Re: [PATCH V3 6/9] objtool: Report inconsistent stack changes in alternative

[Peter Zijlstra]

On Tue, Apr 14, 2020 at 12:36:15PM +0200, Alexandre Chartre wrote:
> To allow a valid stack unwinding, an alternative should have code
> where the same stack changes happens at the same places as in the
> original code. Add a check in objtool to validate that stack changes
> in alternative are effectively consitent with the original code.

This thing is completely buggered, it warns all over the place, even for
obviously correct alternatives like:

0000000000000310 <return_to_handler>:
 310:   48 83 ec 18             sub    $0x18,%rsp
 314:   48 89 04 24             mov    %rax,(%rsp)
 318:   48 89 54 24 08          mov    %rdx,0x8(%rsp)
 31d:   48 89 ef                mov    %rbp,%rdi
 320:   e8 00 00 00 00          callq  325 <return_to_handler+0x15>
                        321: R_X86_64_PLT32     ftrace_return_to_handler-0x4
 325:   48 89 c7                mov    %rax,%rdi
 328:   48 8b 54 24 08          mov    0x8(%rsp),%rdx
 32d:   48 8b 04 24             mov    (%rsp),%rax
 331:   48 83 c4 18             add    $0x18,%rsp
 335:   ff e7                   jmpq   *%rdi
 337:   90                      nop
 338:   90                      nop
 339:   90                      nop


Where 335 has two alternatives:

   0:   e9 00 00 00 00          jmpq   5 <.altinstr_replacement+0x5>
                        1: R_X86_64_PLT32       __x86_retpoline_rdi-0x4

and

   5:   0f ae e8                lfence
   8:   ff e7                   jmpq   *%rdi


And it then comes back with:

  defconfig-build/arch/x86/kernel/ftrace_64.o: warning: objtool: .entry.text+0x335: error in alternative
  defconfig-build/arch/x86/kernel/ftrace_64.o: warning: objtool: .altinstr_replacement+0x5: in alternative 2
  defconfig-build/arch/x86/kernel/ftrace_64.o: warning: objtool: .altinstr_replacement+0x8: misaligned alternative state change

which is just utter crap, JMP has no (CFI) state change.