From: Karel Zak <kzak@redhat.com>
Date: Fri, 6 Dec 2024 11:41:09 +0000 (+0100)
Subject: libmount: create EROFS loopdev only after ENOTBLK
X-Git-Tag: v2.42-start~119^2
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=d20fa0c812107b4c1f9412d7e878b5c23a258c54;p=thirdparty%2Futil-linux.git

libmount: create EROFS loopdev only after ENOTBLK

The EROFS can mount regular files with a filesystem image without the
need for a loop device.

For backward compatibility with previous versions of EROFS, a loop
device will only be created if the first attempt to mount results
ENOTBLK error.

Addresses: https://github.com/util-linux/util-linux/pull/3288
Signed-off-by: Karel Zak <kzak@redhat.com>
---

diff --git a/libmount/src/context_mount.c b/libmount/src/context_mount.c
index af2fd7ed4..c64e735da 100644
--- a/libmount/src/context_mount.c
+++ b/libmount/src/context_mount.c
@@ -962,6 +962,24 @@ int mnt_context_finalize_mount(struct libmnt_context *cxt)
 	return rc;
 }
 
+static int is_erofs_regfile(struct libmnt_context *cxt)
+{
+	const char *type = mnt_fs_get_fstype(cxt->fs);
+	const char *src = mnt_fs_get_srcpath(cxt->fs);
+	unsigned long flags = 0;
+	struct stat st;
+
+	if (!type || strcmp(type, "erofs") != 0)
+		return 0;
+	if (mnt_context_get_user_mflags(cxt, &flags))
+		return 0;
+	if (flags & (MNT_MS_LOOP | MNT_MS_OFFSET | MNT_MS_SIZELIMIT))
+		return 0;	/* it's already loopdev */
+	if (!src || stat(src, &st) != 0 || !S_ISREG(st.st_mode))
+		return 0;
+	return 1;
+}
+
 /**
  * mnt_context_mount:
  * @cxt: mount context
@@ -1066,6 +1084,23 @@ again:
 		}
 	}
 
+	/*
+	 * Try mount EROFS image again with loop device.
+	 * See hook_loopdev.c:is_loopdev_required() for more details.
+	 */
+	if (rc && mnt_context_get_syscall_errno(cxt) == ENOTBLK
+	       && is_erofs_regfile(cxt)) {
+		struct libmnt_optlist *ol = mnt_context_get_optlist(cxt);
+
+		mnt_context_reset_status(cxt);
+		DBG(CXT, ul_debugobj(cxt, "enabling loop= for EROFS"));
+		mnt_optlist_append_flags(ol, MNT_MS_LOOP, cxt->map_userspace);
+
+		rc = mnt_context_call_hooks(cxt, MNT_STAGE_PREP_SOURCE);
+		if (!rc)
+			goto again;
+	}
+
 	if (rc == 0)
 		rc = mnt_context_call_hooks(cxt, MNT_STAGE_POST);
 
diff --git a/libmount/src/hook_loopdev.c b/libmount/src/hook_loopdev.c
index a19ddab3b..65d0f739e 100644
--- a/libmount/src/hook_loopdev.c
+++ b/libmount/src/hook_loopdev.c
@@ -413,6 +413,7 @@ static int is_loopdev_required(struct libmnt_context *cxt, struct libmnt_optlist
 {
 	const char *src, *type;
 	unsigned long flags = 0;
+	struct stat st;
 
 	if (cxt->action != MNT_ACT_MOUNT)
 		return 0;
@@ -434,37 +435,55 @@ static int is_loopdev_required(struct libmnt_context *cxt, struct libmnt_optlist
 	if (mnt_context_get_user_mflags(cxt, &flags))
 		return 0;
 
+	/* loop= (sizelimit= or offset=) explicitly specified */
 	if (flags & (MNT_MS_LOOP | MNT_MS_OFFSET | MNT_MS_SIZELIMIT)) {
 		DBG(LOOP, ul_debugobj(cxt, "loopdev specific options detected"));
 		return 1;
 	}
 
-	/* Automatically create a loop device from a regular file if a
-	 * filesystem is not specified or the filesystem is known for libblkid
-	 * (these filesystems work with block devices only). The file size
-	 * should be at least 1KiB, otherwise we will create an empty loopdev with
-	 * no mountable filesystem...
-	 *
-	 * Note that there is no restriction (on kernel side) that would prevent a regular
-	 * file as a mount(2) source argument. A filesystem that is able to mount
-	 * regular files could be implemented.
+	/* Automatically create a loop device from a regular file. The file
+	 * size should be at least 1KiB, otherwise we will create an empty
+	 * loop device with no mountable filesystem.
 	 */
-	type = mnt_fs_get_fstype(cxt->fs);
-
-	if (mnt_fs_is_regularfs(cxt->fs) &&
-	    (!type || strcmp(type, "auto") == 0 || blkid_known_fstype(type))) {
-		struct stat st;
+	if (stat(src, &st) != 0 || !S_ISREG(st.st_mode))
+		return 0;
+	if (st.st_size <= 1024)
+		return 0;
 
-		if (stat(src, &st) == 0 && S_ISREG(st.st_mode) &&
-		    st.st_size > 1024) {
+	type = mnt_fs_get_fstype(cxt->fs);
+	if (!type || strcmp(type, "auto") == 0) {
+		char *autotype = NULL;
+		int rc;
 
-			DBG(LOOP, ul_debugobj(cxt, "automatically enabling loop= option"));
-			mnt_optlist_append_flags(ol, MNT_MS_LOOP, cxt->map_userspace);
-			return 1;
+		rc = mnt_context_guess_srcpath_fstype(cxt, &autotype);
+		if (rc) {
+			DBG(CXT, ul_debugobj(cxt, "failed to guess regfile FS type [rc=%d]", rc));
+			return 0;
+		}
+		if (autotype) {
+			__mnt_fs_set_fstype_ptr(cxt->fs, autotype);
+			type = mnt_fs_get_fstype(cxt->fs);
 		}
 	}
 
-	return 0;
+	/* The EROFS kernel driver can be compiled with EROFS_FS_BACKED_BY_FILE,
+	 * allowing for the mounting of a regular file as a filesystem without
+	 * a loop device.
+	 */
+	if (type && strcmp(type, "erofs") == 0)
+		return 0;
+
+	/* Note that there is no restriction (on kernel side) that would
+	 * prevent a regular file as a mount(2) source argument. A filesystem
+	 * that is able to mount regular files could be implemented. For this
+	 * reason we do not enable loop device for unknown filesystems.
+	 */
+	if (type && !blkid_known_fstype(type))
+		return 0;
+
+	DBG(LOOP, ul_debugobj(cxt, "automatically enabling loop= option"));
+	mnt_optlist_append_flags(ol, MNT_MS_LOOP, cxt->map_userspace);
+	return 1;
 }
 
 /* call after mount(2) */