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

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Message-ID: <20241001071841.yrc7cxdp2unnzju7@treble>
Date: Tue, 1 Oct 2024 00:18:41 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
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Subject: Re: [RFC PATCH 24/28] tools/objtool: Treat indirect ftrace calls as
 direct calls

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