Re: [PATCH] bpf: Refuse to mount bpffs on the same mount point multiple times

[Yonghong Song <yhs@fb.com>]

On 2/23/22 5:18 AM, Yafang Shao wrote:
> We monitored an unexpected behavoir that bpffs is mounted on a same mount
> point lots of times on some of our production envrionments. For example,
> $ mount -t bpf
> bpffs /sys/fs/bpf bpf rw,relatime 0 0
> bpffs /sys/fs/bpf bpf rw,relatime 0 0
> bpffs /sys/fs/bpf bpf rw,relatime 0 0
> bpffs /sys/fs/bpf bpf rw,relatime 0 0
> ...
> 
> That was casued by a buggy user script which didn't check the mount
> point correctly before mounting bpffs. But it also drives us to think more
> about if it is okay to allow mounting bpffs on the same mount point
> multiple times. After investigation we get the conclusion that it is bad
> to allow that behavior, because it can cause unexpected issues, for
> example it can break bpftool, which depends on the mount point to get
> the pinned files.
> 
> Below is the test case wrt bpftool.
> First, let's mount bpffs on /var/run/ltcp/bpf multiple times.
> $ mount -t bpf
> bpffs on /run/ltcp/bpf type bpf (rw,relatime)
> bpffs on /run/ltcp/bpf type bpf (rw,relatime)
> bpffs on /run/ltcp/bpf type bpf (rw,relatime)
> 
> After pinning some bpf progs on this mount point, let's check the pinned
> files with bpftool,
> $ bpftool prog list -f
> 87: sock_ops  name bpf_sockmap  tag a04f5eef06a7f555  gpl
>          loaded_at 2022-02-23T16:27:38+0800  uid 0
>          xlated 16B  jited 15B  memlock 4096B
>          pinned /run/ltcp/bpf/bpf_sockmap
>          pinned /run/ltcp/bpf/bpf_sockmap
>          pinned /run/ltcp/bpf/bpf_sockmap
>          btf_id 243
> 89: sk_msg  name bpf_redir_proxy  tag 57cd311f2e27366b  gpl
>          loaded_at 2022-02-23T16:27:38+0800  uid 0
>          xlated 16B  jited 18B  memlock 4096B
>          pinned /run/ltcp/bpf/bpf_redir_proxy
>          pinned /run/ltcp/bpf/bpf_redir_proxy
>          pinned /run/ltcp/bpf/bpf_redir_proxy
>          btf_id 244
> 
> The same pinned file will be showed multiple times.
> Finnally after mounting bpffs on the same mount point again, we can't
> get the pinnned files via bpftool,
> $ bpftool prog list -f
> 87: sock_ops  name bpf_sockmap  tag a04f5eef06a7f555  gpl
>          loaded_at 2022-02-23T16:27:38+0800  uid 0
>          xlated 16B  jited 15B  memlock 4096B
>          btf_id 243
> 89: sk_msg  name bpf_redir_proxy  tag 57cd311f2e27366b  gpl
>          loaded_at 2022-02-23T16:27:38+0800  uid 0
>          xlated 16B  jited 18B  memlock 4096B
>          btf_id 244
> 
> We should better refuse to mount bpffs on the same mount point. Before
> making this change, I also checked why it is allowed in the first place.
> The related commits are commit e27f4a942a0e
> ("bpf: Use mount_nodev not mount_ns to mount the bpf filesystem") and
> commit b2197755b263 ("bpf: add support for persistent maps/progs").
> Unfortunately they didn't explain why it is allowed. But there should be
> no use case which requires to mount bpffs on a same mount point multiple
> times, so let's just refuse it.
> 
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> ---
>   kernel/bpf/inode.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
> index 4f841e16779e..58374db9376f 100644
> --- a/kernel/bpf/inode.c
> +++ b/kernel/bpf/inode.c
> @@ -763,7 +763,7 @@ static int bpf_fill_super(struct super_block *sb, struct fs_context *fc)
>   
>   static int bpf_get_tree(struct fs_context *fc)
>   {
> -	return get_tree_nodev(fc, bpf_fill_super);
> +	return get_tree_single(fc, bpf_fill_super);

This is not right. get_tree_nodev is intentional to allow bpffs could be
mounted in different places with different contents. get_tree_single
permits a single shared bpffs tree which is not what we want.

In your particular case, you probably should improve your tools.
in my opinion, with get_tree_nodev, it is user space's responsibility
to coordinate with different bpffs mounts.

>   }
>   
>   static void bpf_free_fc(struct fs_context *fc)