[PATCH bpf-next v1 6/7] tools: bpftool: fix bpffs mount when pinning subdirectories

From: Alban Crequy
Date: Wed Mar 20 2019 - 13:35:48 EST

From: Alban Crequy <alban@xxxxxxxxxx>

When trying to pin at a path such as /sys/fs/bpf/subdir/mymap while the
bpffs is mounted at the standard path and the "subdir" directory didn't
exist, bpftool was failing and attempting to mount the bpffs on
/sys/fs/bpf/subdir/. The mount obviously failed, since "subdir" didn't

Commit 3fc27b71b894 ("tools: bpftool: try to mount bpffs if required for
pinning objects") says:

The code for mnt_bpffs() is a copy of function bpf_mnt_fs() from
iproute2 package (under lib/bpf.c, taken at commit 4b73d52f8a81), with
modifications regarding handling of error messages.

However, the function bpf_mnt_fs() from iproute2 only works when its
parameter is the root of the bpffs (e.g. "/sys/fs/bfs").

iproute2/tc actually only mounts at the standard path "/sys/fs/bfs" or
at the path from the environment variable $TC_BPF_MNT. This patch
implements a similar strategy but uses the environment variable name

This patch still will not do the mkdir automatically. But at least,
there will be no error message about the mount.

Fixes: 3fc27b71b894 ("tools: bpftool: try to mount bpffs if required for pinning objects")
Cc: Quentin Monnet <quentin.monnet@xxxxxxxxxxxxx>
Signed-off-by: Alban Crequy <alban@xxxxxxxxxx>
tools/bpf/bpftool/common.c | 20 ++++++++++++++++----
tools/bpf/bpftool/main.h | 6 ++++++
2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/tools/bpf/bpftool/common.c b/tools/bpf/bpftool/common.c
index e560cd8f66bc..4783cbbe8037 100644
--- a/tools/bpf/bpftool/common.c
+++ b/tools/bpf/bpftool/common.c
@@ -168,6 +168,8 @@ int mount_bpffs_for_pin(const char *name)
char *file;
char *dir;
int err = 0;
+ const char *mnt_env = getenv(BPF_DIR_MNT_ENV);
+ static const char *mnt;

file = malloc(strlen(name) + 1);
strcpy(file, name);
@@ -183,13 +185,23 @@ int mount_bpffs_for_pin(const char *name)
goto out_free;

- err = mnt_fs(dir, "bpf", err_str, ERR_MAX_LEN);
+ mnt = mnt_env ? : BPF_DIR_MNT;
+ // Don't try to mount if wrong prefix: we don't want a leftover mount that is
+ // not going to help.
+ if (!is_prefix(mnt, dir)) {
+ p_err("refuse to mount BPF file system (%s) to pin the object (%s): wrong directory",
+ mnt, name);
+ err = -1;
+ goto out_free;
+ }
+ err = mnt_fs(mnt, "bpf", err_str, ERR_MAX_LEN);
if (err) {
err_str[ERR_MAX_LEN - 1] = '\0';
- p_err("can't mount BPF file system to pin the object (%s): %s",
- name, err_str);
+ p_err("can't mount BPF file system (%s) to pin the object (%s): %s",
+ mnt, name, err_str);
return err;
diff --git a/tools/bpf/bpftool/main.h b/tools/bpf/bpftool/main.h
index 7fc2973446d0..a2e28e600b72 100644
--- a/tools/bpf/bpftool/main.h
+++ b/tools/bpf/bpftool/main.h
@@ -18,6 +18,12 @@

#define ptr_to_u64(ptr) ((__u64)(unsigned long)(ptr))

+/* If bpffs is not mounted, it can be automatically mounted at this location.
+ * The location can be changed with the environment variable $BPFTOOL_BPF_MNT.
+ */
+#define BPF_DIR_MNT "/sys/fs/bpf"
#define NEXT_ARG() ({ argc--; argv++; if (argc < 0) usage(); })
#define NEXT_ARGP() ({ (*argc)--; (*argv)++; if (*argc < 0) usage(); })
#define BAD_ARG() ({ p_err("what is '%s'?", *argv); -1; })