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 12A6D23BF9F; Thu, 24 Sep 2026 01:06:57 +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=1790212019; cv=none; b=hoPWCpU2uyesDvHE7MoyN1KBd+skmOulKPulSxL+K6lAVga8/JNs6xTeGhQ3wvhUCZGk3cCbuZV46rxbVT+6V/MPAhNr4E4o/GL2wInKI1yYBQ6bOLr3ClTgmd48zzKgnShf7uYbIUxEGq0fWjZjPQ9K455f04rdR33/jOrtDcM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790212019; c=relaxed/simple; bh=OFG7solwFJRuwBszQ7Wnwkkpl8Opqn5d4oDAoeNgtfw=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=IE1YdEwv2fU9Y0DRGrLoV8XrT6kNG++d/itSC+fjUgQmfdPRwh9gMd3tijbDu+djy/Wc1q1+vFld7aTU9grXYCHNQGBB2y+u19k14Wx8lX3KjCq+fMArJfXSEIeQkmNNinJKDMQYc6xsdKP8pZ/0zNxHnyoMGGdJeJvQVFRQAWs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=XPXk9yS1; 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="XPXk9yS1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 118E71F000FF; Thu, 24 Sep 2026 01:06:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790212017; bh=CJkNWkNrY+y078XLIOaRAPfEesb+E74HSkQALPyaJbM=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=XPXk9yS1BU/M0jfJbkv2L68IC9GWxkDZPn86zB7oC5cQh18ZiHUGlqB2ftmrLBchk gmiKBPLFlEzNQ2r7wFaVyaUEpo9jhnumi4Sttonwdn4EHXcaZbsIutNGLwgMNFuht8 aUTvgzv8movXQzUUz+Z/gb9DVXxhfT8efFmmPacY47xk5Ue1NRgNiWU7wGBzRFiE2F Zdk3YKG2SiMHNNKfwPgpv5j3XaULtb8Zp8N64fqzzgdSjA8e9wwby94BgEc95Y1EYj LZhLW17QsCsNUJXrQVQhQcwBBxpcs2SGPeL12wBTzy/GNy3F2TYslkuJxumnkgPY6B Lvvc8MwoJ/hww== Date: Thu, 24 Sep 2026 10:06:54 +0900 From: Masami Hiramatsu (Google) To: Donggeun Yoo Cc: Steven Rostedt , Mathieu Desnoyers , linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v1] ring-buffer: Fix the sub-buffer array base in rb_meta_subbuf_idx() Message-Id: <20260924100654.49809fad606e577ee39f3086@kernel.org> In-Reply-To: <20260918071507.1019251-1-donggeunyoo.kernel@gmail.com> References: <20260918071507.1019251-1-donggeunyoo.kernel@gmail.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Fri, 18 Sep 2026 16:15:07 +0900 Donggeun Yoo wrote: > On a persistent ring buffer, rb_meta_subbuf_idx() can return an index one > sub-buffer too high. rb_setup_ids_meta_page() then fills subbuf_ids[] > shifted by one and leaves subbuf_ids[0] NULL, which __rb_map_vma() > dereferences when userspace maps trace_pipe_raw. rb_update_meta_reader() > writes two such indices into meta->buffers[], which rb_cpu_meta_valid() > would reject on the next boot, discarding the previous boot's trace. > > A CPU's sub-buffer array starts after its meta header and the nr_subbufs > integers that follow, aligned up to a sub-buffer. rb_meta_subbuf_idx() > inverts that calculation but skips only the integers, not the header, so > its base is one sub-buffer low whenever the omitted header size crosses > the alignment boundary. > > Invert the calculation with rb_subbufs_from_meta() rather than repeating > it. rb_range_buffer() maps an index back to an address through the same > helper, so the two directions can no longer disagree. > Ah, nice catch! Acked-by: Masami Hiramatsu (Google) Thanks, > Cc: > Fixes: b14d032973d4 ("ring-buffer: Add ring_buffer_meta data") > Signed-off-by: Donggeun Yoo > --- > x86_64_defconfig plus KASAN, FTRACE, TRACING and TRACER_SNAPSHOT, on > 5dd1818b15d9, booted with reserve_mem=:0x1000:trace > trace_instance=boot_mapped@trace. head_buffer is from > instances/boot_mapped/per_cpu/cpuN/buffer_meta, the map of that CPU's > trace_pipe_raw. > > nr_cpus=1, 24 reserved sizes 4096 bytes apart: > > nr_subbufs head_buffer map head_buffer map > unpatched patched > 997 .. 1004 1 ok 1 ok > 1005 .. 1012 2 oops 1 ok > 1013 .. 1019 1 ok 1 ok > > nr_cpus=4, nr_subbufs 1017, one boot: > > cpu 0 1 ok 1 ok > cpu 1, 2, 3 2 oops 1 ok > > cpu 0's meta sits after the buffer-wide header and the scratch area and is not > sub-buffer aligned; every other cpu's is, so the affected nr_subbufs differ > between them. > > Unpatched oops: > > Oops: general protection fault, probably for non-canonical address 0xdffffc0000000007: 0000 [#1] SMP KASAN NOPTI > KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f] > RIP: 0010:__rb_map_vma+0x418/0xa20 > > With no reserved range at all, both kernels map the global buffer. On both > arms, tools/testing/selftests/ring-buffer passes 6/6 with no skips and > CONFIG_RING_BUFFER_STARTUP_TEST reports "Ring buffer PASSED!". ftracetest > gives identical per-test verdicts on the two arms -- 153 pass, 6 fail, 6 > unresolved, 10 unsupported, 2 xfail. The six failures are the same on both > arms and their cause was not established; the unresolved and unsupported ones > need pahole, a hypervisor trace remote, or userspace this initramfs does not > have. > > The discarded-trace path is not measured: a fresh QEMU boot gets fresh guest > memory, so this harness cannot carry a persistent buffer across a reboot. > > kernel/trace/ring_buffer.c | 4 +--- > 1 file changed, 1 insertion(+), 3 deletions(-) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index 04bb94c29f58..fa2e2ce683aa 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -3627,10 +3627,8 @@ static void rb_inc_iter(struct ring_buffer_iter *iter) > /* Return the index into the sub-buffers for a given sub-buffer */ > static int rb_meta_subbuf_idx(struct ring_buffer_cpu_meta *meta, void *subbuf) > { > - void *subbuf_array; > + void *subbuf_array = rb_subbufs_from_meta(meta); > > - subbuf_array = (void *)meta + sizeof(int) * meta->nr_subbufs; > - subbuf_array = (void *)ALIGN((unsigned long)subbuf_array, meta->subbuf_size); > return (subbuf - subbuf_array) / meta->subbuf_size; > } > > > base-commit: 5dd1818b15d98d4a20806cd00b1b40320b06004f > -- > 2.53.0 > -- Masami Hiramatsu (Google)