Re: [PATCH] ext2: Handle fiemap on empty files to prevent EINVAL

[Jan Kara]

On Thu 12-06-25 12:29:16, Jan Kara wrote:
> On Thu 12-06-25 10:28:55, Wei Gao wrote:
> > Previously, ext2_fiemap would unconditionally apply "len = min_t(u64, len,
> > i_size_read(inode));", When inode->i_size was 0 (for an empty file), this
> > would reduce the requested len to 0. Passing len = 0 to iomap_fiemap could
> > then result in an -EINVAL error, even for valid queries on empty files.
> > The new validation logic directly references ext4_fiemap_check_ranges.
> > 
> > Link: https://github.com/linux-test-project/ltp/issues/1246
> > Signed-off-by: Wei Gao <wegao@xxxxxxxx>
> 
> Thanks for the fix. Some comments below:
> 
> > diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> > index 30f8201c155f..e5cc61088f21 100644
> > --- a/fs/ext2/inode.c
> > +++ b/fs/ext2/inode.c
> > @@ -895,10 +895,30 @@ int ext2_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> >  		u64 start, u64 len)
> >  {
> >  	int ret;
> > +	u64 maxbytes;
> >  
> >  	inode_lock(inode);
> > -	len = min_t(u64, len, i_size_read(inode));
> 
> So I agree this 'len' reduction to i_size looks bogus and can be removed.
> But why are you adding those maxbytes checks when they are done inside
> iomap_fiemap() shorly later?

After experimenting a bit I see the purpose of trimming to i_size -
otherwise with large range iomap will iterate all available inode offsets
block by block wasting a lot of CPU. This is because ext2_get_blocks() is
always reporting holes as a single block long. So we either need to keep
the i_size limitation (and just treat i_size == 0 specially) or we need to
improve ext2_get_blocks() to better report length of holes (at least
whatever is in the currently loaded indirect block) - not sure if that is
worth it.

								Honza

> > +	maxbytes = inode->i_sb->s_maxbytes;
> > +
> > +	if (len == 0) {
> > +		ret = -EINVAL;
> > +		goto unlock_inode;
> > +	}
> > +
> > +	if (start > maxbytes) {
> > +		ret = -EFBIG;
> > +		goto unlock_inode;
> > +	}
> > +
> > +	/*
> > +	 * Shrink request scope to what the fs can actually handle.
> > +	 */
> > +	if (len > maxbytes || (maxbytes - len) < start)
> > +		len = maxbytes - start;
> > +
> >  	ret = iomap_fiemap(inode, fieinfo, start, len, &ext2_iomap_ops);
> > +
> > +unlock_inode:
> >  	inode_unlock(inode);
> >  
> >  	return ret;
> > -- 
> > 2.49.0
> > 
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR