Re: [PATCH] ext4: turn off DAX on new files when encryption is set

[Jan Kara]

On Mon 27-07-26 20:13:39, Eric Biggers wrote:
> On Mon, Jul 27, 2026 at 07:43:50PM +0000, Eric Biggers wrote:
> > On Mon, Jul 27, 2026 at 09:06:23PM +0200, Jan Kara wrote:
> > > On Thu 23-07-26 14:26:48, Ojaswin Mujoo wrote:
> > > > Currently, when setting the S_ENCRYPTED flag on a new regular inode in
> > > > a -o dax=always mounted FS, we seem to be erroneously retaining the S_DAX
> > > > flag. This is because the newly created inode ends up with the following:
> > > > 
> > > >   __ext4_new_inode()
> > > >     ext4_set_inode_flags(init=true) // sets S_DAX
> > > >       fscrypt_set_context()
> > > >         ext4_set_context()
> > > >           ext4_set_inode_flags(init=false) // sets S_ENCRYPTED but
> > > >                                               doesn't clears S_DAX
> > > >       ext4_set_aops
> > > >         inode->i_mapping->a_ops = &ext4_dax_aops;
> > > > 
> > > > Due to the S_DAX flag, the excrypted inode gets ext4_dax_aops and it
> > > > silently ends up bypassing encryption completely. This is reflected in
> > > > multiple xfstests failures like generic/548. To fix this, ensure we disable
> > > > S_DAX correctly when S_ENCRYPTED is being set on a newly created inode. It
> > > > is safe to change DAX state there because ext4_set_aops() later can
> > > > correctly detect S_DAX unset and assign the correct aops.
> > > > 
> > > > The fix was actually the intended behavior however it seemed to have
> > > > silently changed in 043546e46dc7.
> > > > 
> > > > Fixes: 043546e46dc7 ("fs/ext4: Only change S_DAX on inode load")
> > > > Reported-by: Disha Goel <disgoel@xxxxxxxxxxxxx>
> > > > Signed-off-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
> > > > ---
> > > >  fs/ext4/crypto.c | 15 +++++++++++----
> > > >  fs/ext4/inode.c  | 10 ++++++----
> > > >  2 files changed, 17 insertions(+), 8 deletions(-)
> > > > 
> > > > diff --git a/fs/ext4/crypto.c b/fs/ext4/crypto.c
> > > > index f41f320f4437..2d409114e02e 100644
> > > > --- a/fs/ext4/crypto.c
> > > > +++ b/fs/ext4/crypto.c
> > > > @@ -134,6 +134,7 @@ static int ext4_set_context(struct inode *inode, const void *ctx, size_t len,
> > > >  {
> > > >  	handle_t *handle = fs_data;
> > > >  	int res, res2, credits, retries = 0;
> > > > +	bool init = S_ISREG(inode->i_mode);
> > > 
> > > Hum, I have some comments: IMHO the change is confusing because the meaning
> > > of 'init' argument of ext4_set_inode_flags() doesn't mean the first setting
> > > of flags any longer. And the reliance on ext4_set_context() getting called
> > > in a safe place exactly for regular files is rather subtle as well.
> > > 
> > > Also does the change to ext4_set_inode_flags() really work? As far as I'm
> > > reading the ext4 crypto implementation, I don't see where we'd be setting
> > > EXT4_INODE_ENCRYPT for inodes with already enabled encryption (where we
> > > load the context from xattrs through ext4_get_context()) and thus DAX would
> > > still get enabled for such inodes? What am I missing?
> > 
> > Maybe __ext4_new_inode() should set the encryption flag on the inode
> > earlier?  Currently it decides whether the inode will be encrypted, then
> > calls ext4_set_inode_flags(), then calls fscrypt_set_context() (and thus
> > ext4_set_context()).  That last step sets the flag and calls
> > ext4_set_inode_flags() *again*.
> > 
> > I don't remember if there's a reason for that redundancy, but it might
> > just be for historical reasons.  ext4_set_context() is called in two
> > different cases: when a new inode is created and it's encrypted from the
> > beginning, and when FS_IOC_SET_ENCRYPTION_POLICY is executed on an empty
> > directory.  The latter does have to set the encrypt flag there.
> >  
> > - Eric
> 
> Here's an idea.  I haven't tested it though.
> 
> Note that this would align ext4 closer with f2fs, which already sets the
> encrypt flag before it calls fscrypt_set_context().

