 ef 00 00 00 ea  ..-..p..........
    rel 12+4 t=6 runtime.rt0_go+eafffffe

The instruction at 0x0000 is encoded as e92d0003, which is what was reported as the correct encoding, so I believe this is fixed (that and the arm builders are working).