Changes in uspace/srv/fs/fat/fat_fat.c [5f116e7:d70d80ed] in mainline

File:

• uspace/srv/fs/fat/fat_fat.c (modified) (19 diffs)

uspace/srv/fs/fat/fat_fat.c

@@ -46 +46 @@
 #include <align.h>
 #include <assert.h>
-#include <fibril_sync.h>
+#include <fibril_synch.h>
 #include <mem.h>
+
+/*
+ * Convenience macros for computing some frequently used values from the
+ * primitive boot sector members.
+ */
+#define RDS(bs)         ((sizeof(fat_dentry_t) * RDE((bs))) / BPS((bs))) + \
+                        (((sizeof(fat_dentry_t) * RDE((bs))) % BPS((bs))) != 0)
+#define SSA(bs)         (RSCNT((bs)) + FATCNT((bs)) * SF((bs)) + RDS(bs))
+
+#define CLBN2PBN(bs, cl, bn) \
+        (SSA((bs)) + ((cl) - FAT_CLST_FIRST) * SPC((bs)) + (bn) % SPC((bs)))
 
 /**
@@ -59 +70 @@
  *
  * @param bs            Buffer holding the boot sector for the file.
- * @param dev_handle    Device handle of the device with the file.
+ * @param devmap_handle Device handle of the device with the file.
  * @param firstc        First cluster to start the walk with.
- * @param lastc         If non-NULL, output argument hodling the last cluster number visited.
+ * @param lastc         If non-NULL, output argument hodling the last cluster
+ *                      number visited.
+ * @param numc          If non-NULL, output argument holding the number of
+ *                      clusters seen during the walk.
  * @param max_clusters  Maximum number of clusters to visit.    
  *
- * @return              Number of clusters seen during the walk.
- */
-uint16_t 
-fat_cluster_walk(fat_bs_t *bs, dev_handle_t dev_handle, fat_cluster_t firstc,
-    fat_cluster_t *lastc, uint16_t max_clusters)
+ * @return              EOK on success or a negative error code.
+ */
+int 
+fat_cluster_walk(fat_bs_t *bs, devmap_handle_t devmap_handle, fat_cluster_t firstc,
+    fat_cluster_t *lastc, uint16_t *numc, uint16_t max_clusters)
 {
         block_t *b;
-        unsigned bps;
-        unsigned rscnt;         /* block address of the first FAT */
         uint16_t clusters = 0;
         fat_cluster_t clst = firstc;
-
-        bps = uint16_t_le2host(bs->bps);
-        rscnt = uint16_t_le2host(bs->rscnt);
+        int rc;
 
         if (firstc == FAT_CLST_RES0) {
@@ -83 +93 @@
                 if (lastc)
                         *lastc = firstc;
-                return 0;
+                if (numc)
+                        *numc = 0;
+                return EOK;
         }
 
         while (clst < FAT_CLST_LAST1 && clusters < max_clusters) {
-                bn_t fsec;      /* sector offset relative to FAT1 */
+                aoff64_t fsec;  /* sector offset relative to FAT1 */
                 unsigned fidx;  /* FAT1 entry index */
 
@@ -93 +105 @@
                 if (lastc)
                         *lastc = clst;  /* remember the last cluster number */
-                fsec = (clst * sizeof(fat_cluster_t)) / bps;
-                fidx = clst % (bps / sizeof(fat_cluster_t));
+                fsec = (clst * sizeof(fat_cluster_t)) / BPS(bs);
+                fidx = clst % (BPS(bs) / sizeof(fat_cluster_t));
                 /* read FAT1 */
-                b = block_get(dev_handle, rscnt + fsec, BLOCK_FLAGS_NONE);
+                rc = block_get(&b, devmap_handle, RSCNT(bs) + fsec,
+                    BLOCK_FLAGS_NONE);
+                if (rc != EOK)
+                        return rc;
                 clst = uint16_t_le2host(((fat_cluster_t *)b->data)[fidx]);
                 assert(clst != FAT_CLST_BAD);
-                block_put(b);
+                rc = block_put(b);
+                if (rc != EOK)
+                        return rc;
                 clusters++;
         }
@@ -105 +122 @@
         if (lastc && clst < FAT_CLST_LAST1)
                 *lastc = clst;
-
-        return clusters;
+        if (numc)
+                *numc = clusters;
+
+        return EOK;
 }
 
