From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1C9C5531AE1 for ; Tue, 8 Sep 2026 11:48:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788868128; cv=none; b=LvPd/clSFeehHH69JkVnJq/YRHHwT5+jO3gO034KsaUxcGCPijcZxbriaXuC7aKiS6M2X42JAUSeuzJ9xjZNk5se/tLEuQNR9h2cKD30PFFjet8YXjmLYgWBfYEVf5smnL0bEHVQfUIcy7/FEm3AsiPMUTA8USTyxF+9whPUjOQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788868128; c=relaxed/simple; bh=hnH3gMBGnOW7h94oe1zXTVsfPHZyQQNTaBmvbejrytc=; h=From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type:Date; b=Vfxzi6YHo6HvcAvMsc0gy2I1enJGwI6IwUHKiPi6GEYpSMDmHU+UAMMTjQZhLqupSS+OS++UNQ983EYzA6uFswzmrl1eqsuBalaiXmNKCvuDvC0/1BLDypUsV7EpLDnzUwNwVdyWiaYSwmMOIlEtD8pMiA7X+H0d7pGUgPs3MtM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kDWxzC+t; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kDWxzC+t" Received: by smtp.kernel.org (Postfix) with UTF8SMTPSA id 0E81F1F00A3A; Tue, 8 Sep 2026 11:48:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788868113; bh=Hhk6cdAwfbJvTdC7g0kMCgPSOArKJZCvUTCT+SS2VYw=; h=From:To:Cc:Subject:Date; b=kDWxzC+tY68D/8/UOWYf/bD0uXCkas3pqrsc2TaHZC3BYI1p/L0vYoQDI9ruWaxL/ 5e9lvc0PbskHehqmR84Mmti2Q8hkCEhZzVtRNAsZiuJ4cOvafq82AqPr9SFhQLgVyB YJY/q6aDlg78czSYz3oBR3KZaejSiAkZXGjFQHZvo1iVFHZBlbfjGGE+eI36/kn0Zh Oh2YdBpexBzCyPdC9uA2wOCszXe4UfDtOJmVtvlqdQcJoAjvbaw/ueYW0BUMlv8NLM Bw/Wv0rhWEtjjfq07Cx9WDbh7AwpAI4aYg7AEuNUEyOIhHa7fCY6o4CEIn8eKGrscy jjpK6/FXJ2Mzw== From: "syzbot" To: syzkaller-upstream-moderation@googlegroups.com Cc: syzbot@lists.linux.dev Subject: [PATCH RFC] ring-buffer: Fix false warning in ring_buffer_map_get_reader() Message-ID: <97fb4855-693b-429f-9e75-b4c2a0348ef0@mail.kernel.org> Precedence: bulk X-Mailing-List: syzbot@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Date: Tue, 8 Sep 2026 11:48:33 +0000 (UTC) In ring_buffer_map_get_reader(), an unconditional WARN_ON(!reader) is triggered when rb_get_reader_page() returns NULL: WARNING: CPU: 1 PID: 5906 at kernel/trace/ring_buffer.c:7998 ring_buffer_map_get_reader+0x940/0x9d0 CPU: 1 UID: 0 PID: 5906 Comm: task Not tainted RIP: 0010:ring_buffer_map_get_reader+0x940/0x9d0 kernel/trace/ring_buffer.c:7998 Call Trace: tracing_buffers_ioctl+0x258/0x300 kernel/trace/trace.c:7381 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84 entry_SYSCALL_64_after_hwframe+0x77/0x7f This warning is triggered due to a race between a writer committing events and a reader mapping the ring buffer via TRACE_MMAP_IOCTL_GET_READER. When a writer commits an event in rb_set_commit_to_write(), it advances cpu_buffer->commit_page to cpu_buffer->tail_page in its first loop before updating the commit counter (commit_page->page->commit) in the second loop. If a reader invokes ring_buffer_map_get_reader() at this moment, the initial check cpu_buffer->reader_page == cpu_buffer->commit_page is false, and it calls rb_get_reader_page(). Inside __rb_get_reader_page(), the reader swaps reader_page with the head page (which is the new commit_page). Because the writer has not yet updated the commit count on the new page, rb_page_size() is zero and reader_page->read < rb_page_size() evaluates to false. __rb_get_reader_page() then checks if cpu_buffer->commit_page == cpu_buffer->reader_page. Since both now point to the swapped page, the condition evaluates to true and rb_get_reader_page() legitimately returns NULL to indicate the reader caught up to the writer. Because WARN_ON must not be used for conditions that can legitimately happen, ring_buffer_map_get_reader() should not unconditionally warn when rb_get_reader_page() returns NULL. Fix this by only warning if rb_get_reader_page() returns NULL while cpu_buffer->reader_page != cpu_buffer->commit_page. Fixes: 117c39200d9d ("ring-buffer: Introducing ring-buffer mapping functions") Assisted-by: Gemini:gemini-3.7-flash Gemini:gemini-3.1-pro-preview syzbot Reported-by: syzbot+de3d7f9bcc9212f3fae1@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=de3d7f9bcc9212f3fae1 Link: https://syzkaller.appspot.com/ai_job?id=f119d7de-b21d-4d27-b61e-5503e31edccd To: To: "Masami Hiramatsu" To: "Steven Rostedt" To: "Vincent Donnefort" Cc: Cc: "Mathieu Desnoyers" --- diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 9c03a555a..98130182e 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -7995,8 +7995,10 @@ int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu) goto out; reader = rb_get_reader_page(cpu_buffer); - if (WARN_ON(!reader)) + if (!reader) { + WARN_ON(cpu_buffer->reader_page != cpu_buffer->commit_page); goto out; + } /* Check if any events were dropped */ missed_events = cpu_buffer->lost_events; base-commit: df2908090cda368b01ff43709f51890076c56157 -- This is an AI-generated patch subject to moderation. 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