linux-kernel - Re: kernel crashes in BPF JIT code with kCFI and clang on x86

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Message-ID: <20260107154404.GJ3708021@noisy.programming.kicks-ass.net>
Date: Wed, 7 Jan 2026 16:44:04 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Rustam Kovhaev <rkovhaev@...il.com>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
	Alexei Starovoitov <ast@...nel.org>, linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org
Subject: Re: kernel crashes in BPF JIT code with kCFI and clang on x86

On Wed, Jan 07, 2026 at 10:36:39AM +0100, Peter Zijlstra wrote:
> On Mon, Dec 22, 2025 at 07:43:32PM -0800, Rustam Kovhaev wrote:

> > After switching to clang kbuild always generates these huge paddings in my kernel config:
> > rusty@...10:~/code/kbuild_rust$ grep -e IBT -e PADDING .config
> > CONFIG_CC_HAS_IBT=y
> > CONFIG_X86_KERNEL_IBT=y
> > CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING=0xa
> > CONFIG_CC_HAS_ENTRY_PADDING=y
> > CONFIG_FUNCTION_PADDING_CFI=59
> > CONFIG_FUNCTION_PADDING_BYTES=59
> > CONFIG_CALL_PADDING=y
> > CONFIG_FINEIBT=y
> 
> Oh gawd, you have FUNCTION_ALIGNMENT_64B. Yeah, I suppose that wasn't
> tested very well.
> 
> Let me go check all that code.

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