Re: [RFC PATCH] relay: avoid relay_open_buf inproperly fails in buffer-only mode

[Philo Lu]

Sorry, the email address of Andrew was wrongly spelled, so CC again.

On 2023/12/20 15:47, Philo Lu wrote:
> In buffer-only mode, relay_open(NULL, NULL, ...) is used to create the
> buffer first, where chan->has_base_filename is not set. Though we still
> need to call chan->cb->create_buf_file in relay_open_buf() to retrieve
> global info for global buffer, the create_buf_file callback should
> return NULL. With IS_ERR_OR_NULL() check, relay_open fails because of
> the returned NULL dentry, so this patch reverts back to the WARN_ON()
> version and add a comment for this behavior.
>
> Here is an example after fix:
> ```
> struct dentry *my_create_buf_file(const char *filename,
>              struct dentry *parent, umode_t mode,
>              struct rchan_buf *buf, int *is_global)
> {
>      if (!filename)
>          return NULL;
>
>      return debugfs_create_file(filename, mode, parent, buf,
>                  &relay_file_operations);
> }
>
> relay_cb.create_buf_file = my_create_buf_file
> relay_chan = relay_open(NULL, NULL,
>                      subbuf_size, subbuf_num,
>                      &relay_cb, NULL);
> relay_late_setup_files(relay_chan, filename, parent);
> ```
>
> But before fix, the create_buf_file callback must be something like:
> ```
> struct dentry *my_create_buf_file(const char *filename,
>              struct dentry *parent, umode_t mode,
>              struct rchan_buf *buf, int *is_global)
> {
>      if (!filename)
>          return ERR_PTR(1); // a valid ptr is necessary for relay_open
>
>      return debugfs_create_file(filename, mode, parent, buf,
>                  &relay_file_operations);
> }
> ```
>
> I'm not sure if this revertion proper because it may break existing use
> cases. I think we can also remove the WARN_ON check instead.
>
> Fixes: 2c1cf00eeacb ("relay: check return of create_buf_file() properly")
> Signed-off-by: Philo Lu <lulie@xxxxxxxxxxxxxxxxx>
> ---
>   kernel/relay.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/relay.c b/kernel/relay.c
> index 83fe0325cde1..0700745447c1 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -395,7 +395,8 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
>   		dentry = chan->cb->create_buf_file(NULL, NULL,
>   						   S_IRUSR, buf,
>   						   &chan->is_global);
> -		if (IS_ERR_OR_NULL(dentry))
> +		/* has_base_filename not set, so dentry should be NULL */
> +		if (WARN_ON(dentry))
>   			goto free_buf;
>   	}
>
> --
> 2.32.0.3.g01195cf9f