2024-02-20 10:58:55

by Philo Lu

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

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. However, with IS_ERR_OR_NULL() checking, relay_open fails
because the returned dentry == NULL.

This patch remove the IS_ERR_OR_NULL() for compatibility and update the
comment to explain the behavior.

Here is a create_buf_file callback 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 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);
}
```

Fixes: 2c1cf00eeacb ("relay: check return of create_buf_file() properly")
Signed-off-by: Philo Lu <[email protected]>
---
kernel/relay.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/relay.c b/kernel/relay.c
index a8e90e98bf2c..0d8b8325530a 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -391,12 +391,13 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
goto free_buf;
relay_set_buf_dentry(buf, dentry);
} else {
- /* Only retrieve global info, nothing more, nothing less */
+ /* In buffer-only mode, relay_open_buf is called with
+ * filename=NULL, but create_buf_file is still needed to
+ * retrieve is_global info. So dentry should be NULL here.
+ */
dentry = chan->cb->create_buf_file(NULL, NULL,
S_IRUSR, buf,
&chan->is_global);
- if (IS_ERR_OR_NULL(dentry))
- goto free_buf;
}

buf->cpu = cpu;
--
2.32.0.3.g01195cf9f

2024-03-15 02:42:31

by zhangzhengming

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

> diff --git a/kernel/relay.c b/kernel/relay.c
> index a8e90e98bf2c..0d8b8325530a 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -391,12 +391,13 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
> goto free_buf;
> relay_set_buf_dentry(buf, dentry);
> } else {
> - /* Only retrieve global info, nothing more, nothing less */
> + /* In buffer-only mode, relay_open_buf is called with
> + * filename=NULL, but create_buf_file is still needed to
> + * retrieve is_global info. So dentry should be NULL here.
> + */
> dentry = chan->cb->create_buf_file(NULL, NULL,
> S_IRUSR, buf,
> &chan->is_global);
> - if (IS_ERR_OR_NULL(dentry))
> - goto free_buf;
> }
>
> buf->cpu = cpu;
I suggest replacing IS_ERR_OR_NULL with IS_ERR instead of removing it due to the possibility of create_buf_file callback returning an error.

2024-03-15 03:23:13

by Philo Lu

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

On 2024/3/15 10:39, zhangzhengming wrote:
>> diff --git a/kernel/relay.c b/kernel/relay.c
>> index a8e90e98bf2c..0d8b8325530a 100644
>> --- a/kernel/relay.c
>> +++ b/kernel/relay.c
>> @@ -391,12 +391,13 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
>> goto free_buf;
>> relay_set_buf_dentry(buf, dentry);
>> } else {
>> - /* Only retrieve global info, nothing more, nothing less */
>> + /* In buffer-only mode, relay_open_buf is called with
>> + * filename=NULL, but create_buf_file is still needed to
>> + * retrieve is_global info. So dentry should be NULL here.
>> + */
>> dentry = chan->cb->create_buf_file(NULL, NULL,
>> S_IRUSR, buf,
>> &chan->is_global);
>> - if (IS_ERR_OR_NULL(dentry))
>> - goto free_buf;
>> }
>>
>> buf->cpu = cpu;
> I suggest replacing IS_ERR_OR_NULL with IS_ERR instead of removing it due to the possibility of create_buf_file callback returning an error.
>

This would be fine. AFAICT the return value here is un-important. So
reverting to `WARN_ON(dentry)` maybe another choice.