 /** Read block from file located on a FAT file system.
  *
+ * @param block         Pointer to a block pointer for storing result.
  * @param bs            Buffer holding the boot sector of the file system.
- * @param dev_handle    Device handle of the file system.
- * @param firstc        First cluster used by the file. Can be zero if the file
- *                      is empty.
+ * @param nodep         FAT node.
  * @param bn            Block number.
  * @param flags         Flags passed to libblock.
  *
- * @return              Block structure holding the requested block.
- */
-block_t *
-_fat_block_get(fat_bs_t *bs, dev_handle_t dev_handle, fat_cluster_t firstc,
-    bn_t bn, int flags)
-{
-        block_t *b;
-        unsigned bps;
-        unsigned rscnt;         /* block address of the first FAT */
-        unsigned rde;
-        unsigned rds;           /* root directory size */
-        unsigned sf;
-        unsigned ssa;           /* size of the system area */
-        unsigned clusters, max_clusters;
-        fat_cluster_t lastc;
-
-        bps = uint16_t_le2host(bs->bps);
-        rscnt = uint16_t_le2host(bs->rscnt);
-        rde = uint16_t_le2host(bs->root_ent_max);
-        sf = uint16_t_le2host(bs->sec_per_fat);
-
-        rds = (sizeof(fat_dentry_t) * rde) / bps;
-        rds += ((sizeof(fat_dentry_t) * rde) % bps != 0);
-        ssa = rscnt + bs->fatcnt * sf + rds;
-
-        if (firstc == FAT_CLST_ROOT) {
+ * @return              EOK on success or a negative error code.
+ */
+int
+fat_block_get(block_t **block, struct fat_bs *bs, fat_node_t *nodep,
+    aoff64_t bn, int flags)
+{
+        fat_cluster_t firstc = nodep->firstc;
+        fat_cluster_t currc;
+        aoff64_t relbn = bn;
+        int rc;
+
+        if (!nodep->size)
+                return ELIMIT;
+
+        if (nodep->firstc == FAT_CLST_ROOT) 
+                goto fall_through;
+
+        if (((((nodep->size - 1) / BPS(bs)) / SPC(bs)) == bn / SPC(bs)) &&
+            nodep->lastc_cached_valid) {
+                /*
+                 * This is a request to read a block within the last cluster
+                 * when fortunately we have the last cluster number cached.
+                 */
+                return block_get(block, nodep->idx->devmap_handle,
+                    CLBN2PBN(bs, nodep->lastc_cached_value, bn), flags);
+        }
+
+        if (nodep->currc_cached_valid && bn >= nodep->currc_cached_bn) {
+                /*
+                 * We can start with the cluster cached by the previous call to
+                 * fat_block_get().
+                 */
+                firstc = nodep->currc_cached_value;
+                relbn -= (nodep->currc_cached_bn / SPC(bs)) * SPC(bs);
+        }
+
+fall_through:
+        rc = _fat_block_get(block, bs, nodep->idx->devmap_handle, firstc,
+            &currc, relbn, flags);
+        if (rc != EOK)
+                return rc;
+        
+        /*
+         * Update the "current" cluster cache.
+         */
+        nodep->currc_cached_valid = true;
+        nodep->currc_cached_bn = bn;
+        nodep->currc_cached_value = currc;
+
+        return rc;
+}
+
+/** Read block from file located on a FAT file system.
+ *
+ * @param block         Pointer to a block pointer for storing result.
+ * @param bs            Buffer holding the boot sector of the file system.
+ * @param devmap_handle Device handle of the file system.
+ * @param fcl           First cluster used by the file. Can be zero if the file
+ *                      is empty.
+ * @param clp           If not NULL, address where the cluster containing bn
+ *                      will be stored.
+ *                      stored 
+ * @param bn            Block number.
+ * @param flags         Flags passed to libblock.
+ *
+ * @return              EOK on success or a negative error code.
+ */
+int
+_fat_block_get(block_t **block, fat_bs_t *bs, devmap_handle_t devmap_handle,
+    fat_cluster_t fcl, fat_cluster_t *clp, aoff64_t bn, int flags)
+{
+        uint16_t clusters;
+        unsigned max_clusters;
+        fat_cluster_t c;
+        int rc;
+
+        /*
+         * This function can only operate on non-zero length files.
+         */
+        if (fcl == FAT_CLST_RES0)
+                return ELIMIT;
+
+        if (fcl == FAT_CLST_ROOT) {
                 /* root directory special case */
-                assert(bn < rds);
-                b = block_get(dev_handle, rscnt + bs->fatcnt * sf + bn, flags);
-                return b;
-        }
-
-        max_clusters = bn / bs->spc;
-        clusters = fat_cluster_walk(bs, dev_handle, firstc, &lastc,
-            max_clusters);
+                assert(bn < RDS(bs));
+                rc = block_get(block, devmap_handle,
+                    RSCNT(bs) + FATCNT(bs) * SF(bs) + bn, flags);
+                return rc;
+        }
+
+        max_clusters = bn / SPC(bs);
+        rc = fat_cluster_walk(bs, devmap_handle, fcl, &c, &clusters, max_clusters);
+        if (rc != EOK)
+                return rc;
         assert(clusters == max_clusters);
 
