linux-kernel - Re: [PATCH] bpftool: Fix undefined behavior caused by shifting into the sign bit

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Message-ID: <b6bef740-8feb-4a50-91a0-e705b5361df9@kernel.org>
Date: Tue, 10 Sep 2024 10:21:08 +0100
From: Quentin Monnet <qmo@...nel.org>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
 yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org,
 sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org,
 jserv@...s.ncku.edu.tw, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpftool: Fix undefined behavior caused by shifting into
 the sign bit

2024-09-09 04:52 UTC+0800 ~ Kuan-Wei Chiu <visitorckw@...il.com>
> On Sun, Sep 08, 2024 at 08:48:40PM +0100, Quentin Monnet wrote:
>> On 08/09/2024 15:00, Kuan-Wei Chiu wrote:
>>> Replace shifts of '1' with '1U' in bitwise operations within
>>> __show_dev_tc_bpf() to prevent undefined behavior caused by shifting
>>> into the sign bit of a signed integer. By using '1U', the operations
>>> are explicitly performed on unsigned integers, avoiding potential
>>> integer overflow or sign-related issues.
>>>
>>> Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
>>
>>
>> Looks good, thank you.
>>
>> Acked-by: Quentin Monnet <qmo@...nel.org>
>>
>> How did you find these?
> 
> TL;DR: I discovered this issue through code review.
> 
> I am a student developer trying to contribute to the Linux kernel. I
> was attempting to compile bpftool with ubsan enabled, and while running
> ./bpftool net list, I encountered the following error message:
> 
> net.c:827:2: runtime error: null pointer passed as argument 1, which is declared to never be null
> 
> This prompted me to review the code in net.c, and during that process,
> I unexpectedly came across the bug that this patch addresses.


Nice


> 
> As for the ubsan complaint mentioned above, it was triggered because
> qsort is being called as qsort(NULL, 0, ...) when netfilter has no
> entries to display. In glibc, qsort is marked with __nonnull ((1, 4)).
> However, I found conflicting information on cppreference.com [1], which
> states that when count is zero, both ptr and comp can be NULL. This
> confused me, so I will need to check the C standard to clarify this. If
> it turns out that qsort(NULL, 0, ...) is invalid, I will submit a
> separate patch to fix it.


OK, thanks for looking into it!


> 
> BTW, should this patch include a Fixes tag and a Cc @stable?
> 

We could maybe have used a Fixes:, but the patch is merged already so 
never mind. As for stable, I don't think this is necessary. I don't 
believe we can hit the undefined behaviour today; and we encourage 
people to package bpftool from the GitHub mirror anyway, where your 
patch will land soon.

Thanks,
Quentin