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[87.205.15.91]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c2aae687318sm266221466b.44.2026.09.24.01.49.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 24 Sep 2026 01:49:52 -0700 (PDT) From: Krystian Kaniewski To: Steven Rostedt , Vincent Donnefort , Masami Hiramatsu , linux-trace-kernel@vger.kernel.org Cc: Mathieu Desnoyers , linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, syzbot@lists.linux.dev, syzbot+de3d7f9bcc9212f3fae1@syzkaller.appspotmail.com Subject: [PATCH v2] ring-buffer: Fix false warning in ring_buffer_map_get_reader() Date: Thu, 24 Sep 2026 10:49:42 +0200 Message-ID: <20260924084946.20104-1-krystianmkaniewski@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <15bc282f-669e-4a94-911d-bb435513461f@mail.kernel.org> References: <15bc282f-669e-4a94-911d-bb435513461f@mail.kernel.org> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The mmap reader can warn when it catches up with a writer that is still committing events. rb_get_reader_page() returns NULL in this case, but ring_buffer_map_get_reader() treats that result as an error. The initial rb_per_cpu_empty() check filters out an empty buffer, but it does not guarantee that the next reader page has committed data. Writers do not take reader_lock and can advance commit_page before publishing the committed length on the new page. rb_get_reader_page() can swap to that page, catch up with the writer and return NULL. Handle NULL through the existing no-data exit without warning. Remove the caller's reader_page == commit_page check as well, since the reader-page helper already handles that case. The existing metadata update and return path remain in place. Fixes: 117c39200d9d ("ring-buffer: Introducing ring-buffer mapping functions") Assisted-by: Gemini:gemini-3.8-flash syzbot Reported-by: syzbot+de3d7f9bcc9212f3fae1@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=de3d7f9bcc9212f3fae1 Link: https://syzkaller.appspot.com/ai_job?id=488adc18-a10e-42d2-a205-25327c38837f Signed-off-by: Krystian Kaniewski --- Changes since v1: - Rewrite the commit message to explain the race more concisely. - Clarify why the earlier empty-buffer check does not exclude this case. - Remove the redundant reader_page == commit_page check, as discussed with Vincent. Let rb_get_reader_page() handle the caught-up reader. v1: https://lore.kernel.org/all/15bc282f-669e-4a94-911d-bb435513461f@mail.kernel.org/ kernel/trace/ring_buffer.c | 6 +----- 1 file changed, 1 insertion(+), 5 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 9c03a555a6ba..9222fc881bc2 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -7990,12 +7990,8 @@ consume: goto out; } - /* Did the reader catch up with the writer? */ - if (cpu_buffer->reader_page == cpu_buffer->commit_page) - goto out; - reader = rb_get_reader_page(cpu_buffer); - if (WARN_ON(!reader)) + if (!reader) goto out; /* Check if any events were dropped */ base-commit: df2908090cda368b01ff43709f51890076c56157 -- 2.53.0