-        b = block_get(dev_handle, ssa + (lastc - FAT_CLST_FIRST) * bs->spc +
-            bn % bs->spc, flags);
-
-        return b;
+        rc = block_get(block, devmap_handle, CLBN2PBN(bs, c, bn), flags);
+
+        if (clp)
+                *clp = c;
+
+        return rc;
 }
 
@@ -170 +249 @@
  *                      this argument is ignored.
  * @param pos           Position in the last node block.
- */
-void fat_fill_gap(fat_bs_t *bs, fat_node_t *nodep, fat_cluster_t mcl, off_t pos)
-{
-        uint16_t bps;
-        unsigned spc;
+ *
+ * @return              EOK on success or a negative error code.
+ */
+int fat_fill_gap(fat_bs_t *bs, fat_node_t *nodep, fat_cluster_t mcl, aoff64_t pos)
+{
         block_t *b;
-        off_t o, boundary;
-
-        bps = uint16_t_le2host(bs->bps);
-        spc = bs->spc;
-        
-        boundary = ROUND_UP(nodep->size, bps * spc);
+        aoff64_t o, boundary;
+        int rc;
+
+        boundary = ROUND_UP(nodep->size, BPS(bs) * SPC(bs));
 
         /* zero out already allocated space */
         for (o = nodep->size; o < pos && o < boundary;
-            o = ALIGN_DOWN(o + bps, bps)) {
-                int flags = (o % bps == 0) ?
+            o = ALIGN_DOWN(o + BPS(bs), BPS(bs))) {
+                int flags = (o % BPS(bs) == 0) ?
                     BLOCK_FLAGS_NOREAD : BLOCK_FLAGS_NONE;
-                b = fat_block_get(bs, nodep, o / bps, flags);
-                memset(b->data + o % bps, 0, bps - o % bps);
+                rc = fat_block_get(&b, bs, nodep, o / BPS(bs), flags);
+                if (rc != EOK)
+                        return rc;
+                memset(b->data + o % BPS(bs), 0, BPS(bs) - o % BPS(bs));
                 b->dirty = true;                /* need to sync node */
-                block_put(b);
+                rc = block_put(b);
+                if (rc != EOK)
+                        return rc;
         }
         
         if (o >= pos)
-                return;
+                return EOK;
         
