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Tue, 25 Aug 2026 18:22:08 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Tue, 25 Aug 2026 18:22:08 -0700 Message-Id: Cc: , Subject: Re: [PATCH] bpf, x86: Sign-extend narrow signed kfunc returns From: "Alexei Starovoitov" To: =?utf-8?q?J=C3=A9r=C3=A9my_Jean?= , "Alexei Starovoitov" , "Daniel Borkmann" , "Andrii Nakryiko" , "Eduard Zingerman" , "Kumar Kartikeya Dwivedi" , "Martin KaFai Lau" X-Mailer: aerc References: <20260823203944.2084994-2-Jeremy.Jean@oss.cyber.gouv.fr> In-Reply-To: <20260823203944.2084994-2-Jeremy.Jean@oss.cyber.gouv.fr> On Sun Aug 23, 2026 at 1:39 PM PDT, J=C3=A9r=C3=A9my 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