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

[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);
 
 	for (id = 0; id < nf_link_count; id++) {
-- 
2.34.1