         /* zero out the initial part of the new cluster chain */
-        for (o = boundary; o < pos; o += bps) {
-                b = _fat_block_get(bs, nodep->idx->dev_handle, mcl,
-                    (o - boundary) / bps, BLOCK_FLAGS_NOREAD);
-                memset(b->data, 0, min(bps, pos - o));
+        for (o = boundary; o < pos; o += BPS(bs)) {
+                rc = _fat_block_get(&b, bs, nodep->idx->devmap_handle, mcl,
+                    NULL, (o - boundary) / BPS(bs), BLOCK_FLAGS_NOREAD);
+                if (rc != EOK)
+                        return rc;
+                memset(b->data, 0, min(BPS(bs), pos - o));
                 b->dirty = true;                /* need to sync node */
-                block_put(b);
-        }
+                rc = block_put(b);
+                if (rc != EOK)
+                        return rc;
+        }
+
+        return EOK;
 }
 
@@ -210 +297 @@
  *
  * @param bs            Buffer holding the boot sector for the file system.
- * @param dev_handle    Device handle for the file system.
+ * @param devmap_handle Device handle for the file system.
  * @param clst          Cluster which to get.
- *
- * @return              Value found in the cluster.
- */
-fat_cluster_t
-fat_get_cluster(fat_bs_t *bs, dev_handle_t dev_handle, fat_cluster_t clst)
+ * @param value         Output argument holding the value of the cluster.
+ *
+ * @return              EOK or a negative error code.
+ */
+int
+fat_get_cluster(fat_bs_t *bs, devmap_handle_t devmap_handle, unsigned fatno,
+    fat_cluster_t clst, fat_cluster_t *value)
 {
         block_t *b;
-        uint16_t bps;
-        uint16_t rscnt;
-        fat_cluster_t *cp, value;
-
-        bps = uint16_t_le2host(bs->bps);
-        rscnt = uint16_t_le2host(bs->rscnt);
-
-        b = block_get(dev_handle, rscnt + (clst * sizeof(fat_cluster_t)) / bps,
-            BLOCK_FLAGS_NONE);
-        cp = (fat_cluster_t *)b->data + clst % (bps / sizeof(fat_cluster_t));
-        value = uint16_t_le2host(*cp);
-        block_put(b);
+        fat_cluster_t *cp;
+        int rc;
+
+        rc = block_get(&b, devmap_handle, RSCNT(bs) + SF(bs) * fatno +
+            (clst * sizeof(fat_cluster_t)) / BPS(bs), BLOCK_FLAGS_NONE);
+        if (rc != EOK)
+                return rc;
+        cp = (fat_cluster_t *)b->data +
+            clst % (BPS(bs) / sizeof(fat_cluster_t));
+        *value = uint16_t_le2host(*cp);
+        rc = block_put(b);
         
-        return value;
+        return rc;
 }
 
@@ -238 +326 @@
  *
  * @param bs            Buffer holding the boot sector for the file system.
- * @param dev_handle    Device handle for the file system.
+ * @param devmap_handle Device handle for the file system.
  * @param fatno         Number of the FAT instance where to make the change.
  * @param clst          Cluster which is to be set.
  * @param value         Value to set the cluster with.
- */
-void
-fat_set_cluster(fat_bs_t *bs, dev_handle_t dev_handle, unsigned fatno,
+ *
+ * @return              EOK on success or a negative error code.
+ */
+int
+fat_set_cluster(fat_bs_t *bs, devmap_handle_t devmap_handle, unsigned fatno,
     fat_cluster_t clst, fat_cluster_t value)
 {
         block_t *b;
-        uint16_t bps;
-        uint16_t rscnt;
-        uint16_t sf;
         fat_cluster_t *cp;
-
-        bps = uint16_t_le2host(bs->bps);
-        rscnt = uint16_t_le2host(bs->rscnt);
-        sf = uint16_t_le2host(bs->sec_per_fat);
-
-        assert(fatno < bs->fatcnt);
-        b = block_get(dev_handle, rscnt + sf * fatno +
-            (clst * sizeof(fat_cluster_t)) / bps, BLOCK_FLAGS_NONE);
-        cp = (fat_cluster_t *)b->data + clst % (bps / sizeof(fat_cluster_t));
+        int rc;
+
+        assert(fatno < FATCNT(bs));
+        rc = block_get(&b, devmap_handle, RSCNT(bs) + SF(bs) * fatno +
+            (clst * sizeof(fat_cluster_t)) / BPS(bs), BLOCK_FLAGS_NONE);
+        if (rc != EOK)
+                return rc;
+        cp = (fat_cluster_t *)b->data +
+            clst % (BPS(bs) / sizeof(fat_cluster_t));
         *cp = host2uint16_t_le(value);
         b->dirty = true;                /* need to sync block */
-        block_put(b);
+        rc = block_put(b);
+        return rc;
 }
 
