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

Peter Zijlstra
2020-04-16T14:33:38Z

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.