libbpf: fix libbpf_strerror_r() handling unknown errors

strerror_r(), used from libbpf-specific libbpf_strerror_r() wrapper is
documented to return error in two different ways, depending on glibc
version. Take that into account when handling strerror_r()'s own errors,
which happens when we pass some non-standard (internal) kernel error to
it. Before this patch we'd have "ERROR: strerror_r(524)=22", which is
quite confusing. Now for the same situation we'll see a bit less
visually scary "unknown error (-524)".

At least we won't confuse user with irrelevant EINVAL (22).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240507001335.1445325-5-andrii@kernel.org
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

diff --git a/tools/lib/bpf/str_error.c b/tools/lib/bpf/str_error.c
index 146da01979c7fc52b3c5dae00909b5d53663d04c..5e6a1e27ddf90b2e01ebaeb23a9f2b3d43ecc5b1 100644 (file)
--- a/tools/lib/bpf/str_error.c
+++ b/tools/lib/bpf/str_error.c
@@ -2,6 +2,7 @@
 #undef _GNU_SOURCE
 #include <string.h>
 #include <stdio.h>
+#include <errno.h>
 #include "str_error.h"
 
 /* make sure libbpf doesn't use kernel-only integer typedefs */
@@ -15,7 +16,18 @@
 char *libbpf_strerror_r(int err, char *dst, int len)
 {
        int ret = strerror_r(err < 0 ? -err : err, dst, len);
-       if (ret)
-               snprintf(dst, len, "ERROR: strerror_r(%d)=%d", err, ret);
+       /* on glibc <2.13, ret == -1 and errno is set, if strerror_r() can't
+        * handle the error, on glibc >=2.13 *positive* (errno-like) error
+        * code is returned directly
+        */
+       if (ret == -1)
+               ret = errno;
+       if (ret) {
+               if (ret == EINVAL)
+                       /* strerror_r() doesn't recognize this specific error */
+                       snprintf(dst, len, "unknown error (%d)", err < 0 ? err : -err);
+               else
+                       snprintf(dst, len, "ERROR: strerror_r(%d)=%d", err, ret);
+       }
        return dst;
 }