@@ -269 +357 @@
  *
  * @param bs            Buffer holding the boot sector of the file system.
- * @param dev_handle    Device handle of the file system.
+ * @param devmap_handle Device handle of the file system.
  * @param lifo          Chain of allocated clusters.
  * @param nclsts        Number of clusters in the lifo chain.
- */
-void fat_alloc_shadow_clusters(fat_bs_t *bs, dev_handle_t dev_handle,
+ *
+ * @return              EOK on success or a negative error code.
+ */
+int fat_alloc_shadow_clusters(fat_bs_t *bs, devmap_handle_t devmap_handle,
     fat_cluster_t *lifo, unsigned nclsts)
 {
         uint8_t fatno;
         unsigned c;
+        int rc;
 
         for (fatno = FAT1 + 1; fatno < bs->fatcnt; fatno++) {
                 for (c = 0; c < nclsts; c++) {
-                        fat_set_cluster(bs, dev_handle, fatno, lifo[c],
+                        rc = fat_set_cluster(bs, devmap_handle, fatno, lifo[c],
                             c == 0 ? FAT_CLST_LAST1 : lifo[c - 1]);
+                        if (rc != EOK)
+                                return rc;
                 }
         }
+
+        return EOK;
 }
 
@@ -295 +390 @@
  *
  * @param bs            Buffer holding the boot sector of the file system.
- * @param dev_handle    Device handle of the file system.
+ * @param devmap_handle Device handle of the file system.
  * @param nclsts        Number of clusters to allocate.
  * @param mcl           Output parameter where the first cluster in the chain
@@ -305 +400 @@
  */
 int
-fat_alloc_clusters(fat_bs_t *bs, dev_handle_t dev_handle, unsigned nclsts,
+fat_alloc_clusters(fat_bs_t *bs, devmap_handle_t devmap_handle, unsigned nclsts,
     fat_cluster_t *mcl, fat_cluster_t *lcl)
 {
-        uint16_t bps;
-        uint16_t rscnt;
-        uint16_t sf;
         block_t *blk;
         fat_cluster_t *lifo;    /* stack for storing free cluster numbers */ 
         unsigned found = 0;     /* top of the free cluster number stack */
         unsigned b, c, cl; 
+        int rc;
 
         lifo = (fat_cluster_t *) malloc(nclsts * sizeof(fat_cluster_t));
         if (!lifo)
                 return ENOMEM;
-        
-        bps = uint16_t_le2host(bs->bps);
-        rscnt = uint16_t_le2host(bs->rscnt);
-        sf = uint16_t_le2host(bs->sec_per_fat);
         