Thanks! This definitely looks much more obvious than Ojaswin's fix.
Ojaswin, does this fix work for you?

								Honza

> 
> diff --git a/fs/ext4/crypto.c b/fs/ext4/crypto.c
> index f41f320f4437..c4bfe18fd527 100644
> --- a/fs/ext4/crypto.c
> +++ b/fs/ext4/crypto.c
> @@ -144,7 +144,7 @@ static int ext4_set_context(struct inode *inode, const void *ctx, size_t len,
>  	if (inode->i_ino == EXT4_ROOT_INO)
>  		return -EPERM;
>  
> -	if (WARN_ON_ONCE(IS_DAX(inode) && i_size_read(inode)))
> +	if (WARN_ON_ONCE(IS_DAX(inode)))
>  		return -EINVAL;
>  
>  	if (ext4_test_inode_flag(inode, EXT4_INODE_DAX))
> @@ -163,6 +163,14 @@ static int ext4_set_context(struct inode *inode, const void *ctx, size_t len,
>  	 */
>  
>  	if (handle) {
> +		/*
> +		 * ext4 should have set the encrypt flag on the inode already,
> +		 * and it should not have enabled inline data.
> +		 */
> +		if (WARN_ON_ONCE(!IS_ENCRYPTED(inode)))
> +			return -EINVAL;
> +		if (WARN_ON_ONCE(ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)))
> +			return -EINVAL;
>  		/*
>  		 * Since the inode is new it is ok to pass the
>  		 * XATTR_CREATE flag. This is necessary to match the
> @@ -170,21 +178,10 @@ static int ext4_set_context(struct inode *inode, const void *ctx, size_t len,
>  		 * function with the credits allocated for the new
>  		 * inode.
>  		 */
> -		res = ext4_xattr_set_handle(handle, inode,
> -					    EXT4_XATTR_INDEX_ENCRYPTION,
> -					    EXT4_XATTR_NAME_ENCRYPTION_CONTEXT,
> -					    ctx, len, XATTR_CREATE);
> -		if (!res) {
> -			ext4_set_inode_flag(inode, EXT4_INODE_ENCRYPT);
> -			ext4_clear_inode_state(inode,
> -					EXT4_STATE_MAY_INLINE_DATA);
> -			/*
> -			 * Update inode->i_flags - S_ENCRYPTED will be enabled,
> -			 * S_DAX may be disabled
> -			 */
> -			ext4_set_inode_flags(inode, false);
> -		}
> -		return res;
> +		return ext4_xattr_set_handle(handle, inode,
> +					     EXT4_XATTR_INDEX_ENCRYPTION,
> +					     EXT4_XATTR_NAME_ENCRYPTION_CONTEXT,
> +					     ctx, len, XATTR_CREATE);
>  	}
>  
>  	res = dquot_initialize(inode);
> diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c
> index a40cb27f8116..3848244af77a 100644
> --- a/fs/ext4/ialloc.c
> +++ b/fs/ext4/ialloc.c
> @@ -997,6 +997,8 @@ struct inode *__ext4_new_inode(struct mnt_idmap *idmap,
>  		err = fscrypt_prepare_new_inode(dir, inode, &encrypt);
>  		if (err)
>  			goto out;
> +		if (encrypt)
> +			i_flags |= EXT4_ENCRYPT_FL;
>  	}
>  
>  	err = dquot_initialize(inode);
> @@ -1306,6 +1308,7 @@ struct inode *__ext4_new_inode(struct mnt_idmap *idmap,
>  	ei->i_extra_isize = sbi->s_want_extra_isize;
>  	ei->i_inline_off = 0;
>  	if (ext4_has_feature_inline_data(sb) &&
> +	    !(ei->i_flags & EXT4_ENCRYPT_FL) &&
>  	    (!(ei->i_flags & (EXT4_DAX_FL|EXT4_EA_INODE_FL)) || S_ISDIR(mode)))
>  		ext4_set_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA);
>  	ret = inode;
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR