fs_mgr: Support filesystem labels in fstab

When mounting fstab entries it is practical to be able to specify
for example LABEL=data instead of a specific block device.

This is particularly important for the new Android Emulator code base,
which uses virtio block devices for the various partitions
(system,data,cache), because there is no defined ordering between the
way the filesystems are specified to the emulator and how they are
enumerated by the android kernel as /dev/vdX.

Change-Id: I7aef95f12e8f7b02ac2e33804ba7897fdcb9ad7f
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>

1 files changed, 91 insertions, 0 deletions

diff --git a/fs_mgr/fs_mgr.c b/fs_mgr/fs_mgr.c
index 40878c1..d6edd29 100644
--- a/fs_mgr/fs_mgr.c
+++ b/fs_mgr/fs_mgr.c
@@ -28,6 +28,9 @@
 #include <libgen.h>
 #include <time.h>
 #include <sys/swap.h>
+#include <dirent.h>
+#include <ext4.h>
+#include <ext4_sb.h>
 
 #include <linux/loop.h>
 #include <private/android_filesystem_config.h>
@@ -325,6 +328,83 @@ static int mount_with_alternatives(struct fstab *fstab, int start_idx, int *end_
     return 0;
 }
 
+static int translate_ext_labels(struct fstab_rec *rec)
+{
+    DIR *blockdir = NULL;
+    struct dirent *ent;
+    char *label;
+    size_t label_len;
+    int ret = -1;
+
+    if (strncmp(rec->blk_device, "LABEL=", 6))
+        return 0;
+
+    label = rec->blk_device + 6;
+    label_len = strlen(label);
+
+    if (label_len > 16) {
+        ERROR("FS label is longer than allowed by filesystem\n");
+        goto out;
+    }
+
+
+    blockdir = opendir("/dev/block");
+    if (!blockdir) {
+        ERROR("couldn't open /dev/block\n");
+        goto out;
+    }
+
+    while ((ent = readdir(blockdir))) {
+        int fd;
+        char super_buf[1024];
+        struct ext4_super_block *sb;
+
+        if (ent->d_type != DT_BLK)
+            continue;
+
+        fd = openat(dirfd(blockdir), ent->d_name, O_RDONLY);
+        if (fd < 0) {
+            ERROR("Cannot open block device /dev/block/%s\n", ent->d_name);
+            goto out;
+        }
+
+        if (TEMP_FAILURE_RETRY(lseek(fd, 1024, SEEK_SET)) < 0 ||
+            TEMP_FAILURE_RETRY(read(fd, super_buf, 1024)) != 1024) {
+            /* Probably a loopback device or something else without a readable
+             * superblock.
+             */
+            close(fd);
+            continue;
+        }
+
+        sb = (struct ext4_super_block *)super_buf;
+        if (sb->s_magic != EXT4_SUPER_MAGIC) {
+            INFO("/dev/block/%s not ext{234}\n", ent->d_name);
+            continue;
+        }
+
+        if (!strncmp(label, sb->s_volume_name, label_len)) {
+            char *new_blk_device;
+
+            if (asprintf(&new_blk_device, "/dev/block/%s", ent->d_name) < 0) {
+                ERROR("Could not allocate block device string\n");
+                goto out;
+            }
+
+            INFO("resolved label %s to %s\n", rec->blk_device, new_blk_device);
+
+            free(rec->blk_device);
+            rec->blk_device = new_blk_device;
+            ret = 0;
+            break;
+        }
+    }
+
+out:
+    closedir(blockdir);
+    return ret;
+}
+
 /* When multiple fstab records share the same mount_point, it will
  * try to mount each one in turn, and ignore any duplicates after a
  * first successful mount.
@@ -356,6 +436,17 @@ int fs_mgr_mount_all(struct fstab *fstab)
             continue;
         }
 
+        /* Translate LABEL= file system labels into block devices */
+        if (!strcmp(fstab->recs[i].fs_type, "ext2") ||
+            !strcmp(fstab->recs[i].fs_type, "ext3") ||
+            !strcmp(fstab->recs[i].fs_type, "ext4")) {
+            int tret = translate_ext_labels(&fstab->recs[i]);
+            if (tret < 0) {
+                ERROR("Could not translate label to block device\n");
+                continue;
+            }
+        }
+
         if (fstab->recs[i].fs_mgr_flags & MF_WAIT) {
             wait_for_file(fstab->recs[i].blk_device, WAIT_TIMEOUT);
         }