         /*
@@ -328 +417 @@
          */
         fibril_mutex_lock(&fat_alloc_lock);
-        for (b = 0, cl = 0; b < sf; b++) {
-                blk = block_get(dev_handle, rscnt + b, BLOCK_FLAGS_NONE);
-                for (c = 0; c < bps / sizeof(fat_cluster_t); c++, cl++) {
+        for (b = 0, cl = 0; b < SF(bs); b++) {
+                rc = block_get(&blk, devmap_handle, RSCNT(bs) + b,
+                    BLOCK_FLAGS_NONE);
+                if (rc != EOK)
+                        goto error;
+                for (c = 0; c < BPS(bs) / sizeof(fat_cluster_t); c++, cl++) {
+                        /*
+                         * Check if the entire cluster is physically there.
+                         * This check becomes necessary when the file system is
+                         * created with fewer total sectors than how many is
+                         * inferred from the size of the file allocation table
+                         * or when the last cluster ends beyond the end of the
+                         * device.
+                         */
+                        if ((cl >= FAT_CLST_FIRST) &&
+                            CLBN2PBN(bs, cl, SPC(bs) - 1) >= TS(bs)) {
+                                rc = block_put(blk);
+                                if (rc != EOK)
+                                        goto error;
+                                goto out;
+                        }
+
                         fat_cluster_t *clst = (fat_cluster_t *)blk->data + c;
                         if (uint16_t_le2host(*clst) == FAT_CLST_RES0) {
@@ -344 +452 @@
                                 if (++found == nclsts) {
                                         /* we are almost done */
-                                        block_put(blk);
+                                        rc = block_put(blk);
+                                        if (rc != EOK)
+                                                goto error;
                                         /* update the shadow copies of FAT */
-                                        fat_alloc_shadow_clusters(bs,
-                                            dev_handle, lifo, nclsts);
+                                        rc = fat_alloc_shadow_clusters(bs,
+                                            devmap_handle, lifo, nclsts);
+                                        if (rc != EOK)
+                                                goto error;
                                         *mcl = lifo[found - 1];
                                         *lcl = lifo[0];
@@ -356 +468 @@
                         }
                 }
-                block_put(blk);
-        }
+                rc = block_put(blk);
+                if (rc != EOK) {
+error:
+                        fibril_mutex_unlock(&fat_alloc_lock);
+                        free(lifo);
+                        return rc;
+                }
+        }
+out:
         fibril_mutex_unlock(&fat_alloc_lock);
 
@@ -365 +484 @@
          */
         while (found--) {
-                fat_set_cluster(bs, dev_handle, FAT1, lifo[found],
+                rc = fat_set_cluster(bs, devmap_handle, FAT1, lifo[found],
                     FAT_CLST_RES0);
+                if (rc != EOK) {
+                        free(lifo);
+                        return rc;
+                }
         }
         
@@ -376 +499 @@
  *
  * @param bs            Buffer hodling the boot sector of the file system.
- * @param dev_handle    Device handle of the file system.
+ * @param devmap_handle Device handle of the file system.
  * @param firstc        First cluster in the chain which is to be freed.
- */
-void
-fat_free_clusters(fat_bs_t *bs, dev_handle_t dev_handle, fat_cluster_t firstc)
+ *
+ * @return              EOK on success or a negative return code.
+ */
+int
+fat_free_clusters(fat_bs_t *bs, devmap_handle_t devmap_handle, fat_cluster_t firstc)
 {
         unsigned fatno;
         fat_cluster_t nextc;
+        int rc;
 
         /* Mark all clusters in the chain as free in all copies of FAT. */
         while (firstc < FAT_CLST_LAST1) {
                 assert(firstc >= FAT_CLST_FIRST && firstc < FAT_CLST_BAD);
-                nextc = fat_get_cluster(bs, dev_handle, firstc);
-                for (fatno = FAT1; fatno < bs->fatcnt; fatno++)
-                        fat_set_cluster(bs, dev_handle, fatno, firstc,
+                rc = fat_get_cluster(bs, devmap_handle, FAT1, firstc, &nextc);
+                if (rc != EOK)
+                        return rc;
+                for (fatno = FAT1; fatno < bs->fatcnt; fatno++) {
+                        rc = fat_set_cluster(bs, devmap_handle, fatno, firstc,
                             FAT_CLST_RES0);
+                        if (rc != EOK)
+                                return rc;
+                }
+
                 firstc = nextc;
         }
+
+        return EOK;
 }
 
