linux-kernel - Re: [f2fs-dev] [PATCH 2/6] f2fs: do not expose unwritten blocks to user by DIO

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Message-ID: <YakMtRif1f0cXT6R@google.com>
Date:   Thu, 2 Dec 2021 10:13:09 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <chao@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH 2/6] f2fs: do not expose unwritten blocks to
 user by DIO

On 12/02, Chao Yu wrote:
> On 2021/11/17 5:45, Jaegeuk Kim wrote:
> > DIO preallocates physical blocks before writing data, but if an error occurrs
> > or power-cut happens, we can see block contents from the disk. This patch tries
> > to fix it by 1) turning to buffered writes for DIO into holes, 2) truncating
> > unwritten blocks from error or power-cut.
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > ---
> >   fs/f2fs/data.c |  5 ++++-
> >   fs/f2fs/f2fs.h |  5 +++++
> >   fs/f2fs/file.c | 24 +++++++++++++++++++++++-
> >   3 files changed, 32 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index 3b27fb7daa8b..7ac1a39fcad2 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -1543,8 +1543,11 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map,
> >   					flag != F2FS_GET_BLOCK_DIO);
> >   				err = __allocate_data_block(&dn,
> >   							map->m_seg_type);
> > -				if (!err)
> > +				if (!err) {
> > +					if (flag == F2FS_GET_BLOCK_PRE_DIO)
> > +						file_need_truncate(inode);
> >   					set_inode_flag(inode, FI_APPEND_WRITE);
> > +				}
> >   			}
> >   			if (err)
> >   				goto sync_out;
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index be871a79c634..14bea669f87e 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -654,6 +654,7 @@ enum {
> >   #define FADVISE_KEEP_SIZE_BIT	0x10
> >   #define FADVISE_HOT_BIT		0x20
> >   #define FADVISE_VERITY_BIT	0x40
> > +#define FADVISE_TRUNC_BIT	0x80
> >   #define FADVISE_MODIFIABLE_BITS	(FADVISE_COLD_BIT | FADVISE_HOT_BIT)
> > @@ -681,6 +682,10 @@ enum {
> >   #define file_is_verity(inode)	is_file(inode, FADVISE_VERITY_BIT)
> >   #define file_set_verity(inode)	set_file(inode, FADVISE_VERITY_BIT)
> > +#define file_should_truncate(inode)	is_file(inode, FADVISE_TRUNC_BIT)
> > +#define file_need_truncate(inode)	set_file(inode, FADVISE_TRUNC_BIT)
> > +#define file_dont_truncate(inode)	clear_file(inode, FADVISE_TRUNC_BIT)
> > +
> >   #define DEF_DIR_LEVEL		0
> >   enum {
> > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > index 4bf77a5bf998..ec8de0662437 100644
> > --- a/fs/f2fs/file.c
> > +++ b/fs/f2fs/file.c
> > @@ -960,10 +960,21 @@ int f2fs_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
> >   		down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> >   		filemap_invalidate_lock(inode->i_mapping);
> > +		/*
> > +		 * Truncate stale preallocated blocks used by the previous DIO.
> > +		 */
> > +		if (file_should_truncate(inode)) {
> > +			err = f2fs_truncate(inode);
> > +			if (err)
> > +				goto out_unlock;
> > +			file_dont_truncate(inode);
> > +		}
> > +
> >   		truncate_setsize(inode, attr->ia_size);
> >   		if (attr->ia_size <= old_size)
> >   			err = f2fs_truncate(inode);
> > +out_unlock:
> >   		/*
> >   		 * do not trim all blocks after i_size if target size is
> >   		 * larger than i_size.
> > @@ -4257,6 +4268,13 @@ static int f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *iter)
> >   	/* If it will be an out-of-place direct write, don't bother. */
> >   	if (dio && f2fs_lfs_mode(sbi))
> >   		return 0;
> > +	/*
> > +	 * Don't preallocate holes aligned to DIO_SKIP_HOLES which turns into
> > +	 * buffered IO, if DIO meets any holes.
> > +	 */
> > +	if (dio && i_size_read(inode) &&
> > +		(F2FS_BYTES_TO_BLK(pos) < F2FS_BLK_ALIGN(i_size_read(inode))))
> > +		return 0;
> >   	/* No-wait I/O can't allocate blocks. */
> >   	if (iocb->ki_flags & IOCB_NOWAIT)
> > @@ -4366,10 +4384,14 @@ static ssize_t f2fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
> >   		if (preallocated > 0 && i_size_read(inode) < target_size) {
> >   			down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> >   			filemap_invalidate_lock(inode->i_mapping);
> > -			f2fs_truncate(inode);
> > +			if (!f2fs_truncate(inode))
> > +				file_dont_truncate(inode);
> >   			filemap_invalidate_unlock(inode->i_mapping);
> >   			up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> > +		} else {
> > +			file_dont_truncate(inode);
> 
> How about this case:
> 
> - touch file
> - DIO write [0, 8kb] to file
>  - preallocate 2 physical blocks
>  - set FADVISE_TRUNC_BIT
>  - SPO
> - BUFIO write [0, 4kb] to file
>  - file_dont_truncate -- it leaks unwritten [4kb, 8kb] to user after
> truncating file to 8kb

i_size should be 4kb, no?

> 
> Thanks,
> 
> >   		}
> > +
> >   		clear_inode_flag(inode, FI_PREALLOCATED_ALL);
> >   		if (ret > 0)
> >