[PATCH bpf-next v2] libbpf: Print hint about ulimit when getting permission denied error

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Probably the single most common error newcomers to XDP are stumped by is
the 'permission denied' error they get when trying to load their program
and 'ulimit -l' is set too low. For examples, see [0], [1].

Since the error code is UAPI, we can't change that. Instead, this patch
adds a few heuristics in libbpf and outputs an additional hint if they are
met: If an EPERM is returned on map create or program load, and geteuid()
shows we are root, and the current RLIMIT_MEMLOCK is not infinity, we
output a hint about raising 'ulimit -l' as an additional log line.

[0] https://marc.info/?l=xdp-newbies&m=157043612505624&w=2
[1] https://github.com/xdp-project/xdp-tutorial/issues/86

Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
---
v2:
  - Format current output as KiB/MiB
  - It's ulimit -l, not ulimit -r
  
 tools/lib/bpf/libbpf.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index a2cc7313763a..3fe42d6b0c2f 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -41,6 +41,7 @@
 #include <sys/types.h>
 #include <sys/vfs.h>
 #include <sys/utsname.h>
+#include <sys/resource.h>
 #include <tools/libc_compat.h>
 #include <libelf.h>
 #include <gelf.h>
@@ -100,6 +101,32 @@ void libbpf_print(enum libbpf_print_level level, const char *format, ...)
 	va_end(args);
 }
 
+static void pr_perm_msg(int err)
+{
+	struct rlimit limit;
+	char buf[100];
+
+	if (err != -EPERM || geteuid() != 0)
+		return;
+
+	err = getrlimit(RLIMIT_MEMLOCK, &limit);
+	if (err)
+		return;
+
+	if (limit.rlim_cur == RLIM_INFINITY)
+		return;
+
+	if (limit.rlim_cur < 1024)
+		snprintf(buf, sizeof(buf), "%lu bytes", limit.rlim_cur);
+	else if (limit.rlim_cur < 1024*1024)
+		snprintf(buf, sizeof(buf), "%.1f KiB", (double)limit.rlim_cur / 1024);
+	else
+		snprintf(buf, sizeof(buf), "%.1f MiB", (double)limit.rlim_cur / (1024*1024));
+
+	pr_warn("permission error while running as root; try raising 'ulimit -l'? current value: %s\n",
+		buf);
+}
+
 #define STRERR_BUFSIZE  128
 
 /* Copied from tools/perf/util/util.h */
@@ -2983,6 +3010,7 @@ bpf_object__create_maps(struct bpf_object *obj)
 			cp = libbpf_strerror_r(err, errmsg, sizeof(errmsg));
 			pr_warn("failed to create map (name: '%s'): %s(%d)\n",
 				map->name, cp, err);
+			pr_perm_msg(err);
 			for (j = 0; j < i; j++)
 				zclose(obj->maps[j].fd);
 			return err;
@@ -4381,6 +4409,7 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 	ret = -errno;
 	cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
 	pr_warn("load bpf program failed: %s\n", cp);
+	pr_perm_msg(ret);
 
 	if (log_buf && log_buf[0] != '\0') {
 		ret = -LIBBPF_ERRNO__VERIFY;
-- 
2.24.1