1 files changed, 27 insertions, 11 deletions

diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index f3f8d8c066e4..937a215f626c 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -174,15 +174,15 @@ static int erofs_init_device(struct erofs_buf *buf, struct super_block *sb,
 		if (!erofs_is_fileio_mode(sbi)) {
 			dif->dax_dev = fs_dax_get_by_bdev(file_bdev(file),
 					&dif->dax_part_off, NULL, NULL);
-			if (!dif->dax_dev && test_opt(&sbi->opt, DAX_ALWAYS)) {
-				erofs_info(sb, "DAX unsupported by %s. Turning off DAX.",
-					   dif->path);
-				clear_opt(&sbi->opt, DAX_ALWAYS);
-			}
 		} else if (!S_ISREG(file_inode(file)->i_mode)) {
 			fput(file);
 			return -EINVAL;
 		}
+		if (!dif->dax_dev && test_opt(&sbi->opt, DAX_ALWAYS)) {
+			erofs_info(sb, "DAX unsupported by %s. Turning off DAX.",
+				   dif->path);
+			clear_opt(&sbi->opt, DAX_ALWAYS);
+		}
 		dif->file = file;
 	}
 
@@ -215,13 +215,13 @@ static int erofs_scan_devices(struct super_block *sb,
 			  ondisk_extradevs, sbi->devs->extra_devices);
 		return -EINVAL;
 	}
-	if (!ondisk_extradevs) {
-		if (test_opt(&sbi->opt, DAX_ALWAYS) && !sbi->dif0.dax_dev) {
-			erofs_info(sb, "DAX unsupported by block device. Turning off DAX.");
-			clear_opt(&sbi->opt, DAX_ALWAYS);
-		}
-		return 0;
+
+	if (test_opt(&sbi->opt, DAX_ALWAYS) && !sbi->dif0.dax_dev) {
+		erofs_info(sb, "DAX unsupported by block device. Turning off DAX.");
+		clear_opt(&sbi->opt, DAX_ALWAYS);
 	}
+	if (!ondisk_extradevs)
+		return 0;
 
 	if (!sbi->devs->extra_devices && !erofs_is_fscache_mode(sb))
 		sbi->devs->flatdev = true;
@@ -639,6 +639,22 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 
 	sbi->blkszbits = PAGE_SHIFT;
 	if (!sb->s_bdev) {
+		/*
+		 * (File-backed mounts) EROFS claims it's safe to nest other
+		 * fs contexts (including its own) due to self-controlled RO
+		 * accesses/contexts and no side-effect changes that need to
+		 * context save & restore so it can reuse the current thread
+		 * context.  However, it still needs to bump `s_stack_depth` to
+		 * avoid kernel stack overflow from nested filesystems.
+		 */
+		if (erofs_is_fileio_mode(sbi)) {
+			sb->s_stack_depth =
+				file_inode(sbi->dif0.file)->i_sb->s_stack_depth + 1;
+			if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
+				erofs_err(sb, "maximum fs stacking depth exceeded");
+				return -ENOTBLK;
+			}
+		}
 		sb->s_blocksize = PAGE_SIZE;
 		sb->s_blocksize_bits = PAGE_SHIFT;