[PATCH v2] bpf: fix mount source show for bpffs

[PATCH v2] bpf: fix mount source show for bpffs

[Yafang Shao]

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 enrioment is
not match, 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,

- kenrel 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>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>

---
v1->v2:
use the helper vfs_parse_fs_param_source() instead of open-coded (Daniel)
---
 kernel/bpf/inode.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 80da1db47c68..5a8d9f7467bf 100644
--- 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:
-- 
2.17.1

Re: [PATCH v2] bpf: fix mount source show for bpffs

[Christian Brauner]

On Sat, Jan 08, 2022 at 01:46:23PM +0000, Yafang Shao wrote:
> 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 enrioment is
> not match, 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,
> 
> - kenrel 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>
> Cc: David Howells <dhowells@redhat.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> 
> ---
> v1->v2:
> use the helper vfs_parse_fs_param_source() instead of open-coded (Daniel)
> ---

Looks good,
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

Re: [PATCH v2] bpf: fix mount source show for bpffs

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf.git (master)
by Daniel Borkmann <daniel@iogearbox.net>:

On Sat,  8 Jan 2022 13:46:23 +0000 you wrote:
> 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 enrioment is
> not match, 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,
> 
> [...]

Here is the summary with links:
  - [v2] bpf: fix mount source show for bpffs
    https://git.kernel.org/bpf/bpf/c/1e9d74660d4d

You are awesome, thank you!
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