@@ -401 +535 @@
  * @param nodep         Node representing the file.
  * @param mcl           First cluster of the cluster chain to append.
- */
-void fat_append_clusters(fat_bs_t *bs, fat_node_t *nodep, fat_cluster_t mcl)
-{
-        dev_handle_t dev_handle = nodep->idx->dev_handle;
-        fat_cluster_t lcl;
+ * @param lcl           Last cluster of the cluster chain to append.
+ *
+ * @return              EOK on success or a negative error code.
+ */
+int
+fat_append_clusters(fat_bs_t *bs, fat_node_t *nodep, fat_cluster_t mcl,
+    fat_cluster_t lcl)
+{
+        devmap_handle_t devmap_handle = nodep->idx->devmap_handle;
+        fat_cluster_t lastc;
         uint8_t fatno;
-
-        if (fat_cluster_walk(bs, dev_handle, nodep->firstc, &lcl,
-            (uint16_t) -1) == 0) {
+        int rc;
+
+        if (nodep->firstc == FAT_CLST_RES0) {
                 /* No clusters allocated to the node yet. */
                 nodep->firstc = mcl;
-                nodep->dirty = true;            /* need to sync node */
-                return;
-        }
-
-        for (fatno = FAT1; fatno < bs->fatcnt; fatno++)
-                fat_set_cluster(bs, nodep->idx->dev_handle, fatno, lcl, mcl);
+                nodep->dirty = true;    /* need to sync node */
+        } else {
+                if (nodep->lastc_cached_valid) {
+                        lastc = nodep->lastc_cached_value;
+                        nodep->lastc_cached_valid = false;
+                } else {
+                        rc = fat_cluster_walk(bs, devmap_handle, nodep->firstc,
+                            &lastc, NULL, (uint16_t) -1);
+                        if (rc != EOK)
+                                return rc;
+                }
+
+                for (fatno = FAT1; fatno < bs->fatcnt; fatno++) {
+                        rc = fat_set_cluster(bs, nodep->idx->devmap_handle, fatno,
+                            lastc, mcl);
+                        if (rc != EOK)
+                                return rc;
+                }
+        }
+
+        nodep->lastc_cached_valid = true;
+        nodep->lastc_cached_value = lcl;
+
+        return EOK;
 }
 
@@ -424 +581 @@
  * @param bs            Buffer holding the boot sector of the file system.
  * @param nodep         FAT node where the chopping will take place.
- * @param lastc         Last cluster which will remain in the node. If this
+ * @param lcl           Last cluster which will remain in the node. If this
  *                      argument is FAT_CLST_RES0, then all clusters will
  *                      be chopped off.
- */
-void fat_chop_clusters(fat_bs_t *bs, fat_node_t *nodep, fat_cluster_t lastc)
-{
-        dev_handle_t dev_handle = nodep->idx->dev_handle;
-        if (lastc == FAT_CLST_RES0) {
+ *
+ * @return              EOK on success or a negative return code.
+ */
+int fat_chop_clusters(fat_bs_t *bs, fat_node_t *nodep, fat_cluster_t lcl)
+{
+        int rc;
+        devmap_handle_t devmap_handle = nodep->idx->devmap_handle;
+
+        /*
+         * Invalidate cached cluster numbers.
+         */
+        nodep->lastc_cached_valid = false;
+        if (nodep->currc_cached_value != lcl)
+                nodep->currc_cached_valid = false;
+
+        if (lcl == FAT_CLST_RES0) {
                 /* The node will have zero size and no clusters allocated. */
-                fat_free_clusters(bs, dev_handle, nodep->firstc);
+                rc = fat_free_clusters(bs, devmap_handle, nodep->firstc);
+                if (rc != EOK)
+                        return rc;
                 nodep->firstc = FAT_CLST_RES0;
                 nodep->dirty = true;            /* need to sync node */
@@ -440 +610 @@
                 unsigned fatno;
 
-                nextc = fat_get_cluster(bs, dev_handle, lastc);
+                rc = fat_get_cluster(bs, devmap_handle, FAT1, lcl, &nextc);
+                if (rc != EOK)
+                        return rc;
 
                 /* Terminate the cluster chain in all copies of FAT. */
-                for (fatno = FAT1; fatno < bs->fatcnt; fatno++)
-                        fat_set_cluster(bs, dev_handle, fatno, lastc, FAT_CLST_LAST1);
+                for (fatno = FAT1; fatno < bs->fatcnt; fatno++) {
+                        rc = fat_set_cluster(bs, devmap_handle, fatno, lcl,
+                            FAT_CLST_LAST1);
+                        if (rc != EOK)
+                                return rc;
+                }
 
