libmount: support MS_RDONLY on write-protected devices

This feature is supported by mount(8) only. It seems better move
this code to libmount. The results is more simple mount(8) and the
feature is accessible for all libmount users.

Signed-off-by: Karel Zak <kzak@redhat.com>

6 files changed, 89 insertions, 1 deletions

diff --git a/libmount/docs/libmount-sections.txt b/libmount/docs/libmount-sections.txt
index 78d89cae7..1d21879a0 100644
--- a/libmount/docs/libmount-sections.txt
+++ b/libmount/docs/libmount-sections.txt
@@ -35,8 +35,10 @@ mnt_context_enable_fork
 mnt_context_enable_lazy
 mnt_context_enable_loopdel
 mnt_context_enable_rdonly_umount
+mnt_context_enable_rwonly_mount;
 mnt_context_enable_sloppy
 mnt_context_enable_verbose
+mnt_context_forced_rdonly;
 mnt_context_get_cache
 mnt_context_get_fs
 mnt_context_get_fstab
@@ -72,6 +74,7 @@ mnt_context_is_nomtab
 mnt_context_is_parent
 mnt_context_is_rdonly_umount
 mnt_context_is_restricted
+mnt_context_is_rwonly_mount;
 mnt_context_is_sloppy
 mnt_context_is_swapmatch
 mnt_context_is_verbose
diff --git a/libmount/src/context.c b/libmount/src/context.c
index e731749b4..6a7c7d351 100644
--- a/libmount/src/context.c
+++ b/libmount/src/context.c
@@ -172,6 +172,7 @@ int mnt_reset_context(struct libmnt_context *cxt)
 	cxt->flags |= (fl & MNT_FL_FORCE);
 	cxt->flags |= (fl & MNT_FL_NOCANONICALIZE);
 	cxt->flags |= (fl & MNT_FL_RDONLY_UMOUNT);
+	cxt->flags |= (fl & MNT_FL_RWONLY_MOUNT);
 	cxt->flags |= (fl & MNT_FL_NOSWAPMATCH);
 	cxt->flags |= (fl & MNT_FL_TABPATHS_CHECKED);
 	return 0;
@@ -478,6 +479,48 @@ int mnt_context_is_rdonly_umount(struct libmnt_context *cxt)
 }
 
 /**
+ * mnt_context_enable_rwonly_mount:
+ * @cxt: mount context
+ * @enable: TRUE or FALSE
+ *
+ * Force read-write mount; if enabled libmount will never try MS_RDONLY
+ * after failed mount(2) EROFS. (See mount(8) man page, option -w).
+ *
+ * Returns: 0 on success, negative number in case of error.
+ */
+int mnt_context_enable_rwonly_mount(struct libmnt_context *cxt, int enable)
+{
+	return set_flag(cxt, MNT_FL_RWONLY_MOUNT, enable);
+}
+
+/**
+ * mnt_context_is_rwonly_mount
+ * @cxt: mount context
+ *
+ * See also mnt_context_enable_rwonly_mount() and mount(8) man page,
+ * option -w.
+ *
+ * Returns: 1 if only read-write mount is allowed.
+ */
+int mnt_context_is_rwonly_mount(struct libmnt_context *cxt)
+{
+	return cxt->flags & MNT_FL_RWONLY_MOUNT ? 1 : 0;
+}
+
+/**
+ * mnt_context_forced_rdonly:
+ * @cxt: mount context
+ *
+ * See also mnt_context_enable_rwonly_mount().
+ *
+ * Returns: 1 if mounted read-only on write-protected device.
+ */
+int mnt_context_forced_rdonly(struct libmnt_context *cxt)
+{
+	return cxt->flags & MNT_FL_FORCED_RDONLY ? 1 : 0;
+}
+
+/**
  * mnt_context_disable_helpers:
  * @cxt: mount context
  * @disable: TRUE or FALSE
@@ -2002,8 +2045,10 @@ int mnt_context_apply_fstab(struct libmnt_context *cxt)
 	if (!cxt || !cxt->fs)
 		return -EINVAL;
 
-	if (mnt_context_tab_applied(cxt))	/* already applied */
+	if (mnt_context_tab_applied(cxt)) {	/* already applied */
+		DBG(CXT, ul_debugobj(cxt, "fstab already applied -- skip"));
 		return 0;
+	}
 
 	if (mnt_context_is_restricted(cxt)) {
 		DBG(CXT, ul_debugobj(cxt, "force fstab usage for non-root users!"));
diff --git a/libmount/src/context_mount.c b/libmount/src/context_mount.c
index 6368e9ba8..1c8a62924 100644
--- a/libmount/src/context_mount.c
+++ b/libmount/src/context_mount.c
@@ -1083,6 +1083,7 @@ int mnt_context_mount(struct libmnt_context *cxt)
 	assert(cxt->helper_exec_status == 1);
 	assert(cxt->syscall_status == 1);
 
