Hi Ingo and Josh, On Sat, 10 Jul 2021 00:31:40 +0900 Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > > > +STACK_FRAME_NON_STANDARD(kretprobe_trampoline); > > > +#undef UNWIND_HINT_FUNC > > > +#define UNWIND_HINT_FUNC > > > +#endif > > > /* > > > * When a retprobed function returns, this code saves registers and > > > * calls trampoline_handler() runs, which calls the kretprobe's handler. > > > @@ -1031,6 +1044,7 @@ asm( > > > /* We don't bother saving the ss register */ > > > #ifdef CONFIG_X86_64 > > > " pushq %rsp\n" > > > + UNWIND_HINT_FUNC > > > " pushfq\n" > > > SAVE_REGS_STRING > > > " movq %rsp, %rdi\n" > > > @@ -1041,6 +1055,7 @@ asm( > > > " popfq\n" > > > #else > > > " pushl %esp\n" > > > + UNWIND_HINT_FUNC > > > " pushfl\n" > > > SAVE_REGS_STRING > > > " movl %esp, %eax\n" > > > > Why not provide an appropriate annotation method in <asm/unwind_hints.h>, > > so that other future code can use it too instead of reinventing the wheel? I think I got what you meant. Let me summarize the issue. In case of CONFIG_FRAME_POINTER=n, it is OK just adding UNWIND_HINT_FUNC. In case of CONFIG_FRAME_POINTER=y, without STACK_FRAME_NON_STANDARD(), the objtool complains that a CALL instruction without the frame pointer. --- arch/x86/kernel/kprobes/core.o: warning: objtool: __kretprobe_trampoline()+0x25: call without frame pointer save/setup --- If we just add STACK_FRAME_NON_STANDARD() with UNWIND_HINT_FUNC macro, the objtool complains that non-standard function has unwind hint. --- arch/x86/kernel/kprobes/core.o: warning: objtool: __kretprobe_trampoline()+0x1: BUG: why am I validating an ignored function? --- Thus, add STACK_FRAME_NON_STANDARD() and undefine UNWIND_HINT_FUNC macro, the objtool doesn't complain. This means that the STACK_FRAME_NON_STANDARD() and UNWIND_HINT_FUNC macro are mutually exclusive. However, those macros are used different way. The STACK_FRAME_NON_STANDARD() will have the target symbol and the UNWIND_HINT_FUNC needs to be embedded in the target code. Thus we can not combine them in general. If we can have something like UNWIND_HINT_FUNC_NO_FP, it may solve this issue without ugly #ifdef and #undef. Is that correct? Maybe I can add UNWIND_HINT_TYPE_FUNC_NO_FP for UNWIND_HINT and make objtool ignore the call without frame pointer. This makes an exception that the kretprobe_trampoline will be noted in '.discard.unwind_hints' section instead of '.discard.func_stack_frame_non_standard' section. Or another idea is to introduce ANNOTATE_NO_FP_FUNCTION_CALL with a new '.discard.no_fp_function_calls' section. What do you think these ideas? Thank you, -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
