ext4: fix trim length underflow with small trim length

In 0f0a25b, we adjust 'len' with s_first_data_block - start, but
it could underflow in case blocksize=1K, fstrim_range.len=512 and
fstrim_range.start = 0. In this case, when we run the code:
len -= first_data_blk - start; len will be underflow to -1ULL.
In the end, although we are safe that last_group check later will limit
the trim to the whole volume, but that isn't what the user really want.

So this patch fix it. It also adds the check for 'start' like ext3 so that
we can break immediately if the start is invalid.

Cc: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 1900ec7a15794b635224433c1f08561fa942f46a..b189cb4ff20f161c5ee765c42d0c6651e899c58e 100644 (file)
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4902,6 +4902,8 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
 
        if (unlikely(minlen > EXT4_BLOCKS_PER_GROUP(sb)))
                return -EINVAL;
+       if (start + len <= first_data_blk)
+               goto out;
        if (start < first_data_blk) {
                len -= first_data_blk - start;
                start = first_data_blk;
@@ -4950,5 +4952,6 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
        }
        range->len = trimmed * sb->s_blocksize;
 
+out:
        return ret;
 }