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.