From: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>
To: "Jérémy Jean" <Jeremy.Jean@oss.cyber.gouv.fr>,
"Alexei Starovoitov" <ast@kernel.org>,
"Daniel Borkmann" <daniel@iogearbox.net>,
"Andrii Nakryiko" <andrii@kernel.org>,
"Eduard Zingerman" <eddyz87@gmail.com>,
"Kumar Kartikeya Dwivedi" <memxor@gmail.com>,
"Martin KaFai Lau" <martin.lau@linux.dev>
Cc: <bpf@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] bpf, x86: Sign-extend narrow signed kfunc returns
Date: Tue, 25 Aug 2026 18:22:08 -0700 [thread overview]
Message-ID: <DKYHCWYCB5XV.3MSN6ZJGOW2R7@gmail.com> (raw)
In-Reply-To: <20260823203944.2084994-2-Jeremy.Jean@oss.cyber.gouv.fr>
On Sun Aug 23, 2026 at 1:39 PM PDT, Jérémy Jean wrote:
> bpf_res_spin_lock() returns a 32-bit int. On failure, the verifier
> models R0 as a signed 64-bit value in [-MAX_ERRNO, -1].
>
> On x86-64, returning -EDEADLK writes 0xffffffdd to EAX and clears the
> upper half of RAX. Since the JIT leaves the native return value as-is,
> BPF sees 0x00000000ffffffdd instead of the sign-extended
> 0xffffffffffffffdd. A 64-bit signed comparison therefore treats the
> value as positive, while the verifier treats it as -35.
>
> As a result, a signed comparison against zero can take one path during
> verification and another at run time. With rqspinlock aliases, this can
> lead to unmatched bpf_res_spin_unlock() calls, corrupting the per-CPU
> rqspinlock state and unbalancing the preemption count.
Is that a theory? I'm pretty sure we have selftests for negative
error codes from res_spin_lock.
Pls craft a selftest if the issue is real.
pw-bot: cr
next prev parent reply other threads:[~2026-08-26 1:22 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-23 20:39 [PATCH] bpf, x86: Sign-extend narrow signed kfunc returns Jérémy Jean
2026-08-23 20:52 ` sashiko-bot
2026-08-23 21:29 ` bot+bpf-ci
2026-08-26 1:22 ` Alexei Starovoitov [this message]
2026-08-26 14:20 ` Jérémy Jean
2026-10-04 18:22 ` Jérémy Jean
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