create-diff-object: Fix function pointer rela on s390

[sumanthkorikkar]

Typically, symbols within the same object file can be referenced directly for better performance. However, when the patched code stores a function pointer (R_390_GOTENT and rela symbol type is STT_FUNC), using a dynrela is a safer approach. This ensures that if the function is used as a asynchronous callback, the kernel does not attempt to execute an invalid pointer after the module is unloaded. Instead, with a dynrela, the kernel will invoke the original function, preventing potential crashes.

Test program: Test if the original function ptr address is printed during patch load / unload.
iii-i/linux@0f400db

Patched code (function ptr rela):
void *patchme(void) { printk(KERN_NOTICE "patched\n"); return patchme; }
0: c0 04 00 00 00 00 jgnop 0 <patchme>
6: eb ef f0 88 00 24 stmg %r14,%r15,136(%r15)
c: c4 28 00 00 00 00 lgrl %r2,c <patchme+0xc>
e: R_390_GOTENT .LC0+0x2
12: b9 04 00 ef lgr %r14,%r15
16: e3 f0 ff e8 ff 71 lay %r15,-24(%r15)
1c: e3 e0 f0 98 00 24 stg %r14,152(%r15)
22: c0 e5 00 00 00 00 brasl %r14,22 <patchme+0x22>
24: R_390_PLT32DBL _printk+0x2
28: c4 28 00 00 00 00 lgrl %r2,28 <patchme+0x28>
2a: R_390_GOTENT patchme+0x2 <- func ptr
2e: eb ef f0 a0 00 04 lmg %r14,%r15,160(%r15)
34: c0 f4 00 00 00 00 jg 34 <patchme+0x34>
36: R_390_PC32DBL __s390_indirect_jump_r14+0x2
3a: 07 07 nopr %r7
3c: 07 07 nopr %r7
3e: 07 07 nopr %r7

Reference:

1. Question about pointer equality #1437
2. kpatch-build, x86: do not use the patched functions as callbacks directly #755

[joe-lawrence]

Seems reasonable to me, and thanks for untangling that massive return line. Arch-specific switch may be longer to read, but IMHO way more readable for non-PowerPC cases.

Before merging, I'm trying a few more tests patching an OOT kernel module as trying to find perfect kernel code to test combinations (function pointers in .text, .data, etc.) for an integration test is difficult (as you found noticed). The best compromise we may currently have is to use an OOT module to generate a unit test (cached .o files), we'll see.