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@gmail.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);
 
 	for (id = 0; id < nf_link_count; id++) {
-- 
2.34.1

2024-09-10 20:58 UTC+0800 ~ Kuan-Wei Chiu <visitorckw@gmail.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@gmail.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