Correct size detection for special block devices

When writing directly to a special block device instead of a regular
file image, the size reported by fstat(2) returns 0, which is incorrect.
This commit introduces a check to identify if the target file is a
special block device. If it is, the commit uses the ioctl(2) to
accurately determine the size of the block device.

mkfs.c

@@ -28,13 +28,13 @@ _Static_assert(sizeof(struct superblock) == SIMPLEFS_BLOCK_SIZE);
  */
 #define DIV_ROUND_UP(n, d) (((n) + (d) -1) / (d))
 
-static struct superblock *write_superblock(int fd, struct stat *fstats)
+static struct superblock *write_superblock(int fd, int64_t partition_size)
 {
     struct superblock *sb = malloc(sizeof(struct superblock));
     if (!sb)
         return NULL;
 
-    uint32_t nr_blocks = fstats->st_size / SIMPLEFS_BLOCK_SIZE;
+    uint32_t nr_blocks = partition_size / SIMPLEFS_BLOCK_SIZE;
     uint32_t nr_inodes = nr_blocks;
     uint32_t mod = nr_inodes % SIMPLEFS_INODES_PER_BLOCK;
     if (mod)
@@ -246,14 +246,14 @@ int main(int argc, char **argv)
         return EXIT_FAILURE;
     }
 
-    /* Open disk image */
+    /* Open disk partition */
     int fd = open(argv[1], O_RDWR);
     if (fd == -1) {
         perror("open():");
         return EXIT_FAILURE;
     }
 
-    /* Get image size */
+    /* Get partition size */
     struct stat stat_buf;
     int ret = fstat(fd, &stat_buf);
     if (ret) {
@@ -263,28 +263,31 @@ int main(int argc, char **argv)
     }
 
     /* Get block device size */
-    if ((stat_buf.st_mode & S_IFMT) == S_IFBLK) {
-        long int blk_size = 0;
-        ret = ioctl(fd, BLKGETSIZE64, &blk_size);
+    uint64_t partition_size = 0;
+    if (S_ISBLK(stat_buf.st_mode)) {
+        /* a block device */
+        ret = ioctl(fd, BLKGETSIZE64, partition_size);
         if (ret != 0) {
             perror("BLKGETSIZE64:");
             ret = EXIT_FAILURE;
             goto fclose;
         }
-        stat_buf.st_size = blk_size;
+    } else {
+        /* a regular file */
+        partition_size = stat_buf.st_size;
     }
 
-    /* Verify if the file system image has sufficient size. */
+    /* Verify if the partition has sufficient size. */
     long int min_size = 100 * SIMPLEFS_BLOCK_SIZE;
-    if (stat_buf.st_size < min_size) {
+    if (partition_size <= min_size) {
         fprintf(stderr, "File is not large enough (size=%ld, min size=%ld)\n",
                 stat_buf.st_size, min_size);
         ret = EXIT_FAILURE;
         goto fclose;
     }
 
     /* Write superblock (block 0) */
-    struct superblock *sb = write_superblock(fd, &stat_buf);
+    struct superblock *sb = write_superblock(fd, partition_size);
     if (!sb) {
         perror("write_superblock():");
         ret = EXIT_FAILURE;