ext4: add debugging trigger for ext4_error()

Make it easy to test whether or not the error handling subsystem in
ext4 is working correctly.  This allows us to simulate an ext4_error()
by echoing a string to /sys/fs/ext4/<dev>/trigger_fs_error.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: ksumrall@google.com

fs/ext4/ext4.h

@@ -1197,6 +1197,7 @@ struct ext4_sb_info {
 	struct proc_dir_entry *s_proc;
 	struct kobject s_kobj;
 	struct completion s_kobj_unregister;
+	struct super_block *s_sb;
 
 	/* Journaling */
 	struct journal_s *s_journal;

fs/ext4/super.c

@@ -2473,6 +2473,23 @@ static ssize_t sbi_ui_store(struct ext4_attr *a,
 	return count;
 }
 
+static ssize_t trigger_test_error(struct ext4_attr *a,
+				  struct ext4_sb_info *sbi,
+				  const char *buf, size_t count)
+{
+	int len = count;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (len && buf[len-1] == '\n')
+		len--;
+
+	if (len)
+		ext4_error(sbi->s_sb, "%.*s", len, buf);
+	return count;
+}
+
 #define EXT4_ATTR_OFFSET(_name,_mode,_show,_store,_elname) \
 static struct ext4_attr ext4_attr_##_name = {			\
 	.attr = {.name = __stringify(_name), .mode = _mode },	\
@@ -2503,6 +2520,7 @@ EXT4_RW_ATTR_SBI_UI(mb_order2_req, s_mb_order2_reqs);
 EXT4_RW_ATTR_SBI_UI(mb_stream_req, s_mb_stream_request);
 EXT4_RW_ATTR_SBI_UI(mb_group_prealloc, s_mb_group_prealloc);
 EXT4_RW_ATTR_SBI_UI(max_writeback_mb_bump, s_max_writeback_mb_bump);
+EXT4_ATTR(trigger_fs_error, 0200, NULL, trigger_test_error);
 
 static struct attribute *ext4_attrs[] = {
 	ATTR_LIST(delayed_allocation_blocks),
@@ -2517,6 +2535,7 @@ static struct attribute *ext4_attrs[] = {
 	ATTR_LIST(mb_stream_req),
 	ATTR_LIST(mb_group_prealloc),
 	ATTR_LIST(max_writeback_mb_bump),
+	ATTR_LIST(trigger_fs_error),
 	NULL,
 };
 
@@ -3087,6 +3106,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		goto out_free_orig;
 	}
 	sb->s_fs_info = sbi;
+	sbi->s_sb = sb;
 	sbi->s_mount_opt = 0;
 	sbi->s_resuid = EXT4_DEF_RESUID;
 	sbi->s_resgid = EXT4_DEF_RESGID;