Re: [PATCH v3 1/3] debugfs: Add write support to debugfs_create_str()

[Georgi Djakov <djakov@kernel.org>]

On 7.08.23 17:29, 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/
> 

Hi Greg,

Looks like the original code was reviewed two years ago (not sure if it
counts). But in any case, i need an ack from you to apply this.

There is no build dependency with the rest of the patches (but there is a
functional one). It should be also fine if you apply it directly, if you
prefer so?

Thanks,
Georgi

> Signed-off-by: Mike Tipton <quic_mdtipton@quicinc.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;
>   }
>   
>   static const struct file_operations fops_str = {