linux-kernel - Re: [PATCH 6/6] vfs: Disallow copy_file

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Date:   Thu, 11 Feb 2021 20:53:47 -0800
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Nicolas Boichat <drinkcat@...omium.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Ian Lance Taylor <iant@...gle.com>,
        Luis Lozano <llozano@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] vfs: Disallow copy_file_range on generated file
 systems

On Fri, Feb 12, 2021 at 12:44:05PM +0800, Nicolas Boichat wrote:
> copy_file_range (which calls generic_copy_file_checks) uses the
> inode file size to adjust the copy count parameter. This breaks
> with special filesystems like procfs/sysfs/debugfs/tracefs, where
> the file size appears to be zero, but content is actually returned
> when a read operation is performed. Other issues would also
> happen on partial writes, as the function would attempt to seek
> in the input file.
> 
> Use the newly introduced FS_GENERATED_CONTENT filesystem flag
> to return -EOPNOTSUPP: applications can then retry with a more
> usual read/write based file copy (the fallback code is usually
> already present to handle older kernels).
> 
> Signed-off-by: Nicolas Boichat <drinkcat@...omium.org>
> ---
> 
>  fs/read_write.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 0029ff2b0ca8..80322e89fb0a 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1485,6 +1485,9 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
>  	if (flags != 0)
>  		return -EINVAL;
>  
> +	if (file_inode(file_in)->i_sb->s_type->fs_flags & FS_GENERATED_CONTENT)
> +		return -EOPNOTSUPP;

Why not declare a dummy copy_file_range_nop function that returns
EOPNOTSUPP and point all of these filesystems at it?

(Or, I guess in these days where function pointers are the enemy,
create a #define that is a cast of 0x1, and fix do_copy_file_range to
return EOPNOTSUPP if it sees that?)

--D

> +
>  	ret = generic_copy_file_checks(file_in, pos_in, file_out, pos_out, &len,
>  				       flags);
>  	if (unlikely(ret))
> -- 
> 2.30.0.478.g8a0d178c01-goog
>