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

[Disha Goel <disgoel@linux.ibm.com>]

On 28/07/26 1:43 am, 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@linux.ibm.com>
>>>> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
>>>> ---
>>>>   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().

Hi Eric,

I have tested it in my setup. All failing encryption tests in the
generic group passed.

> 
> 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;

-- 
Regards,
Disha