Btrfs: Fix use after free during compressed reads

Yan's fix to use the correct file offset during compressed reads used the
extent_map struct pointer after it had been freed.  This saves the
fields we want for later use instead.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 8e7a78a..b582c6f 100644 (file)
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -505,6 +505,8 @@ int btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
        struct block_device *bdev;
        struct bio *comp_bio;
        u64 cur_disk_byte = (u64)bio->bi_sector << 9;
+       u64 em_len;
+       u64 em_start;
        struct extent_map *em;
        int ret;
 
@@ -525,7 +527,10 @@ int btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
 
        cb->start = em->orig_start;
        compressed_len = em->block_len;
+       em_len = em->len;
+       em_start = em->start;
        free_extent_map(em);
+       em = NULL;
 
        cb->len = uncompressed_len;
        cb->compressed_len = compressed_len;
@@ -543,7 +548,7 @@ int btrfs_submit_compressed_read(struct inode *inode, struct bio *bio,
        }
        cb->nr_pages = nr_pages;
 
-       add_ra_bio_pages(inode, em->start + em->len, cb);
+       add_ra_bio_pages(inode, em_start + em_len, cb);
 
        if (!btrfs_test_opt(root, NODATASUM) &&
            !btrfs_test_flag(inode, NODATASUM)) {