ext4: correctly handle queries for metadata mappings

Currently, our handling of metadata is _ambiguous_ in some scenarios,
that is, we end up returning unknown if the range only covers the
mapping partially.

For example, in the following case:

$ xfs_io -c fsmap -d

  0: 254:16 [0..7]: static fs metadata 8
  1: 254:16 [8..15]: special 102:1 8
  2: 254:16 [16..5127]: special 102:2 5112
  3: 254:16 [5128..5255]: special 102:3 128
  4: 254:16 [5256..5383]: special 102:4 128
  5: 254:16 [5384..70919]: inodes 65536
  6: 254:16 [70920..70967]: unknown 48
  ...

$ xfs_io -c fsmap -d 24 33

  0: 254:16 [24..39]: unknown 16  <--- incomplete reporting

$ xfs_io -c fsmap -d 24 33  (With patch)

    0: 254:16 [16..5127]: special 102:2 5112

This is because earlier in ext4_getfsmap_meta_helper, we end up ignoring
any extent that starts before our queried range, but overlaps it. While
the man page [1] is a bit ambiguous on this, this fix makes the output
make more sense since we are anyways returning an "unknown" extent. This
is also consistent to how XFS does it:

$ xfs_io -c fsmap -d

  ...
  6: 254:16 [104..127]: free space 24
  7: 254:16 [128..191]: inodes 64
  ...

$ xfs_io -c fsmap -d 137 150

  0: 254:16 [128..191]: inodes 64   <-- full extent returned

 [1] https://man7.org/linux/man-pages/man2/ioctl_getfsmap.2.html

Reported-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Message-ID: <023f37e35ee280cd9baac0296cbadcbe10995cab.1757058211.git.ojaswin@linux.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: OjaswinM

fs/ext4/fsmap.c

@@ -74,7 +74,8 @@ static int ext4_getfsmap_dev_compare(const void *p1, const void *p2)
 static bool ext4_getfsmap_rec_before_low_key(struct ext4_getfsmap_info *info,
 					     struct ext4_fsmap *rec)
 {
-	return rec->fmr_physical < info->gfi_low.fmr_physical;
+	return rec->fmr_physical + rec->fmr_length <=
+	       info->gfi_low.fmr_physical;
 }
 
 /*
@@ -200,15 +201,18 @@ static int ext4_getfsmap_meta_helper(struct super_block *sb,
 			  ext4_group_first_block_no(sb, agno));
 	fs_end = fs_start + EXT4_C2B(sbi, len);
 
-	/* Return relevant extents from the meta_list */
+	/*
+	 * Return relevant extents from the meta_list. We emit all extents that
+	 * partially/fully overlap with the query range
+	 */
 	list_for_each_entry_safe(p, tmp, &info->gfi_meta_list, fmr_list) {
-		if (p->fmr_physical < info->gfi_next_fsblk) {
+		if (p->fmr_physical + p->fmr_length <= info->gfi_next_fsblk) {
 			list_del(&p->fmr_list);
 			kfree(p);
 			continue;
 		}
-		if (p->fmr_physical <= fs_start ||
-		    p->fmr_physical + p->fmr_length <= fs_end) {
+		if (p->fmr_physical <= fs_end &&
+		    p->fmr_physical + p->fmr_length > fs_start) {
 			/* Emit the retained free extent record if present */
 			if (info->gfi_lastfree.fmr_owner) {
 				error = ext4_getfsmap_helper(sb, info,