linux-kernel - Re: [RFC PATCH v2 2/3] fs: debugfs: Add write functionality to debugfs blobs

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Message-ID: <a310a133-8d4e-0870-adef-0b7b47a452e7@amd.com>
Date:   Thu, 8 Jun 2023 14:02:29 +1000
From:   Alexey Kardashevskiy <aik@....com>
To:     Avadhut Naik <Avadhut.Naik@....com>, rafael@...nel.org,
        gregkh@...uxfoundation.org, lenb@...nel.org,
        linux-acpi@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc:     avadnaik@....com, yazen.ghannam@....com,
        alexey.kardashevskiy@....com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 2/3] fs: debugfs: Add write functionality to
 debugfs blobs



On 26/5/23 06:44, Avadhut Naik wrote:
> Currently, debugfs_create_blob() creates read-only debugfs binary blob
> files.
> 
> In some cases, however, userspace tools need to write variable length
> data structures into predetermined memory addresses. An example is when
> injecting Vendor-defined error types through the einj module. In such
> cases, the functionality to write to these blob files in debugfs would
> be desired since the mapping aspect can be handled within the modules
> with userspace tools only needing to write into the blob files.
> 
> Implement a write callback to enable writing to these blob files in
> debugfs.
> 
> Signed-off-by: Avadhut Naik <Avadhut.Naik@....com>

Reviewed-by: Alexey Kardashevskiy <aik@....com>

> ---
>   fs/debugfs/file.c | 28 +++++++++++++++++++++++-----
>   1 file changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> index 1f971c880dde..fab5a562b57c 100644
> --- a/fs/debugfs/file.c
> +++ b/fs/debugfs/file.c
> @@ -973,17 +973,35 @@ static ssize_t read_file_blob(struct file *file, char __user *user_buf,
>   	return r;
>   }
>   
> +static ssize_t write_file_blob(struct file *file, const char __user *user_buf,
> +			       size_t count, loff_t *ppos)
> +{
> +	struct debugfs_blob_wrapper *blob = file->private_data;
> +	struct dentry *dentry = F_DENTRY(file);
> +	ssize_t r;
> +
> +	r = debugfs_file_get(dentry);
> +	if (unlikely(r))
> +		return r;
> +	r = simple_write_to_buffer(blob->data, blob->size, ppos, user_buf,
> +				   count);
> +
> +	debugfs_file_put(dentry);
> +	return r;
> +}
> +
>   static const struct file_operations fops_blob = {
>   	.read =		read_file_blob,
> +	.write =	write_file_blob,
>   	.open =		simple_open,
>   	.llseek =	default_llseek,
>   };
>   
>   /**
> - * debugfs_create_blob - create a debugfs file that is used to read a binary blob
> + * debugfs_create_blob - create a debugfs file that is used to read and write
> + * a binary blob
>    * @name: a pointer to a string containing the name of the file to create.
> - * @mode: the read permission that the file should have (other permissions are
> - *	  masked out)
> + * @mode: the permission that the file should have
>    * @parent: a pointer to the parent dentry for this file.  This should be a
>    *          directory dentry if set.  If this parameter is %NULL, then the
>    *          file will be created in the root of the debugfs filesystem.
> @@ -992,7 +1010,7 @@ static const struct file_operations fops_blob = {
>    *
>    * This function creates a file in debugfs with the given name that exports
>    * @blob->data as a binary blob. If the @mode variable is so set it can be
> - * read from. Writing is not supported.
> + * read from and written to.
>    *
>    * This function will return a pointer to a dentry if it succeeds.  This
>    * pointer must be passed to the debugfs_remove() function when the file is
> @@ -1007,7 +1025,7 @@ struct dentry *debugfs_create_blob(const char *name, umode_t mode,
>   				   struct dentry *parent,
>   				   struct debugfs_blob_wrapper *blob)
>   {
> -	return debugfs_create_file_unsafe(name, mode & 0444, parent, blob, &fops_blob);
> +	return debugfs_create_file_unsafe(name, mode, parent, blob, &fops_blob);
>   }
>   EXPORT_SYMBOL_GPL(debugfs_create_blob);
>   

-- 
Alexey