[v2] bpftool: Fix undefined behavior in qsort(NULL, 0, ...)

Commit Message

Kuan-Wei Chiu Sept. 10, 2024, 3:02 p.m. UTC

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_info is NULL to prevent
calling qsort with a NULL pointer.

Signed-off-by: Kuan-Wei Chiu <[e-mail]>
---
Changes in v2:
- Change from checking nf_link_count to checking nf_link_info.

 tools/bpf/bpftool/net.c | 3 +++
 1 file changed, 3 insertions(+)

Comments

Quentin Monnet Sept. 10, 2024, 5:11 p.m. UTC | #1

2024-09-10 23:02 UTC+0800 ~ Kuan-Wei Chiu <[e-mail]>
> 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_info is NULL to prevent
> calling qsort with a NULL pointer.
> 
> Signed-off-by: Kuan-Wei Chiu <[e-mail]>
> ---
> Changes in v2:
> - Change from checking nf_link_count to checking nf_link_info.
> 
>   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..0ad684e810f3 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_info)
> +		return;
> +
>   	qsort(nf_link_info, nf_link_count, sizeof(*nf_link_info), netfilter_link_compar);
>   
>   	for (id = 0; id < nf_link_count; id++) {


Reviewed-by: Quentin Monnet <[e-mail]>

Thanks!

[e-mail] Sept. 10, 2024, 6:50 p.m. UTC | #2

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <[e-mail]>:

On Tue, 10 Sep 2024 23:02:07 +0800 you wrote:
> 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:
> 
> [...]

Here is the summary with links:
  - [v2] bpftool: Fix undefined behavior in qsort(NULL, 0, ...)
    https://git.kernel.org/bpf/bpf-next/c/f04e2ad394e2

You are awesome, thank you!

Patch

diff --git a/tools/bpf/bpftool/net.c b/tools/bpf/bpftool/net.c
index 968714b4c3d4..0ad684e810f3 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_info)
+		return;
+
 	qsort(nf_link_info, nf_link_count, sizeof(*nf_link_info), netfilter_link_compar);
 
 	for (id = 0; id < nf_link_count; id++) {