fs/fat: Improve error handling

The FAT filesystem fails silently in inexplicable ways when given a
filesystem with a block-size that does not match the device sector size.
In theory this is not an unsupportable combination but requires a major
rewrite of a lot of the filesystem.  Until that occurs, the filesystem
should detect that scenario and display a helpful error message.

This scenario in particular occurred on a 512-byte blocksize FAT fs
stored in an El-Torito boot volume on a CD-ROM (2048-byte sector size).

Additionally, in many circumstances the ->block_read method will not
return a negative number to indicate an error but instead return 0 to
indicate the number of blocks successfully read (IE: None).

The FAT filesystem should defensively check to ensure that it got all of
the sectors that it asked for when reading.

Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>

diff --git a/fs/fat/fat.c b/fs/fat/fat.c
index 2bb7adfcf156424e48b9df634473c6fb6d6bca54..1f95eb4cafe350cb4d69888710de77298e2535aa 100644 (file)
--- a/fs/fat/fat.c
+++ b/fs/fat/fat.c
@@ -274,6 +274,8 @@ get_cluster (fsdata *mydata, __u32 clustnum, __u8 *buffer,
 {
        __u32 idx = 0;
        __u32 startsect;
+       __u32 nr_sect;
+       int ret;
 
        if (clustnum > 0) {
                startsect = mydata->data_begin +
@@ -284,16 +286,19 @@ get_cluster (fsdata *mydata, __u32 clustnum, __u8 *buffer,
 
        debug("gc - clustnum: %d, startsect: %d\n", clustnum, startsect);
 
-       if (disk_read(startsect, size / mydata->sect_size, buffer) < 0) {
-               debug("Error reading data\n");
+       nr_sect = size / mydata->sect_size;
+       ret = disk_read(startsect, nr_sect, buffer);
+       if (ret != nr_sect) {
+               debug("Error reading data (got %d)\n", ret);
                return -1;
        }
        if (size % mydata->sect_size) {
                __u8 tmpbuf[mydata->sect_size];
 
                idx = size / mydata->sect_size;
-               if (disk_read(startsect + idx, 1, tmpbuf) < 0) {
-                       debug("Error reading data\n");
+               ret = disk_read(startsect + idx, 1, tmpbuf);
+               if (ret != 1) {
+                       debug("Error reading data (got %d)\n", ret);
                        return -1;
                }
                buffer += idx * mydata->sect_size;
@@ -803,6 +808,11 @@ do_fat_read (const char *filename, void *buffer, unsigned long maxsize,
 
        mydata->sect_size = (bs.sector_size[1] << 8) + bs.sector_size[0];
        mydata->clust_size = bs.cluster_size;
+       if (mydata->sect_size != cur_part_info.blksz) {
+               printf("Error: FAT sector size mismatch (fs=%hu, dev=%lu)\n",
+                               mydata->sect_size, cur_part_info.blksz);
+               return -1;
+       }
 
        if (mydata->fatsize == 32) {
                mydata->data_begin = mydata->rootdir_sect -