util-linux/0073-libmount-add-utab.act-file.patch

From 2e22136c0ef7fd5744460d783ef570525df42337 Mon Sep 17 00:00:00 2001
From: Karel Zak <kzak@redhat.com>
Date: Mon, 22 Jan 2024 12:56:24 +0100
Subject: libmount: add utab.act file

The file exists when libmount works with utab, and more steps are
expected during a single mount operation (more kernel events, more
updates to utab, etc.).

It is possible to monitor the file through the mnt_monitor_...() API
by a simple access() after any event. No locks are expected in
monitor, making it usable for non-root users without any security
impact. The monitor can ignore events if the file exist.

Addresses: https://issues.redhat.com/browse/RHEL-14612
Upstream: http://github.com/util-linux/util-linux/commit/9218c9678a6aa9b163705364246ed128e94fabb7
Signed-off-by: Karel Zak <kzak@redhat.com>
---
 libmount/src/context.c    |   7 +++
 libmount/src/mountP.h     |   2 +
 libmount/src/tab_update.c | 109 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 118 insertions(+)

diff --git a/libmount/src/context.c b/libmount/src/context.c
index 06ac50dc2..27e06a19c 100644
--- a/libmount/src/context.c
+++ b/libmount/src/context.c
@@ -2261,6 +2261,10 @@ int mnt_context_prepare_update(struct libmnt_context *cxt)
 		rc = mnt_update_set_fs(cxt->update, cxt->mountflags,
 					NULL, cxt->fs);
 
+	if (mnt_update_is_ready(cxt->update)) {
+		DBG(CXT, ul_debugobj(cxt, "update is ready"));
+		mnt_update_start(cxt->update);
+	}
 	return rc < 0 ? rc : 0;
 }
 
@@ -2319,7 +2323,10 @@ int mnt_context_update_tabs(struct libmnt_context *cxt)
 emit:
 	if (rc == 0 && !mnt_context_within_helper(cxt))
 		mnt_update_emit_event(cxt->update);
+
 end:
+	mnt_update_end(cxt->update);
+
 	if (!mnt_context_switch_ns(cxt, ns_old))
 		return -MNT_ERR_NAMESPACE;
 	return rc;
diff --git a/libmount/src/mountP.h b/libmount/src/mountP.h
index eddc82bde..505d70808 100644
--- a/libmount/src/mountP.h
+++ b/libmount/src/mountP.h
@@ -478,6 +478,8 @@ extern int mnt_update_set_filename(struct libmnt_update *upd,
 				   const char *filename, int userspace_only);
 extern int mnt_update_emit_event(struct libmnt_update *upd);
 extern int mnt_update_already_done(struct libmnt_update *upd);
+extern int mnt_update_start(struct libmnt_update *upd);
+extern int mnt_update_end(struct libmnt_update *upd);
 
 #if __linux__
 /* btrfs.c */
diff --git a/libmount/src/tab_update.c b/libmount/src/tab_update.c
index 25a734e9d..7b088c025 100644
--- a/libmount/src/tab_update.c
+++ b/libmount/src/tab_update.c
@@ -38,6 +38,9 @@ struct libmnt_update {
 	unsigned long	mountflags;
 	int		userspace_only;
 
+	int		act_fd;
+	char		*act_filename;
+
 	unsigned int	ready : 1,
 			missing_options : 1;
 
@@ -61,6 +64,7 @@ struct libmnt_update *mnt_new_update(void)
 	if (!upd)
 		return NULL;
 
+	upd->act_fd = -1;
 	DBG(UPDATE, ul_debugobj(upd, "allocate"));
 	return upd;
 }
@@ -81,8 +85,11 @@ void mnt_free_update(struct libmnt_update *upd)
 	mnt_unref_lock(upd->lock);
 	mnt_unref_fs(upd->fs);
 	mnt_unref_table(upd->mountinfo);
+	if (upd->act_fd >= 0)
+		close(upd->act_fd);
 	free(upd->target);
 	free(upd->filename);
+	free(upd->act_filename);
 	free(upd);
 }
 
@@ -1004,6 +1011,8 @@ int mnt_update_emit_event(struct libmnt_update *upd)
 	if (asprintf(&filename, "%s.event", upd->filename) <= 0)
 		return -ENOMEM;
 
