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

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Date:   Thu, 11 Feb 2021 20:59:02 -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 Thu, Feb 11, 2021 at 08:53:47PM -0800, Darrick J. Wong wrote:
> 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?)

Oh, I see, because that doesn't help if the source file is procfs and
the dest file is (say) xfs, because the generic version will try to do
splice magic and *poof*.

I guess the other nit thatI can think of at this late hour is ... what
about the other virtual filesystems like configfs and whatnot?  Should
we have a way to flag them as "this can't be the source of a CFR
request" as well?

Or is it just trace/debug/proc/sysfs that have these "zero size but
readable" speshul behaviors?

--D

> 
> --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
> >