[PATCH bpf-next 3/3] libbpf: Add pin option to automount BPF filesystem before pinning

[Toke Høiland-Jørgensen <toke@xxxxxxxxxx>]

From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>

While the current map pinning functions will check whether the pin path is
contained on a BPF filesystem, it does not offer any options to mount the
file system if it doesn't exist. Since we now have pinning options, add a
new one to automount a BPF filesystem at the pinning path if that is not
already pointing at a bpffs.

The mounting logic itself is copied from the iproute2 BPF helper functions.

Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
---
 tools/lib/bpf/libbpf.c |   47 +++++++++++++++++++++++++++++++++++++++++++++++
 tools/lib/bpf/libbpf.h |    5 ++++-
 2 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index aea3916de341..f527224bb211 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -37,6 +37,7 @@
 #include <sys/epoll.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
+#include <sys/mount.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/vfs.h>
@@ -4072,6 +4073,35 @@ int bpf_map__unpin(struct bpf_map *map, const char *path)
 	return 0;
 }
 
+static int mount_bpf_fs(const char *target)
+{
+	bool bind_done = false;
+
+	while (mount("", target, "none", MS_PRIVATE | MS_REC, NULL)) {
+		if (errno != EINVAL || bind_done) {
+			pr_warning("mount --make-private %s failed: %s\n",
+				   target, strerror(errno));
+			return -1;
+		}
+
+		if (mount(target, target, "none", MS_BIND, NULL)) {
+			pr_warning("mount --bind %s %s failed: %s\n",
+				   target, target, strerror(errno));
+			return -1;
+		}
+
+		bind_done = true;
+	}
+
+	if (mount("bpf", target, "bpf", 0, "mode=0700")) {
+		fprintf(stderr, "mount -t bpf bpf %s failed: %s\n",
+			target, strerror(errno));
+		return -1;
+	}
+
+	return 0;
+}
+
 static int get_pin_path(char *buf, size_t buf_len,
 			struct bpf_map *map, struct bpf_object_pin_opts *opts,
 			bool mkdir)
@@ -4102,6 +4132,23 @@ static int get_pin_path(char *buf, size_t buf_len,
 		err = make_dir(path);
 		if (err)
 			return err;
+
+		if (OPTS_GET(opts, mount_bpf_fs, false)) {
+			struct statfs st_fs;
+			char *cp;
+
+			if (statfs(path, &st_fs)) {
+				char errmsg[STRERR_BUFSIZE];
+
+				cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
+				pr_warning("failed to statfs %s: %s\n", path, cp);
+				return -errno;
+			}
+			if (st_fs.f_type != BPF_FS_MAGIC &&
+			    mount_bpf_fs(path)) {
+				return -EINVAL;
+			}
+		}
 	}
 
 	len = snprintf(buf, buf_len, "%s/%s", path, bpf_map__name(map));
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 2131eeafb18d..76b9a6cc7063 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -143,8 +143,11 @@ struct bpf_object_pin_opts {
 	 * and this type used for all maps instead.
 	 */
 	enum libbpf_pin_type override_type;
+
+	/* Whether to attempt to mount a BPF FS if it's not already mounted */
+	bool mount_bpf_fs;
 };
-#define bpf_object_pin_opts__last_field override_type
+#define bpf_object_pin_opts__last_field mount_bpf_fs
 
 LIBBPF_API int bpf_object__pin_maps(struct bpf_object *obj, const char *path);
 LIBBPF_API int bpf_object__unpin_maps(struct bpf_object *obj,