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

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: 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.

References

• https://git.kernel.org/stable/c/c2d9f9a7837ab29ccae0c42252f17d436bf0a501
• https://git.kernel.org/stable/c/2e0f6f33f2aa87493b365a38a8fd87b8854b7734
• https://git.kernel.org/stable/c/c208b02827eb642758cef65641995fd3f38c89af
• https://git.kernel.org/stable/c/f04e2ad394e2755d0bb2d858ecb5598718bf00d5