Re: [RFC PATCH 24/28] tools/objtool: Treat indirect ftrace calls as direct calls

Josh Poimboeuf
2024-10-01T07:18:44Z

On Wed, Sep 25, 2024 at 05:01:24PM +0200, Ard Biesheuvel wrote:
> + if (insn->type == INSN_CALL_DYNAMIC) {
> + if (!reloc)
> + continue;
> +
> + /*
> + * GCC 13 and older on x86 will always emit the call to
> + * __fentry__ using a relaxable GOT-based symbol
> + * reference when operating in PIC mode, i.e.,
> + *
> + * call *0x0(%rip)
> + * R_X86_64_GOTPCRELX __fentry__-0x4
> + *
> + * where it is left up to the linker to relax this into
> + *
> + * call __fentry__
> + * nop
> + *
> + * if __fentry__ turns out to be DSO local, which is
> + * always the case for vmlinux. Given that this
> + * relaxation is mandatory per the x86_64 psABI, these
> + * calls can simply be treated as direct calls.
> + */
> + if (arch_ftrace_match(reloc->sym->name)) {
> + insn->type = INSN_CALL;
> + add_call_dest(file, insn, reloc->sym, false);
> + }

Can the compiler also do this for non-fentry direct calls? If so would
it make sense to generalize this by converting all
INSN_CALL_DYNAMIC+reloc to INSN_CALL?

And maybe something similar for add_jump_destinations().

--
Josh