linux-kernel - Re: [PATCH] bpftool: Fix undefined behavior in qsort(NULL, 0, ...)

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Message-ID: <c18fefbb-b22f-475f-9912-63162bf84765@kernel.org>
Date: Tue, 10 Sep 2024 15:47:34 +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, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org, jserv@...s.ncku.edu.tw
Subject: Re: [PATCH] bpftool: Fix undefined behavior in qsort(NULL, 0, ...)

2024-09-10 20:58 UTC+0800 ~ Kuan-Wei Chiu <visitorckw@...il.com>
> When netfilter has no entry to display, qsort is called with
> qsort(NULL, 0, ...). This results in undefined behavior, as UBSan
> reports:
> 
> net.c:827:2: runtime error: null pointer passed as argument 1, which is declared to never be null
> 
> Although the C standard does not explicitly state whether calling qsort
> with a NULL pointer when the size is 0 constitutes undefined behavior,
> Section 7.1.4 of the C standard (Use of library functions) mentions:
> 
> "Each of the following statements applies unless explicitly stated
> otherwise in the detailed descriptions that follow: If an argument to a
> function has an invalid value (such as a value outside the domain of
> the function, or a pointer outside the address space of the program, or
> a null pointer, or a pointer to non-modifiable storage when the
> corresponding parameter is not const-qualified) or a type (after
> promotion) not expected by a function with variable number of
> arguments, the behavior is undefined."
> 
> To avoid this, add an early return when nf_link_count is 0 to prevent
> calling qsort with a NULL pointer.
> 
> Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> ---
>   tools/bpf/bpftool/net.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/tools/bpf/bpftool/net.c b/tools/bpf/bpftool/net.c
> index 968714b4c3d4..13e098fa295a 100644
> --- a/tools/bpf/bpftool/net.c
> +++ b/tools/bpf/bpftool/net.c
> @@ -824,6 +824,9 @@ static void show_link_netfilter(void)
>   		nf_link_count++;
>   	}
>   
> +	if (!nf_link_count)
> +		return;
> +
>   	qsort(nf_link_info, nf_link_count, sizeof(*nf_link_info), netfilter_link_compar);

Thanks! As the issue is calling qsort() with a NULL pointer, could you 
please make the check on nf_link_info rather than nf_link_count? I'd 
find it easier to follow.

Quentin