linux-kernel - Re: [PATCH bpf-next v2] add function metadata support

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Message-ID: <20250227165302.GB5880@noisy.programming.kicks-ass.net>
Date: Thu, 27 Feb 2025 17:53:02 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: rostedt@...dmis.org, mark.rutland@....com, alexei.starovoitov@...il.com,
	catalin.marinas@....com, will@...nel.org, mhiramat@...nel.org,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	mathieu.desnoyers@...icios.com, nathan@...nel.org,
	ndesaulniers@...gle.com, morbo@...gle.com, justinstitt@...gle.com,
	dongml2@...natelecom.cn, akpm@...ux-foundation.org, rppt@...nel.org,
	graf@...zon.com, dan.j.williams@...el.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH bpf-next v2] add function metadata support

On Wed, Feb 26, 2025 at 08:15:37PM +0800, Menglong Dong wrote:

> In x86, we need 5-bytes to prepend a "mov %eax xxx" insn, which can hold
> a 4-bytes index. So we have following logic:
> 
> 1. use the head 5-bytes if CFI_CLANG is not enabled
> 2. use the tail 5-bytes if MITIGATION_CALL_DEPTH_TRACKING is not enabled
> 3. compile the kernel with extra 5-bytes padding if
>    MITIGATION_CALL_DEPTH_TRACKING and CFI_CLANG are both enabled.

3) would result in 16+5 bytes padding, what does that do for alignment?

Functions should be 16 byte aligned.

Also, did you make sure all the code in arch/x86/kernel/alternative.c
still works? Because adding extra padding in the CFI_CLANG case moves
where the CFI bytes are emitted and all the CFI rewriting code goes
sideways.