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

JIRA: https://issues.redhat.com/browse/RHEL-63880

commit f04e2ad394e2755d0bb2d858ecb5598718bf00d5
Author: Kuan-Wei Chiu <visitorckw@gmail.com>
Date:   Tue Sep 10 23:02:07 2024 +0800

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

    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 <visitorckw@gmail.com>
    Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
    Reviewed-by: Quentin Monnet <qmo@kernel.org>
    Link: https://lore.kernel.org/bpf/20240910150207.3179306-1-visitorckw@gmail.com

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

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++) {