Re: [PATCH bpf-next v14 2/8] libbpf: Add support for extended BPF syscall

On 13/5/26 00:23, bot+bpf-ci@kernel.org wrote:

quoted

diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
index 5846de364209..9d8740761b7a 100644
--- a/tools/lib/bpf/bpf.c
+++ b/tools/lib/bpf/bpf.c

[ ... ]

quoted

+int probe_sys_bpf_ext(void)
+{
+	const size_t attr_sz = offsetofend(union bpf_attr, prog_token_fd);
+	union bpf_attr attr;
+	int fd;
+
+	memset(&attr, 0, attr_sz);
+	fd = syscall(__NR_bpf, BPF_PROG_LOAD | BPF_COMMON_ATTRS, &attr, attr_sz, NULL,
+		     sizeof(struct bpf_common_attr));
+	if (fd >= 0) {
+		close(fd);
+		return -EINVAL;

                        ^^^^^^

Should probe_sys_bpf_ext() return 1 instead of -EINVAL when fd >= 0?

The probe function detects kernel support for extended BPF syscall
attributes by calling the BPF syscall with the BPF_COMMON_ATTRS flag
and a NULL pointer for common attributes.

The expected outcomes are:
- New kernel with support: errno=EFAULT (can't copy from NULL) -> return 1
- Old kernel without support: errno!=EFAULT (invalid command) -> return 0

If the syscall succeeds (fd >= 0), that definitively proves the kernel
accepts the extended syscall format and supports BPF_COMMON_ATTRS. The
function should return 1 (supported) rather than -EINVAL (error).

Returning -EINVAL causes feat_supported() in features.c to print a
warning "Detection of kernel BPF syscall common attributes support
support failed: Invalid argument" and mark the feature as MISSING,
incorrectly reporting that the kernel doesn't support the feature when
the successful syscall proves it does.

Should not return 1.

This return was discussed in v5 and v6.

Cite Andrii's review in v5:

But fd should really not be >= 0, and if it is -- it's some problem,
so I'd return an error in that case to keep us aware, which is why I'm
saying I'd just return inside if (fd >= 0) { }

Thanks,
Leon

quoted

+	}
+	return errno == EFAULT ? 1 : 0;
+}

[ ... ]


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