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From:   Martin Kepplinger <martink@posteo.de>
To:     hirofumi@mail.parknet.co.jp, angus@akkea.ca
Cc:     linux-kernel@vger.kernel.org, kay.sievers@vrfy.org,
        lennart@poettering.net, harald@redhat.com,
        gregkh@linuxfoundation.org, david@fubar.dk, tytso@mit.edu,
        alan@lxorguk.ukuu.org.uk, akpm@linux-foundation.org,
        Martin Kepplinger <martink@posteo.de>
Subject: [PATCH] fs: fat: Make the volume label writable when mounted
Date:   Thu,  7 Oct 2021 09:56:39 +0000
Message-Id: <20211007095639.5002-1-martink@posteo.de>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

The motivation for this change (probably) has its origin in a whishlist
for the kernel that popped up in 2011, see
https://lore.kernel.org/lkml/1319135973.1020.9.camel@mop/

Subsequently the "eudyptula programming challenge", see
http://eudyptula-challenge.org/ included the task of making a mounted FAT
filesystems' label writable. That's where it found its way to me in 2014.

During the following years I was asked not to publish this in order to help
the challenge be successful. Time has passed and I'm not asked to keep it
secret anymore.

As for the API, I'm sending the original implementation as an ioctl() and
the example program that I've occasionally used over the years. Based on the
previous discussions, it might be less controversal to create a debugfs
dir for the fs and make the label a file there. I'm happy for acceptable
ideas and all feedback:

We add

	#define VFAT_IOCTL_SET_LABEL _IOW('r', 0x14, __u32)

for users to assign a new FAT volume label while the fat volume
is mounted (it works on the mountpoint).

* A pointer to a string as it's attribute changes the label.
* A nullpointer prints the current label strings to the kernel log.
* An empty string clears the label to "NO NAME    ". (I at least found this
  in the FAT16 spec)

I simply dump a test app here for completeness:

	#include <stdio.h>
	#include <stdint.h>
	#include <sys/ioctl.h>
	#include <fcntl.h>
	#include <unistd.h>
	#include <errno.h>
	#include <linux/msdos_fs.h>
	#include <string.h>
	#include <limits.h>

	#ifndef VFAT_IOCTL_SET_LABEL
	#define VFAT_IOCTL_SET_LABEL _IOW('r', 0x14, __u32)
	#endif

	int main(int argc, char **argv)
	{
		int fd;
		char path[LINE_MAX];
		char mylabel[MSDOS_NAME];
		char *label = mylabel;

		if (argc < 2 || argc > 3) {
			printf("Change label:\n   %s [MOUNTPOINT] [LABEL]\n", argv[0]);
			printf("Show label in kernel logs:\n   %s [MOUNTPOINT]\n\n", argv[0]);
			printf("Available filesystems and their mountpoints are:\n");
			printf("   df --output=source,fstype,target | grep -E 'msdos|fat'\n");
			return -EINVAL;
		}
		if (strlen(argv[1]) >= LINE_MAX) {
			printf("Path to mountpoint too long");
			return -EINVAL;
		}

		strcpy(path, argv[1]);
		if (argc == 3) {
			#pragma GCC diagnostic push
			#pragma GCC diagnostic ignored "-Wstringop-truncation"
			strncpy(label, argv[2], MSDOS_NAME);
			#pragma GCC diagnostic pop
		}
		if (argc == 2)
			label = NULL;

		fd = open(path, O_RDONLY);
		if (fd < 0)
			goto err;

		ioctl(fd, VFAT_IOCTL_SET_LABEL, label);

		close(fd);
	err:
		perror(NULL);
		return errno;
	}

Signed-off-by: Martin Kepplinger <martink@posteo.de>
---
 fs/fat/dir.c                  |  22 +++++
 fs/fat/fat.h                  |   3 +
 fs/fat/file.c                 | 164 ++++++++++++++++++++++++++++++++++
 include/uapi/linux/msdos_fs.h |   1 +
 4 files changed, 190 insertions(+)

diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index c4a274285858..7c971821dce5 100644
--- a/fs/fat/dir.c
+++ b/fs/fat/dir.c
@@ -1412,3 +1412,25 @@ int fat_add_entries(struct inode *dir, void *slots, int nr_slots,
 	return err;
 }
 EXPORT_SYMBOL_GPL(fat_add_entries);
+
+/* found at https://lwn.net/Articles/343565/ */
+int fat_get_label_entry(struct inode *root_inode, struct buffer_head **bh,
+			struct msdos_dir_entry **de)
+{
+	loff_t pos;
+
+	if (WARN_ON(root_inode->i_ino != MSDOS_ROOT_INO))
+		return -EINVAL;
+
+	pos = 0;
+	*bh = NULL;
+	while (fat_get_entry(root_inode, &pos, bh, de) >= 0) {
+		/* volume label: note that it is not enough to check only
+		 * whether the ATTR_VOLUME bit is set, since this would yield
+		 * true on any vfat extended entry.
+		 */
+		if ((*de)->attr != ATTR_EXT && ((*de)->attr & ATTR_VOLUME))
+			return 0;
+	}
+	return -ENOENT;
+}
diff --git a/fs/fat/fat.h b/fs/fat/fat.h
index 02d4d4234956..efcf509b905d 100644
--- a/fs/fat/fat.h
+++ b/fs/fat/fat.h
@@ -336,6 +336,9 @@ extern int fat_alloc_new_dir(struct inode *dir, struct timespec64 *ts);
 extern int fat_add_entries(struct inode *dir, void *slots, int nr_slots,
 			   struct fat_slot_info *sinfo);
 extern int fat_remove_entries(struct inode *dir, struct fat_slot_info *sinfo);
+extern int fat_get_label_entry(struct inode *root_inode,
+			       struct buffer_head **bh,
+			       struct msdos_dir_entry **de);
 
 /* fat/fatent.c */
 struct fat_entry {
diff --git a/fs/fat/file.c b/fs/fat/file.c
index 13855ba49cd9..0e97fc48f424 100644
--- a/fs/fat/file.c
+++ b/fs/fat/file.c
@@ -153,6 +153,168 @@ static int fat_ioctl_fitrim(struct inode *inode, unsigned long arg)
 	return 0;
 }
 
