From: Mikhail Malyshev <[email protected]>
mkfs.erofs --tar fails to convert layers that store files underneath a
symlinked directory, a very common "usr-merge" layout in RHEL/UBI/Fedora
based OCI images. For example a base layer contains
lib64 -> usr/lib64 (symlink)
usr/lib64/ (directory)
lib64/libc.so.6 (regular file, path goes THROUGH the symlink)
erofs_rebuild_get_dentry() walks the path component by component and, when
it hits the "lib64" component, finds an existing dentry that is a symlink
rather than a directory. Since there are still components left to resolve
it bails out with -ENOTDIR. In practice this surfaces as a bogus ~2 TiB
image and "Could not format the device", making the whole layer
unconvertible even though nothing is wrong with the tar.
A real filesystem (and containerd's walking differ, which applies the same
layers via fs.RootPath) resolves such a path by following the symlink to
its target directory. Do the same during tar rebuild: when an intermediate
path component resolves to a symlink, follow it to the target directory
(relative targets from the containing directory, absolute targets from the
tree root), chasing chained symlinks up to a bounded depth to avoid loops.
Signed-off-by: Mikhail Malyshev <[email protected]>
---
lib/rebuild.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 70 insertions(+), 2 deletions(-)
diff --git a/lib/rebuild.c b/lib/rebuild.c
index 108a464..febc95c 100644
--- a/lib/rebuild.c
+++ b/lib/rebuild.c
@@ -82,8 +82,60 @@ struct erofs_dentry *erofs_d_lookup(struct erofs_inode *dir,
const char *name)
return NULL;
}
-struct erofs_dentry *erofs_rebuild_get_dentry(struct erofs_inode *pwd,
- char *path, bool aufs, bool *whout, bool *opq, bool to_head)
+/* Bound symlink chains during path resolution, like the kernel's MAXSYMLINKS.
*/
+#define EROFS_REBUILD_SYMLINK_DEPTH 40
+
+static struct erofs_dentry *__erofs_rebuild_get_dentry(struct erofs_inode
*root,
+ struct erofs_inode *pwd, char *path, bool aufs,
+ bool *whout, bool *opq, bool to_head, unsigned int depth);
+
+/*
+ * Resolve the target of an intermediate symlink path component to its
+ * directory inode, following chained symlinks up to a bounded depth. This
+ * lets tar layers that store files under a symlinked directory be converted
+ * the same way a real filesystem extracts them, e.g. usr-merge layouts where
+ * "/lib64 -> usr/lib64" coexists with entries such as "lib64/libc.so.6".
+ */
+static struct erofs_inode *erofs_rebuild_follow_link(struct erofs_inode *root,
+ struct erofs_inode *pwd, struct erofs_inode *link,
+ unsigned int depth)
+{
+ struct erofs_dentry *d;
+ struct erofs_inode *base;
+ bool dumb_wh, dumb_opq;
+ char *target;
+
+ if (depth >= EROFS_REBUILD_SYMLINK_DEPTH)
+ return ERR_PTR(-ELOOP);
+ if (!link->i_link)
+ return ERR_PTR(-ENOENT);
+
+ target = strdup(link->i_link);
+ if (!target)
+ return ERR_PTR(-ENOMEM);
+
+ /* absolute targets are resolved from the tree root */
+ base = target[0] == '/' ? root : pwd;
+ d = __erofs_rebuild_get_dentry(root, base, target, false,
+ &dumb_wh, &dumb_opq, false, depth + 1);
+ free(target);
+ if (IS_ERR(d))
+ return ERR_CAST(d);
+ /* target referred to the root or the base directory itself ("/", ".")
*/
+ if (!d)
+ return base;
+ if (d->type == EROFS_FT_DIR)
+ return d->inode;
+ /* the target is itself a symlink: keep following the chain */
+ if (d->type == EROFS_FT_SYMLINK)
+ return erofs_rebuild_follow_link(root, d->inode->i_parent,
+ d->inode, depth + 1);
+ return ERR_PTR(-ENOTDIR);
+}
+
+static struct erofs_dentry *__erofs_rebuild_get_dentry(struct erofs_inode
*root,
+ struct erofs_inode *pwd, char *path, bool aufs,
+ bool *whout, bool *opq, bool to_head, unsigned int depth)
{
struct erofs_dentry *d = NULL;
char *s = path;
@@ -121,6 +173,15 @@ struct erofs_dentry *erofs_rebuild_get_dentry(struct
erofs_inode *pwd,
}
d = erofs_d_lookup(pwd, s);
+ if (d && slash && d->type == EROFS_FT_SYMLINK) {
+ pwd = erofs_rebuild_follow_link(root, pwd,
+ d->inode, depth);
+ if (IS_ERR(pwd))
+ return ERR_CAST(pwd);
+ *slash = '/';
+ s = slash + 1;
+ continue;
+ }
if (d) {
if (d->type != EROFS_FT_DIR) {
if (slash)
@@ -152,6 +213,13 @@ struct erofs_dentry *erofs_rebuild_get_dentry(struct
erofs_inode *pwd,
return d;
}
+struct erofs_dentry *erofs_rebuild_get_dentry(struct erofs_inode *pwd,
+ char *path, bool aufs, bool *whout, bool *opq, bool to_head)
+{
+ return __erofs_rebuild_get_dentry(pwd, pwd, path, aufs, whout, opq,
+ to_head, 0);
+}
+
static int erofs_rebuild_write_blob_index(struct erofs_sb_info *dst_sb,
struct erofs_inode *inode)
{
--
2.43.0