+	DBG(UPDATE, ul_debugobj(upd, "emitting utab event"));
+
 	fd = open(filename, O_WRONLY|O_CREAT|O_CLOEXEC,
 			    S_IWUSR|S_IRUSR|S_IRGRP|S_IROTH);
 	free(filename);
@@ -1013,6 +1022,106 @@ int mnt_update_emit_event(struct libmnt_update *upd)
 	return 0;
 }
 
+/*
+ * Let's use /run/mount/utab.act file to report to libmount monitor that
+ * libmount is working with utab. In this case, the monitor can ignore all
+ * events from kernel until entire mount (with all steps) is done.
+ *
+ * For example mount NFS with x-* options, means
+ * - create utab.act and mark it as used (by LOCK_SH)
+ * - exec /sbin/mount.nfs
+ *   - call mount(2) (kernel event on /proc/self/mounts)
+ *   - utab update (NFS stuff)
+ * - utab update (add x-* userspace options)
+ * - unlink utab.act (if not use anyone else)
+ * - release event by /run/mount/utab.event
+ *
+ * Note, this is used only when utab is in the game (x-* options).
+ */
+int mnt_update_start(struct libmnt_update *upd)
+{
+	int rc = 0;
+	mode_t oldmask;
+
+	if (!upd || !upd->filename)
+		return -EINVAL;
+
+	if (!upd->act_filename &&
+	    asprintf(&upd->act_filename, "%s.act", upd->filename) <= 0)
+		return -ENOMEM;
+
+	/* Use exclusive lock to avoid some other proces will remove the the
+	 * file before it's marked as used by LOCK_SH (below) */
+	rc = update_init_lock(upd, NULL);
+	if (rc)
+		return rc;
+
+	rc = mnt_lock_file(upd->lock);
+	if (rc)
+		return -MNT_ERR_LOCK;
+
+	DBG(UPDATE, ul_debugobj(upd, "creating act file"));
+
+	oldmask = umask(S_IRGRP|S_IWGRP|S_IXGRP|S_IROTH|S_IWOTH|S_IXOTH);
+	upd->act_fd = open(upd->act_filename, O_WRONLY|O_CREAT|O_CLOEXEC, S_IRUSR|S_IWUSR);
+	umask(oldmask);
+
+	if (upd->act_fd < 0) {
+		rc = -errno;
+		goto fail;
+	}
+
+	/* mark the file as used */
+	rc = flock(upd->act_fd, LOCK_SH);
+	if (rc) {
+		rc = -errno;
+		goto fail;
+	}
+
+	mnt_unlock_file(upd->lock);
+	return 0;
+fail:
+	DBG(UPDATE, ul_debugobj(upd, "act file failed [rc=%d]", rc));
+	mnt_unlock_file(upd->lock);
+	unlink(upd->act_filename);
+	close(upd->act_fd);
+	upd->act_fd = -1;
+	return rc;
+}
+
+int mnt_update_end(struct libmnt_update *upd)
+{
+	int rc;
+
+	if (!upd || upd->act_fd < 0)
+		return -EINVAL;
+
+	DBG(UPDATE, ul_debugobj(upd, "removing act file"));
+
+	/* make sure nobody else will use the file */
+	rc = mnt_lock_file(upd->lock);
+	if (rc)
+		return -MNT_ERR_LOCK;
+
+	/* mark the file as unused */
+	flock(upd->act_fd, LOCK_UN);
+	errno = 0;
+
+	/* check if nobody else need the file (if yes, then the file is under LOCK_SH) )*/
+	if (flock(upd->act_fd, LOCK_EX | LOCK_NB) != 0) {
+		if (errno == EWOULDBLOCK)
+			DBG(UPDATE, ul_debugobj(upd, "act file used, no unlink"));
+	} else {
+		DBG(UPDATE, ul_debugobj(upd, "unlinking act file"));
+		unlink(upd->act_filename);
+	}
+
+	mnt_unlock_file(upd->lock);
+	close(upd->act_fd);
+	upd->act_fd = -1;
+	return 0;
+}
+
 #ifdef TEST_PROGRAM
 
 static int update(const char *target, struct libmnt_fs *fs, unsigned long mountflags)
-- 
2.43.0