+again:
 	rc = mnt_context_prepare_mount(cxt);
 	if (!rc)
 		rc = mnt_context_prepare_update(cxt);
@@ -1090,6 +1091,32 @@ int mnt_context_mount(struct libmnt_context *cxt)
 		rc = mnt_context_do_mount(cxt);
 	if (!rc)
 		rc = mnt_context_update_tabs(cxt);
+
+	/*
+	 * Read-only device; try mount filesystem read-only
+	 */
+	if ((rc == -EROFS && !mnt_context_syscall_called(cxt))	/* before syscall; rdonly loopdev */
+	     || mnt_context_get_syscall_errno(cxt) == EROFS	/* syscall failed with EROFS */
+	     || mnt_context_get_syscall_errno(cxt) == EACCES)	/* syscall failed with EACCES */
+	{
+		unsigned long mflags = 0;
+
+		mnt_context_get_mflags(cxt, &mflags);
+
+		if (!(mflags & MS_RDONLY)			/* not yet RDONLY */
+		    && !(mflags & MS_REMOUNT)			/* not remount */
+		    && !(mflags & MS_BIND)			/* not bin mount */
+		    && !mnt_context_is_rwonly_mount(cxt)) {	/* no explicit read-write */
+
+			assert(!(cxt->flags & MNT_FL_FORCED_RDONLY));
+			DBG(CXT, ul_debugobj(cxt, "write-protected source, trying RDONLY."));
+
+			mnt_context_reset_status(cxt);
+			mnt_context_set_mflags(cxt, mflags | MS_RDONLY);
+			cxt->flags |= MNT_FL_FORCED_RDONLY;
+			goto again;
+		}
+	}
 	return rc;
 }
 
diff --git a/libmount/src/libmount.h.in b/libmount/src/libmount.h.in
index f191a52fe..3f2da3230 100644
--- a/libmount/src/libmount.h.in
+++ b/libmount/src/libmount.h.in
@@ -610,6 +610,7 @@ extern int mnt_context_set_optsmode(struct libmnt_context *cxt, int mode);
 extern int mnt_context_disable_canonicalize(struct libmnt_context *cxt, int disable);
 extern int mnt_context_enable_lazy(struct libmnt_context *cxt, int enable);
 extern int mnt_context_enable_rdonly_umount(struct libmnt_context *cxt, int enable);
+extern int mnt_context_enable_rwonly_mount(struct libmnt_context *cxt, int enable);
 extern int mnt_context_disable_helpers(struct libmnt_context *cxt, int disable);
 extern int mnt_context_enable_sloppy(struct libmnt_context *cxt, int enable);
 extern int mnt_context_enable_fake(struct libmnt_context *cxt, int enable);
@@ -626,6 +627,8 @@ extern int mnt_context_is_lazy(struct libmnt_context *cxt)
 			__ul_attribute__((nonnull));
 extern int mnt_context_is_rdonly_umount(struct libmnt_context *cxt)
 			__ul_attribute__((nonnull));
+extern int mnt_context_is_rwonly_mount(struct libmnt_context *cxt)
+			__ul_attribute__((nonnull));
 extern int mnt_context_is_sloppy(struct libmnt_context *cxt)
 			__ul_attribute__((nonnull));
 extern int mnt_context_is_fake(struct libmnt_context *cxt)
@@ -644,6 +647,8 @@ extern int mnt_context_is_nocanonicalize(struct libmnt_context *cxt)
 			__ul_attribute__((nonnull));
 extern int mnt_context_is_swapmatch(struct libmnt_context *cxt)
 			__ul_attribute__((nonnull));
+extern int mnt_context_forced_rdonly(struct libmnt_context *cxt)
+			__ul_attribute__((nonnull));
 
 extern int mnt_context_is_fork(struct libmnt_context *cxt)
 			__ul_attribute__((nonnull));
diff --git a/libmount/src/libmount.sym b/libmount/src/libmount.sym
index f0210c240..354332b43 100644
--- a/libmount/src/libmount.sym
+++ b/libmount/src/libmount.sym
@@ -315,3 +315,9 @@ MOUNT_2.28 {
 	mnt_table_find_target_with_option;
 	mnt_fs_set_priority;
 } MOUNT_2.26;
+
+MOUNT_2.30 {
+	mnt_context_is_rwonly_mount;
+	mnt_context_forced_rdonly;
+	mnt_context_enable_rwonly_mount;
+} MOUNT_2.28;
diff --git a/libmount/src/mountP.h b/libmount/src/mountP.h
index 2baab55de..42957ecb9 100644
--- a/libmount/src/mountP.h
+++ b/libmount/src/mountP.h
@@ -328,6 +328,7 @@ struct libmnt_context
 #define MNT_FL_RDONLY_UMOUNT	(1 << 11)	/* remount,ro after EBUSY umount(2) */
 #define MNT_FL_FORK		(1 << 12)
 #define MNT_FL_NOSWAPMATCH	(1 << 13)
+#define MNT_FL_RWONLY_MOUNT	(1 << 14)	/* explicit mount -w; never try read-only  */
 
 #define MNT_FL_MOUNTDATA	(1 << 20)
 #define MNT_FL_TAB_APPLIED	(1 << 21)	/* mtab/fstab merged to cxt->fs */
@@ -338,6 +339,7 @@ struct libmnt_context
 #define MNT_FL_LOOPDEV_READY	(1 << 26)	/* /dev/loop<N> initialized by the library */
 #define MNT_FL_MOUNTOPTS_FIXED  (1 << 27)
 #define MNT_FL_TABPATHS_CHECKED	(1 << 28)
+#define MNT_FL_FORCED_RDONLY	(1 << 29)	/* mounted read-only on write-protected device */
 
 /* default flags */
 #define MNT_FL_DEFAULT		0