+/* protect access to the FAT fs label */
+DEFINE_MUTEX(label_mutex);
+
+static int fat_set_directory_volume_label(struct file *file, char *label)
+{
+	struct msdos_dir_entry *de;
+	struct buffer_head *bh;
+	struct inode *root_inode = file_inode(file);
+	struct super_block *sb = root_inode->i_sb;
+	struct msdos_sb_info *sbi = MSDOS_SB(sb);
+	int err;
+	int offset;
+	sector_t blocknr;
+	loff_t i_pos;
+
+	err = 0;
+	i_pos = fat_i_pos_read(sbi, root_inode);
+	if (!i_pos)
+		return 0;
+
+	/* code found in inode.c */
+	fat_get_blknr_offset(sbi, i_pos, &blocknr, &offset);
+	bh = sb_bread(sb, blocknr);
+	if (!bh) {
+		fat_msg(sb, KERN_ERR,
+			"unable to read inode block for updating (i_pos %lld)",
+			i_pos);
+		return -EIO;
+	}
+
+	de = NULL;
+	err = fat_get_label_entry(root_inode, &bh, &de);
+	if (err)
+		return err;
+
+	if (label) {
+		fat_msg(sb, KERN_INFO,
+			"rename directory label from %.11s to %.11s",
+			de->name, label);
+		mutex_lock(&label_mutex);
+		strncpy(de->name, label, MSDOS_NAME);
+		mark_buffer_dirty(bh);
+		sync_dirty_buffer(bh);
+		brelse(bh);
+		mutex_unlock(&label_mutex);
+	} else {
+		fat_msg(sb, KERN_INFO, "directory label is %.11s", de->name);
+	}
+
+	return 0;
+}
+
+static int fat_set_partition_volume_label(struct file *file, char *label)
+{
+	struct fat_boot_sector *b;
+	struct buffer_head *bh;
+	struct inode *inode = file_inode(file);
+	struct super_block *sb = inode->i_sb;
+	struct msdos_sb_info *sbi = inode->i_sb->s_fs_info;
+
+	bh = sb_bread(sb, 0);
+	if (!bh) {
+		fat_msg(sb, KERN_ERR, "unable to read boot sector\n");
+		return -EIO;
+	}
+	b = (struct fat_boot_sector *)bh->b_data;
+
+	if (sbi->fat_bits == 32) {
+		if (label) {
+			fat_msg(sb, KERN_INFO,
+				"rename partition label from %.11s to %.11s",
+				b->fat32.vol_label, label);
+
+			mutex_lock(&MSDOS_SB(sb)->s_lock);
+			b->fat32.state |= FAT_STATE_DIRTY;
+			memcpy(b->fat32.vol_label, label, MSDOS_NAME);
+			mutex_unlock(&MSDOS_SB(sb)->s_lock);
+		} else {
+			b->fat32.state &= ~FAT_STATE_DIRTY;
+			fat_msg(sb, KERN_INFO, "partition label is %.11s",
+				b->fat32.vol_label);
+		}
+	} else {
+		if (label) {
+			fat_msg(sb, KERN_INFO,
+				"rename partition label from %.11s to %.11s ",
+				b->fat16.vol_label, label);
+
+			mutex_lock(&MSDOS_SB(sb)->s_lock);
+			b->fat32.state |= FAT_STATE_DIRTY;
+			memcpy(b->fat32.vol_label, label, MSDOS_NAME);
+			mutex_unlock(&MSDOS_SB(sb)->s_lock);
+		} else {
+			b->fat32.state &= ~FAT_STATE_DIRTY;
+			fat_msg(sb, KERN_INFO, "partition label is %.11s",
+				b->fat16.vol_label);
+		}
+	}
+	mark_buffer_dirty(bh);
+	sync_dirty_buffer(bh);
+	brelse(bh);
+
+	return 0;
+}
+
+static int fat_ioctl_set_label(struct file *file, u32 __user *user_attr)
+{
+	struct inode *inode = file_inode(file);
+	struct super_block *sb = inode->i_sb;
+	unsigned char newlabel[MSDOS_NAME + 1];
+	char *label;
+	int err;
+	int i;
+	int len;
+	unsigned char c;
+
+	if (!user_attr) {
+		label = NULL;
+		goto print_only;
+	}
+
+	label = newlabel;
+	err = copy_from_user(label, (void *)user_attr, MSDOS_NAME);
+	if (err) {
+		fat_msg(sb, KERN_ERR,
+			"copy_from_user failed %d bytes not copied\n", err);
+		return -EFAULT;
+	}
+	label[MSDOS_NAME] = '\0';
+	len = strlen(label);
+
+	if (len == 0) {
+		strncpy(label, "NO NAME    ", MSDOS_NAME);
+	} else {
+		for (i = 0; i < len; i++) {
+			c = label[i];
+			if ((c < 'a' || c > 'z') && (c < 'A' || c > 'Z')) {
+				if ((c < '0' || c > '9') && c != 0x20)
+					return -EINVAL;
+			}
+		}
+	}
+
+print_only:
+	err = fat_set_directory_volume_label(file, label);
+	if (err == -ENOENT) {
+		fat_msg(sb, KERN_ERR, "no label entry. please reformat\n");
+		strncpy(label, "NO NAME    ", MSDOS_NAME);
+	} else if (err) {
+		fat_msg(sb, KERN_ERR, "error setting directory label\n");
+		return err;
+	}
+
+	err = fat_set_partition_volume_label(file, label);
+	if (err) {
+		fat_msg(sb, KERN_ERR, "error setting partition label\n");
+		return err;
+	}
+
+	return 0;
+}
+
 long fat_generic_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 {
 	struct inode *inode = file_inode(filp);
@@ -167,6 +329,8 @@ long fat_generic_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		return fat_ioctl_get_volume_id(inode, user_attr);
 	case FITRIM:
 		return fat_ioctl_fitrim(inode, arg);
+	case VFAT_IOCTL_SET_LABEL:
+		return fat_ioctl_set_label(filp, user_attr);
 	default:
 		return -ENOTTY;	/* Inappropriate ioctl for device */
 	}
diff --git a/include/uapi/linux/msdos_fs.h b/include/uapi/linux/msdos_fs.h
index a5773899f4d9..b666bca09238 100644
--- a/include/uapi/linux/msdos_fs.h
+++ b/include/uapi/linux/msdos_fs.h
@@ -104,6 +104,7 @@ struct __fat_dirent {
 #define FAT_IOCTL_SET_ATTRIBUTES	_IOW('r', 0x11, __u32)
 /*Android kernel has used 0x12, so we use 0x13*/
 #define FAT_IOCTL_GET_VOLUME_ID		_IOR('r', 0x13, __u32)
+#define VFAT_IOCTL_SET_LABEL		_IOW('r', 0x14, __u32)
 
 struct fat_boot_sector {
 	__u8	ignored[3];	/* Boot strap short or near jump */
-- 
2.30.2