Commit 07bfa441 authored by OGAWA Hirofumi's avatar OGAWA Hirofumi Committed by Linus Torvalds
Browse files

fat: work around race with userspace's read via blockdev while mounting 

If userspace reads the buffer via blockdev while mounting,
sb_getblk()+modify can race with buffer read via blockdev.

For example,

            FS                               userspace
    bh = sb_getblk()
    modify bh->b_data
                                  read
				    ll_rw_block(bh)
				      fill bh->b_data by on-disk data
				      /* lost modified data by FS */
				      set_buffer_uptodate(bh)
    set_buffer_uptodate(bh)

Userspace should not use the blockdev while mounting though, the udev
seems to be already doing this.  Although I think the udev should try to
avoid this, workaround the race by small overhead.

Link: http://lkml.kernel.org/r/87pnk7l3sw.fsf_-_@mail.parknet.co.jp


Signed-off-by: default avatarOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reported-by: default avatarJan Stancek <jstancek@redhat.com>
Tested-by: default avatarJan Stancek <jstancek@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 619e17cf

fs/fat/dir.c

+11 −2
Original line number Diff line number Diff line
@@ -1100,8 +1100,11 @@ static int fat_zeroed_cluster(struct inode *dir, sector_t blknr, int nr_used, 
			err = -ENOMEM;

			goto error;

		}

		/* Avoid race with userspace read via bdev */

		lock_buffer(bhs[n]);

		memset(bhs[n]->b_data, 0, sb->s_blocksize);

		set_buffer_uptodate(bhs[n]);

		unlock_buffer(bhs[n]);

		mark_buffer_dirty_inode(bhs[n], dir);



		n++;
@@ -1158,6 +1161,8 @@ int fat_alloc_new_dir(struct inode *dir, struct timespec64 *ts) 
	fat_time_unix2fat(sbi, ts, &time, &date, &time_cs);



	de = (struct msdos_dir_entry *)bhs[0]->b_data;

	/* Avoid race with userspace read via bdev */

	lock_buffer(bhs[0]);

	/* filling the new directory slots ("." and ".." entries) */

	memcpy(de[0].name, MSDOS_DOT, MSDOS_NAME);

	memcpy(de[1].name, MSDOS_DOTDOT, MSDOS_NAME);
@@ -1180,6 +1185,7 @@ int fat_alloc_new_dir(struct inode *dir, struct timespec64 *ts) 
	de[0].size = de[1].size = 0;

	memset(de + 2, 0, sb->s_blocksize - 2 * sizeof(*de));

	set_buffer_uptodate(bhs[0]);

	unlock_buffer(bhs[0]);

	mark_buffer_dirty_inode(bhs[0], dir);



	err = fat_zeroed_cluster(dir, blknr, 1, bhs, MAX_BUF_PER_PAGE);
@@ -1237,11 +1243,14 @@ static int fat_add_new_entries(struct inode *dir, void *slots, int nr_slots, 


			/* fill the directory entry */

			copy = min(size, sb->s_blocksize);

			/* Avoid race with userspace read via bdev */

			lock_buffer(bhs[n]);

			memcpy(bhs[n]->b_data, slots, copy);

			slots += copy;

			size -= copy;

			set_buffer_uptodate(bhs[n]);

			unlock_buffer(bhs[n]);

			mark_buffer_dirty_inode(bhs[n], dir);

			slots += copy;

			size -= copy;

			if (!size)

				break;

			n++;

fs/fat/fatent.c

+3 −0
Original line number Diff line number Diff line
@@ -388,8 +388,11 @@ static int fat_mirror_bhs(struct super_block *sb, struct buffer_head **bhs, 
				err = -ENOMEM;

				goto error;

			}

			/* Avoid race with userspace read via bdev */

			lock_buffer(c_bh);

			memcpy(c_bh->b_data, bhs[n]->b_data, sb->s_blocksize);

			set_buffer_uptodate(c_bh);

			unlock_buffer(c_bh);

			mark_buffer_dirty_inode(c_bh, sbi->fat_inode);

			if (sb->s_flags & SB_SYNCHRONOUS)

				err = sync_dirty_buffer(c_bh);

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