mkfs.vfat: fix a problem with over-estimating FAT size

diff --git a/util-linux/mkfs_vfat.c b/util-linux/mkfs_vfat.c
index 30996b2677e297f5b11fe40a2bd5eecba744daaa..705c75c20dd1bf142f0e358e2091df0bffdc360b 100644 (file)
--- a/util-linux/mkfs_vfat.c
+++ b/util-linux/mkfs_vfat.c
@@ -327,6 +327,7 @@ int mkfs_vfat_main(int argc UNUSED_PARAM, char **argv)
                                        volume_size_bytes >= ((off_t)8)*1024*1024*1024 ? 16 :
                                        volume_size_bytes >=        260*1024*1024 ? 8 : 1;
                } else {
+                       // floppy, loop, or regular file
                        int not_floppy = ioctl(dev, FDGETPRM, &param);
                        if (not_floppy == 0) {
                                // floppy disk
@@ -367,12 +368,14 @@ int mkfs_vfat_main(int argc UNUSED_PARAM, char **argv)
        // Calculate number of clusters, sectors/cluster, sectors/FAT
        // (an initial guess for sect_per_clust should already be set)
        //
-       if ((off_t)(volume_size_sect - reserved_sect) < 16) // arbitrary limit
+       // "mkdosfs -v -F 32 image5k 5" is the minimum:
+       // 2 sectors for FATs and 2 data sectors
+       if ((off_t)(volume_size_sect - reserved_sect) < 4)
                bb_error_msg_and_die("the image is too small for FAT32");
        sect_per_fat = 1;
        while (1) {
                while (1) {
-                       unsigned spf;
+                       int spf_adj;
                        off_t tcl = (volume_size_sect - reserved_sect - NUM_FATS * sect_per_fat) / sect_per_clust;
                        // tcl may be > MAX_CLUST_32 here, but it may be
                        // because sect_per_fat is underestimated,
@@ -382,8 +385,13 @@ int mkfs_vfat_main(int argc UNUSED_PARAM, char **argv)
                        if (tcl > 0x7fffffff)
                                goto next;
                        total_clust = tcl; // fits in uint32_t
-                       spf = ((total_clust + 2) * 4 + bytes_per_sect - 1) / bytes_per_sect;
-                       if (spf <= sect_per_fat) {
+                       spf_adj = ((total_clust + 2) * 4 + bytes_per_sect - 1) / bytes_per_sect - sect_per_fat;
+#if 0
+                       bb_error_msg("sect_per_clust:%u sect_per_fat:%u total_clust:%u",
+                                       sect_per_clust, sect_per_fat, (int)tcl);
+                       bb_error_msg("adjust to sect_per_fat:%d", spf_adj);
+#endif
+                       if (spf_adj <= 0) {
                                // do not need to adjust sect_per_fat.
                                // so, was total_clust too big after all?
                                if (total_clust <= MAX_CLUST_32)
@@ -392,7 +400,8 @@ int mkfs_vfat_main(int argc UNUSED_PARAM, char **argv)
                                goto next;
                        }
                        // adjust sect_per_fat, go back and recalc total_clust
-                       sect_per_fat = spf;
+                       // (note: just "sect_per_fat += spf_adj" isn't ok)
+                       sect_per_fat += ((unsigned)spf_adj / 2) | 1;
                }
  next:
                if (sect_per_clust == 128)
@@ -409,7 +418,7 @@ int mkfs_vfat_main(int argc UNUSED_PARAM, char **argv)
                fprintf(stderr,
                        "Device '%s':\n"
                        "heads:%u, sectors/track:%u, bytes/sector:%u\n"
-                       "media descriptor:0x%02x\n"
+                       "media descriptor:%02x\n"
                        "total sectors:%"OFF_FMT"u, clusters:%u, sectors/cluster:%u\n"
                        "FATs:2, sectors/FAT:%u\n"
                        "volumeID:%08x, label:'%s'\n",