erofs-utils: lib: check NULL from erofs_rebuild_get_dentry()

erofs_rebuild_get_dentry() returns NULL when the input path
normalizes to nothing (".", "/", "//", or paths that collapse via
".."). The tar hardlink branch and the S3 import loop only check
IS_ERR() and then dereference the result.

Reject a hardlink target that resolves to root with -EISDIR, and
treat a root-normalized S3 key as the root inode itself.

Fixes: 95d315fd7958 ("erofs-utils: introduce tarerofs")
Fixes: 29728ba8f6f6 ("erofs-utils: mkfs: support EROFS meta-only image generation from S3")
Signed-off-by: Vansh Choudhary <ch@vnsh.in>
Link: https://lore.kernel.org/r/20260420174752.50132-1-ch@vnsh.in
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

lib/remotes/s3.c

diff --git a/lib/remotes/s3.c b/lib/remotes/s3.c
index e552ed0..d386a32 100644
--- a/lib/remotes/s3.c
+++ b/lib/remotes/s3.c
@@ -1129,7 +1129,10 @@
 			ret = PTR_ERR(d);
 			goto err_iter;
 		}
-		if (d->type == EROFS_FT_DIR) {
+		if (!d) {
+			inode = root;
+			inode->i_mode = S_IFDIR | 0755;
+		} else if (d->type == EROFS_FT_DIR) {
 			inode = d->inode;
 			inode->i_mode = S_IFDIR | 0755;
 		} else {

lib/tar.c

diff --git a/lib/tar.c b/lib/tar.c
index 3379549..16e9c22 100644
--- a/lib/tar.c
+++ b/lib/tar.c
@@ -1043,6 +1043,10 @@
 			ret = PTR_ERR(d2);
 			goto out;
 		}
+		if (!d2) {
+			ret = -EISDIR;
+			goto out;
+		}
 		if (d2->type == EROFS_FT_UNKNOWN) {
 			ret = -ENOENT;
 			goto out;