ocfs2: validate extent block list fields during block read

Add extent list validation to ocfs2_validate_extent_block() so that
corrupted on-disk fields are caught early at block read time rather than
during extent tree traversal.

Two checks are added:

  - l_count must equal the expected value from
    ocfs2_extent_recs_per_eb(), catching blocks with a corrupted record
    count before any array iteration.

  - l_next_free_rec must not exceed l_count, preventing out-of-bounds
    access when iterating over extent records.

Link: https://lkml.kernel.org/r/20260403090803.3860971-4-joseph.qi@linux.alibaba.com
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: Heming Zhao <heming.zhao@suse.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Jun Piao <piaojun@huawei.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Mark Fasheh <mark@fasheh.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: josephhz

fs/ocfs2/alloc.c

@@ -917,11 +917,32 @@ static int ocfs2_validate_extent_block(struct super_block *sb,
 		goto bail;
 	}
 
-	if (le32_to_cpu(eb->h_fs_generation) != OCFS2_SB(sb)->fs_generation)
+	if (le32_to_cpu(eb->h_fs_generation) != OCFS2_SB(sb)->fs_generation) {
 		rc = ocfs2_error(sb,
 				 "Extent block #%llu has an invalid h_fs_generation of #%u\n",
 				 (unsigned long long)bh->b_blocknr,
 				 le32_to_cpu(eb->h_fs_generation));
+		goto bail;
+	}
+
+	if (le16_to_cpu(eb->h_list.l_count) != ocfs2_extent_recs_per_eb(sb)) {
+		rc = ocfs2_error(sb,
+				 "Extent block #%llu has invalid l_count %u (expected %u)\n",
+				 (unsigned long long)bh->b_blocknr,
+				 le16_to_cpu(eb->h_list.l_count),
+				 ocfs2_extent_recs_per_eb(sb));
+		goto bail;
+	}
+
+	if (le16_to_cpu(eb->h_list.l_next_free_rec) > le16_to_cpu(eb->h_list.l_count)) {
+		rc = ocfs2_error(sb,
+				 "Extent block #%llu has invalid l_next_free_rec %u (l_count %u)\n",
+				 (unsigned long long)bh->b_blocknr,
+				 le16_to_cpu(eb->h_list.l_next_free_rec),
+				 le16_to_cpu(eb->h_list.l_count));
+		goto bail;
+	}
+
 bail:
 	return rc;
 }