Fix squashfs failing to load sparse files

SquashFS supports sprase blocks in files - that is, if a given block is
composed only of zeros, it's not written to the output file to save
space and it's on-disk length field is set to zero to indicate that.

Previously the squashfs driver did not recognise that, and would attempt
to read and decompress a zero-sized block, which obviously failed.

The following command may be used to create a file for testing:

cat <(dd if=/dev/urandom of=/dev/stdout bs=1M count=1) \
	<(dd if=/dev/zero of=/dev/stdout bs=1M count=1) \
	<(dd if=/dev/urandom of=/dev/stdout bs=1k count=200) >test_file

Signed-off-by: Campbell Suter <campbell@snapit.group>

diff --git a/fs/squashfs/sqfs.c b/fs/squashfs/sqfs.c
index 5de69ac..dca13bd 100644 (file)
--- a/fs/squashfs/sqfs.c
+++ b/fs/squashfs/sqfs.c
@@ -1310,7 +1310,7 @@ int sqfs_read(const char *filename, void *buf, loff_t offset, loff_t len,
 {
        char *dir = NULL, *fragment_block, *datablock = NULL, *data_buffer = NULL;
        char *fragment = NULL, *file = NULL, *resolved, *data;
-       u64 start, n_blks, table_size, data_offset, table_offset;
+       u64 start, n_blks, table_size, data_offset, table_offset, sparse_size;
        int ret, j, i_number, datablk_count = 0;
        struct squashfs_super_block *sblk = ctxt.sblk;
        struct squashfs_fragment_block_entry frag_entry;
@@ -1444,28 +1444,43 @@ int sqfs_read(const char *filename, void *buf, loff_t offset, loff_t len,
                n_blks = DIV_ROUND_UP(table_size + table_offset,
                                      ctxt.cur_dev->blksz);
 
-               data_buffer = malloc_cache_aligned(n_blks * ctxt.cur_dev->blksz);
+               /* Don't load any data for sparse blocks */
+               if (finfo.blk_sizes[j] == 0) {
+                       n_blks = 0;
+                       table_offset = 0;
+                       data_buffer = NULL;
+                       data = NULL;
+               } else {
+                       data_buffer = malloc_cache_aligned(n_blks * ctxt.cur_dev->blksz);
 
-               if (!data_buffer) {
-                       ret = -ENOMEM;
-                       goto out;
-               }
+                       if (!data_buffer) {
+                               ret = -ENOMEM;
+                               goto out;
+                       }
 
-               ret = sqfs_disk_read(start, n_blks, data_buffer);
-               if (ret < 0) {
-                       /*
-                        * Possible causes: too many data blocks or too large
-                        * SquashFS block size. Tip: re-compile the SquashFS
-                        * image with mksquashfs's -b <block_size> option.
-                        */
-                       printf("Error: too many data blocks to be read.\n");
-                       goto out;
-               }
+                       ret = sqfs_disk_read(start, n_blks, data_buffer);
+                       if (ret < 0) {
+                               /*
+                                * Possible causes: too many data blocks or too large
+                                * SquashFS block size. Tip: re-compile the SquashFS
+                                * image with mksquashfs's -b <block_size> option.
+                                */
+                               printf("Error: too many data blocks to be read.\n");
+                               goto out;
+                       }
 
-               data = data_buffer + table_offset;
+                       data = data_buffer + table_offset;
+               }
 
                /* Load the data */
-               if (SQFS_COMPRESSED_BLOCK(finfo.blk_sizes[j])) {
+               if (finfo.blk_sizes[j] == 0) {
+                       /* This is a sparse block */
+                       sparse_size = get_unaligned_le32(&sblk->block_size);
+                       if ((*actread + sparse_size) > len)
+                               sparse_size = len - *actread;
+                       memset(buf + *actread, 0, sparse_size);
+                       *actread += sparse_size;
+               } else if (SQFS_COMPRESSED_BLOCK(finfo.blk_sizes[j])) {
                        dest_len = get_unaligned_le32(&sblk->block_size);
                        ret = sqfs_decompress(&ctxt, datablock, &dest_len,
                                              data, table_size);
@@ -1484,7 +1499,8 @@ int sqfs_read(const char *filename, void *buf, loff_t offset, loff_t len,
                }
 
                data_offset += table_size;
-               free(data_buffer);
+               if (data_buffer)
+                       free(data_buffer);
                data_buffer = NULL;
                if (*actread >= len)
                        break;