changed section .rela__jump_table not selected for inclusion

[jpoimboe]

~ $ cat jumptablefail.patch 
Index: src/kernel/timer.c
===================================================================
--- src.orig/kernel/timer.c
+++ src/kernel/timer.c
@@ -1370,6 +1370,8 @@ static void run_timer_softirq(struct sof
 {
    struct tvec_base *base = __this_cpu_read(tvec_bases);

+   printk("run_timer_softirq\n");
+
    hrtimer_run_pending();

    if (time_after_eq(jiffies, base->timer_jiffies))
~ $ kpatch-build -d jumptablefail.patch
DEBUG mode enabled
Using cache at /home/jpoimboe/.kpatch/3.13.10-200.fc20.x86_64/src
Testing patch file
patching file kernel/timer.c
Building original kernel
Building patched kernel
Detecting changed objects
Rebuilding changed objects
Extracting new and modified ELF sections
timer.o: changed function: run_timer_softirq
timer.o: changed function: timer_cpu_notify
timer.o: changed section .rela__jump_table not selected for inclusion
/usr/local/libexec/kpatch/create-diff-object: unreconcilable difference
ERROR: kpatch build failed. Check /tmp/kpatch-build-1398801879.log for more details.

[jpoimboe]

This is very similar to the .smp_locks issue, except these relocations are 8 bytes:

Relocation section [73] '.rela__jump_table' for section [72] '__jump_table' at offset 0x11bb0 contains 36 entries:
  Offset              Type            Value               Addend Name
  000000000000000000  X86_64_64       000000000000000000     +43 .text.call_timer_fn
  0x0000000000000008  X86_64_64       000000000000000000    +160 .text.call_timer_fn
  0x0000000000000010  X86_64_64       0x0000000000000280      +8 __tracepoint_timer_expire_entry
  0x0000000000000018  X86_64_64       000000000000000000     +54 .text.call_timer_fn
  0x0000000000000020  X86_64_64       000000000000000000    +112 .text.call_timer_fn
  0x0000000000000028  X86_64_64       0x0000000000000240      +8 __tracepoint_timer_expire_exit
  0x0000000000000030  X86_64_64       000000000000000000    +257 .text.run_timer_softirq
  0x0000000000000038  X86_64_64       000000000000000000    +416 .text.run_timer_softirq
  0x0000000000000040  X86_64_64       0x0000000000000200      +8 __tracepoint_timer_cancel
  0x0000000000000048  X86_64_64       000000000000000000     +20 .text.init_timer_key
  0x0000000000000050  X86_64_64       000000000000000000    +112 .text.init_timer_key
  0x0000000000000058  X86_64_64       0x0000000000000300      +8 __tracepoint_timer_init
  0x0000000000000060  X86_64_64       000000000000000000     +35 .text.detach_if_pending
  0x0000000000000068  X86_64_64       000000000000000000    +128 .text.detach_if_pending
  0x0000000000000070  X86_64_64       0x0000000000000200      +8 __tracepoint_timer_cancel
  0x0000000000000078  X86_64_64       000000000000000000     +32 .text.migrate_timer_list
  0x0000000000000080  X86_64_64       000000000000000000    +120 .text.migrate_timer_list
  0x0000000000000088  X86_64_64       0x0000000000000200      +8 __tracepoint_timer_cancel
  0x0000000000000090  X86_64_64       000000000000000000     +80 .sched.text
  0x0000000000000098  X86_64_64       000000000000000000    +440 .sched.text
  0x00000000000000a0  X86_64_64       0x0000000000000300      +8 __tracepoint_timer_init
  0x00000000000000a8  X86_64_64       000000000000000000    +237 .sched.text
  0x00000000000000b0  X86_64_64       000000000000000000    +504 .sched.text
  0x00000000000000b8  X86_64_64       0x00000000000002c0      +8 __tracepoint_timer_start
  0x00000000000000c0  X86_64_64       000000000000000000    +115 .text.add_timer_on
  0x00000000000000c8  X86_64_64       000000000000000000    +176 .text.add_timer_on
  0x00000000000000d0  X86_64_64       0x00000000000002c0      +8 __tracepoint_timer_start
  0x00000000000000d8  X86_64_64       000000000000000000     +95 .text.mod_timer_pinned
  0x00000000000000e0  X86_64_64       000000000000000000    +232 .text.mod_timer_pinned
  0x00000000000000e8  X86_64_64       0x00000000000002c0      +8 __tracepoint_timer_start
  0x00000000000000f0  X86_64_64       000000000000000000    +128 .text.mod_timer_pending
  0x00000000000000f8  X86_64_64       000000000000000000    +256 .text.mod_timer_pending
  0x0000000000000100  X86_64_64       0x00000000000002c0      +8 __tracepoint_timer_start
  0x0000000000000108  X86_64_64       000000000000000000    +136 .text.mod_timer
  0x0000000000000110  X86_64_64       000000000000000000    +144 .text.mod_timer
  0x0000000000000118  X86_64_64       0x00000000000002c0      +8 __tracepoint_timer_start

[sjenning]

Revised analysis:

__jump_table is a table of struct jump_entry, 24 bytes in size on x86_64, init'ed to 0. All three fields are reloc'ed.

from arch/x86/include/asm/jump_label.h

#ifdef CONFIG_X86_64
typedef u64 jump_label_t;
#else
typedef u32 jump_label_t;
#endif

struct jump_entry {
    jump_label_t code;
    jump_label_t target;
    jump_label_t key;
};

.rela__jump_table looks like this:

Relocation section [73] '.rela__jump_table' for section [72] '__jump_table' at offset 0x11bb0 contains 36 entries:
  Offset              Type            Value               Addend Name
  000000000000000000  X86_64_64       000000000000000000     +43 .text.call_timer_fn
  0x0000000000000008  X86_64_64       000000000000000000    +160 .text.call_timer_fn
  0x0000000000000010  X86_64_64       0x0000000000000280      +8 __tracepoint_timer_expire_entry

The "code" and "target" fields point to locations within the function with the tracepoint. The "key" field points to a symbol "__tracepoint_timer_expire_entry" which references the __tracepoints section. There is a _tracepoint* symbol for each tracepoint in the __tracepoints section.

The __tracepoints section, in turn, is a table of struct tracepoints, 64 bytes in size on x86_64, init'ed to 0.

from include/linux/tracepoint.h

struct tracepoint {
    const char *name;       /* Tracepoint name */
    struct static_key key;
    void (*regfunc)(void);
    void (*unregfunc)(void);
    struct tracepoint_func __rcu *funcs;
};

The "name" field is reloc'ed, referencing an offset in a string table contained in the __tracepoints_strings section:

Relocation section [288] '.rela__tracepoints' for section [287] '__tracepoints' at offset 0x14f88 contains 13 entries:
  Offset              Type            Value               Addend Name
  000000000000000000  X86_64_64       000000000000000000      +0 __tracepoints_strings
  0x0000000000000040  X86_64_64       000000000000000000     +10 __tracepoints_strings
  0x0000000000000080  X86_64_64       000000000000000000     +24 __tracepoints_strings
...

There are _tpstrtab* symbols pointing to each string in the string in the table.

Additionally there is a __tracepoints_ptrs section that is a table of pointers that reference the struct tracepoints in the __tracepoints section. There are also _tracepoint_ptr* symbols that reference the pointers in __tracepoint_ptrs.

So... this rabbit hole goes much deeper than originally thought.