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libbpf: Fix skel_internal.h to set errno on loader retval < 0

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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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210927145941.1383001-11-memxor@gmail.com
This commit is contained in:
Kumar Kartikeya Dwivedi 2021-09-27 20:29:39 +05:30 committed by Alexei Starovoitov
parent 161ecd5379
commit e68ac00827
1 changed files with 4 additions and 2 deletions
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6
tools/lib/bpf/skel_internal.h
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@ -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;
}
goto out;
}
err = 0;