bpf: Fix mount source show for bpffs

commit 1e9d74660d4df625b0889e77018f9e94727ceacd upstream.

We noticed our tc ebpf tools can't start after we upgrade our in-house kernel
version from 4.19 to 5.10. That is because of the behaviour change in bpffs
caused by commit d2935de7e4fd ("vfs: Convert bpf to use the new mount API").

In our tc ebpf tools, we do strict environment check. If the environment is
not matched, we won't allow to start the ebpf progs. One of the check is whether
bpffs is properly mounted. The mount information of bpffs in kernel-4.19 and
kernel-5.10 are as follows:

- kernel 4.19
$ mount -t bpf bpffs /sys/fs/bpf
$ mount -t bpf
bpffs on /sys/fs/bpf type bpf (rw,relatime)

- kernel 5.10
$ mount -t bpf bpffs /sys/fs/bpf
$ mount -t bpf
none on /sys/fs/bpf type bpf (rw,relatime)

The device name in kernel-5.10 is displayed as none instead of bpffs, then our
environment check fails. Currently we modify the tools to adopt to the kernel
behaviour change, but I think we'd better change the kernel code to keep the
behavior consistent.

After this change, the mount information will be displayed the same with the
behavior in kernel-4.19, for example:

$ mount -t bpf bpffs /sys/fs/bpf
$ mount -t bpf
bpffs on /sys/fs/bpf type bpf (rw,relatime)

Fixes: d2935de7e4fd ("vfs: Convert bpf to use the new mount API")
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/bpf/20220108134623.32467-1-laoar.shao@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 80da1db47c6869073cffdba7f5593ca8ff292538..5a8d9f7467bf4d9836773f31ae2aa95c338d861d 100644 (file)
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -648,12 +648,22 @@ static int bpf_parse_param(struct fs_context *fc, struct fs_parameter *param)
        int opt;
 
        opt = fs_parse(fc, bpf_fs_parameters, param, &result);
-       if (opt < 0)
+       if (opt < 0) {
                /* We might like to report bad mount options here, but
                 * traditionally we've ignored all mount options, so we'd
                 * better continue to ignore non-existing options for bpf.
                 */
-               return opt == -ENOPARAM ? 0 : opt;
+               if (opt == -ENOPARAM) {
+                       opt = vfs_parse_fs_param_source(fc, param);
+                       if (opt != -ENOPARAM)
+                               return opt;
+
+                       return 0;
+               }
+
+               if (opt < 0)
+                       return opt;
+       }
 
        switch (opt) {
        case OPT_MODE: