linux-kernel - Re: [RFC PATCH v3 2/3] fs: debugfs: Add write functionality to debugfs blobs

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Message-ID: <1d55a83a-b36a-4319-16bc-c1aa72e361b5@amd.com>
Date:   Tue, 13 Jun 2023 20:05:41 +1000
From:   Alexey Kardashevskiy <aik@....com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Avadhut Naik <Avadhut.Naik@....com>
Cc:     rafael@...nel.org, lenb@...nel.org, linux-acpi@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, avadnaik@....com,
        yazen.ghannam@....com, alexey.kardashevskiy@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 2/3] fs: debugfs: Add write functionality to
 debugfs blobs



On 13/6/23 17:59, Greg KH wrote:
> On Mon, Jun 12, 2023 at 09:51:38PM +0000, Avadhut Naik wrote:
>>   /**
>> - * 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);
> 
> Have you audited all calls to this function to verify that you haven't
> just turned on write access to some debugfs files?

I just did, it is one of S_IRUGO/S_IRUSR/0444/0400/(S_IFREG | 0444). So 
we are quite safe here. Except (S_IFREG | 0444) in 
drivers/platform/chrome/cros_ec_debugfs.c which seems wrong as debugfs 
files are not regular files.

> Why not rename this to debugfs_create_blob_wo() and then make a new
> debugfs_create_blob_rw() call to ensure that it all is ok?

It is already taking the mode for this purpose. imho just 
cros_ec_create_panicinfo()'s debugfs_create_blob("panicinfo", S_IFREG | 
0444,...) needs fixing.

> 
> thanks,
> 
> greg k-h

-- 
Alexey