Re: [PATCH bpf-next v5 10/12] libbpf: Fix skel_internal.h to set errno on loader retval < 0

When the loader indicates an internal error (result of a checked bpf
system call), it returns the result in attr.test.retval. However, tests
that rely on ASSERT_OK_PTR on NULL (returned from light skeleton) may
miss that NULL denotes an error if errno is set to 0. This would result
in skel pointer being NULL, while ASSERT_OK_PTR returning 1, leading to
a SEGV on dereference of skel, because libbpf_get_error relies on the
assumption that errno is always set in case of error for ptr == NULL.

In particular, this was observed for the ksyms_module test. When
executed using `./test_progs -t ksyms`, prior tests manipulated errno
and the test didn't crash when it failed at ksyms_module load, while
using `./test_progs -t ksyms_module` crashed due to errno being
untouched.

Fixes: 67234743736a (libbpf: Generate loader program out of BPF ELF file.)
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 tools/lib/bpf/skel_internal.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/skel_internal.h b/tools/lib/bpf/skel_internal.h
index b22b50c1b173..9cf66702fa8d 100644
--- a/tools/lib/bpf/skel_internal.h
+++ b/tools/lib/bpf/skel_internal.h

@@ -105,10 +105,12 @@ static inline int bpf_load_and_run(struct bpf_load_and_run_opts *opts)
        err = skel_sys_bpf(BPF_PROG_RUN, &attr, sizeof(attr));
        if (err < 0 || (int)attr.test.retval < 0) {
                opts->errstr = "failed to execute loader prog";
-               if (err < 0)
+               if (err < 0) {
                        err = -errno;
-               else
+               } else {
                        err = (int)attr.test.retval;
+                       errno = -err;
+               }