linux-kernel - Re: [PATCH v3 1/3] debugfs: Add write support to debugfs_create

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Message-ID: <44a63cb2-1ae5-f52b-19d2-fe03d48fd44d@kernel.org>
Date:   Fri, 18 Aug 2023 13:05:57 +0300
From:   Georgi Djakov <djakov@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Mike Tipton <quic_mdtipton@...cinc.com>
Cc:     rafael@...nel.org, corbet@....net, linux-pm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, quic_okukatla@...cinc.com,
        quic_viveka@...cinc.com, peterz@...radead.org,
        quic_pkondeti@...cinc.com
Subject: Re: [PATCH v3 1/3] debugfs: Add write support to debugfs_create_str()

Hi Greg,

On 12.08.23 13:40, Greg KH wrote:
> On Mon, Aug 07, 2023 at 07:29:12AM -0700, Mike Tipton wrote:
>> Currently, debugfs_create_str() only supports reading strings from
>> debugfs. Add support for writing them as well.
>>
>> Based on original implementation by Peter Zijlstra [0]. Write support
>> was present in the initial patch version, but dropped in v2 due to lack
>> of users. We have a user now, so reintroduce it.
>>
>> [0] https://lore.kernel.org/all/YF3Hv5zXb%2F6lauzs@hirez.programming.kicks-ass.net/
>>
>> Signed-off-by: Mike Tipton <quic_mdtipton@...cinc.com>
>> ---
>>   fs/debugfs/file.c | 48 +++++++++++++++++++++++++++++++++++++++++++++--
>>   1 file changed, 46 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
>> index b7711888dd17..87b3753aa4b1 100644
>> --- a/fs/debugfs/file.c
>> +++ b/fs/debugfs/file.c
>> @@ -904,8 +904,52 @@ EXPORT_SYMBOL_GPL(debugfs_create_str);
>>   static ssize_t debugfs_write_file_str(struct file *file, const char __user *user_buf,
>>   				      size_t count, loff_t *ppos)
>>   {
>> -	/* This is really only for read-only strings */
>> -	return -EINVAL;
>> +	struct dentry *dentry = F_DENTRY(file);
>> +	char *old, *new = NULL;
>> +	int pos = *ppos;
>> +	int r;
>> +
>> +	r = debugfs_file_get(dentry);
>> +	if (unlikely(r))
>> +		return r;
>> +
>> +	old = *(char **)file->private_data;
>> +
>> +	/* only allow strict concatenation */
>> +	r = -EINVAL;
>> +	if (pos && pos != strlen(old))
>> +		goto error;
>> +
>> +	r = -E2BIG;
>> +	if (pos + count + 1 > PAGE_SIZE)
>> +		goto error;
>> +
>> +	r = -ENOMEM;
>> +	new = kmalloc(pos + count + 1, GFP_KERNEL);
>> +	if (!new)
>> +		goto error;
>> +
>> +	if (pos)
>> +		memcpy(new, old, pos);
>> +
>> +	r = -EFAULT;
>> +	if (copy_from_user(new + pos, user_buf, count))
>> +		goto error;
>> +
>> +	new[pos + count] = '\0';
>> +	strim(new);
>> +
>> +	rcu_assign_pointer(*(char **)file->private_data, new);
>> +	synchronize_rcu();
>> +	kfree(old);
>> +
>> +	debugfs_file_put(dentry);
>> +	return count;
>> +
>> +error:
>> +	kfree(new);
>> +	debugfs_file_put(dentry);
>> +	return r;
>>   }
> 
> So you just added write support for ALL debugfs files that use the
> string interface, what did you just allow to break?

Not true. Write support is added only for debugfs string files that are
created with +w permissions. All existing files are created as read-only
and use the fops_str_ro ops.

> I recommend just using your own debugfs file function instead, as this
> could cause bad problems, right?  Are you sure that all string calls can
> handle the variable be freed underneath it like this call will allow to
> happen?
> 
> So I wouldn't recommend doing this, sorry.
> 

Maybe you missed the fact that the different file ops are already there
and are selected based on permissions:

> static const struct file_operations fops_str = {
>         .read =         debugfs_read_file_str,
>         .write =        debugfs_write_file_str,
>         .open =         simple_open,
>         .llseek =       default_llseek,
> };
> 
> static const struct file_operations fops_str_ro = {
>         .read =         debugfs_read_file_str,
>         .open =         simple_open,
>         .llseek =       default_llseek,
> };
> 
> static const struct file_operations fops_str_wo = {
>         .write =        debugfs_write_file_str,
>         .open =         simple_open,
>         .llseek =       default_llseek,
> };

...so this patch is doing exactly what you suggested? If you agree,
could you ack it again please?

Thanks,
Georgi