                 /* Free all following clusters. */
-                fat_free_clusters(bs, dev_handle, nextc);
-        }
-}
-
-void
-fat_zero_cluster(struct fat_bs *bs, dev_handle_t dev_handle, fat_cluster_t c)
+                rc = fat_free_clusters(bs, devmap_handle, nextc);
+                if (rc != EOK)
+                        return rc;
+        }
+
+        /*
+         * Update and re-enable the last cluster cache.
+         */
+        nodep->lastc_cached_valid = true;
+        nodep->lastc_cached_value = lcl;
+
+        return EOK;
+}
+
+int
+fat_zero_cluster(struct fat_bs *bs, devmap_handle_t devmap_handle, fat_cluster_t c)
 {
         int i;
         block_t *b;
-        unsigned bps;
-
-        bps = uint16_t_le2host(bs->bps);
-        
-        for (i = 0; i < bs->spc; i++) {
-                b = _fat_block_get(bs, dev_handle, c, i, BLOCK_FLAGS_NOREAD);
-                memset(b->data, 0, bps);
+        int rc;
+
+        for (i = 0; i < SPC(bs); i++) {
+                rc = _fat_block_get(&b, bs, devmap_handle, c, NULL, i,
+                    BLOCK_FLAGS_NOREAD);
+                if (rc != EOK)
+                        return rc;
+                memset(b->data, 0, BPS(bs));
                 b->dirty = true;
-                block_put(b);
-        }
+                rc = block_put(b);
+                if (rc != EOK)
+                        return rc;
+        }
+
+        return EOK;
+}
+
+/** Perform basic sanity checks on the file system.
+ *
+ * Verify if values of boot sector fields are sane. Also verify media
+ * descriptor. This is used to rule out cases when a device obviously
+ * does not contain a fat file system.
+ */
+int fat_sanity_check(fat_bs_t *bs, devmap_handle_t devmap_handle)
+{
+        fat_cluster_t e0, e1;
+        unsigned fat_no;
+        int rc;
+
+        /* Check number of FATs. */
+        if (bs->fatcnt == 0)
+                return ENOTSUP;
+
+        /* Check total number of sectors. */
+
+        if (bs->totsec16 == 0 && bs->totsec32 == 0)
+                return ENOTSUP;
+
+        if (bs->totsec16 != 0 && bs->totsec32 != 0 &&
+            bs->totsec16 != bs->totsec32) 
+                return ENOTSUP;
+
+        /* Check media descriptor. Must be between 0xf0 and 0xff. */
+        if ((bs->mdesc & 0xf0) != 0xf0)
+                return ENOTSUP;
+
+        /* Check number of sectors per FAT. */
+        if (bs->sec_per_fat == 0)
+                return ENOTSUP;
+
+        /*
+         * Check that the root directory entries take up whole blocks.
+         * This check is rather strict, but it allows us to treat the root
+         * directory and non-root directories uniformly in some places.
+         * It can be removed provided that functions such as fat_read() are
+         * sanitized to support file systems with this property.
+         */
+        if ((uint16_t_le2host(bs->root_ent_max) * sizeof(fat_dentry_t)) %
+            uint16_t_le2host(bs->bps) != 0)
+                return ENOTSUP;
+
+        /* Check signature of each FAT. */
+
+        for (fat_no = 0; fat_no < bs->fatcnt; fat_no++) {
+                rc = fat_get_cluster(bs, devmap_handle, fat_no, 0, &e0);
+                if (rc != EOK)
+                        return EIO;
+
+                rc = fat_get_cluster(bs, devmap_handle, fat_no, 1, &e1);
+                if (rc != EOK)
+                        return EIO;
+
+                /* Check that first byte of FAT contains the media descriptor. */
+                if ((e0 & 0xff) != bs->mdesc)
+                        return ENOTSUP;
+
+                /*
+                 * Check that remaining bits of the first two entries are
+                 * set to one.
+                 */
+                if ((e0 >> 8) != 0xff || e1 != 0xffff)
+                        return ENOTSUP;
+        }
+
+        return EOK;
 }