diff mbox series

[v5,1/5] libfs: Return ENOSPC when the directory offset range is exhausted

Message ID 20241215185816.1826975-2-cel@kernel.org (mailing list archive)
State New
Headers show
Series Improve simple directory offset wrap behavior | expand

Commit Message

Chuck Lever Dec. 15, 2024, 6:58 p.m. UTC 
From: Chuck Lever <chuck.lever@oracle.com>

Testing shows that the EBUSY error return from mtree_alloc_cyclic()
leaks into user space. The ERRORS section of "man creat(2)" says:

>	EBUSY	O_EXCL was specified in flags and pathname refers
>		to a block device that is in use by the system
>		(e.g., it is mounted).

ENOSPC is closer to what applications expect in this situation.

Note that the normal range of simple directory offset values is
2..2^63, so hitting this error is going to be rare to impossible.

Fixes: 6faddda69f62 ("libfs: Add directory operations for stable offsets")
Cc: <stable@vger.kernel.org> # v6.9+
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/libfs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Comments

David Laight Dec. 15, 2024, 7:35 p.m. UTC | #1 
From: cel@kernel.org
> Sent: 15 December 2024 18:58
> 
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> Testing shows that the EBUSY error return from mtree_alloc_cyclic()
> leaks into user space. The ERRORS section of "man creat(2)" says:
> 
> >	EBUSY	O_EXCL was specified in flags and pathname refers
> >		to a block device that is in use by the system
> >		(e.g., it is mounted).
> 
> ENOSPC is closer to what applications expect in this situation.
> 
> Note that the normal range of simple directory offset values is
> 2..2^63, so hitting this error is going to be rare to impossible.
> 
> Fixes: 6faddda69f62 ("libfs: Add directory operations for stable offsets")
> Cc: <stable@vger.kernel.org> # v6.9+
> Reviewed-by: Jeff Layton <jlayton@kernel.org>
> Reviewed-by: Yang Erkun <yangerkun@huawei.com>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/libfs.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index 748ac5923154..f6d04c69f195 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -292,7 +292,9 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
> 
>  	ret = mtree_alloc_cyclic(&octx->mt, &offset, dentry, DIR_OFFSET_MIN,
>  				 LONG_MAX, &octx->next_offset, GFP_KERNEL);
> -	if (ret < 0)
> +	if (unlikely(ret == -EBUSY))
> +		return -ENOSPC;
> +	if (unlikely(ret < 0))
>  		return ret;

You've just added an extra comparison to a hot path.
Doing:
	if (ret < 0)
		return ret == -EBUSY ? -ENOSPC : ret;
would be better.

	David

> 
>  	offset_set(dentry, offset);
> --
> 2.47.0
> 

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
Pratyush Yadav Dec. 16, 2024, 1:39 p.m. UTC | #2 
On Sun, Dec 15 2024, David Laight wrote:

> From: cel@kernel.org
>> Sent: 15 December 2024 18:58
>> 
>> From: Chuck Lever <chuck.lever@oracle.com>
>> 
>> Testing shows that the EBUSY error return from mtree_alloc_cyclic()
>> leaks into user space. The ERRORS section of "man creat(2)" says:
>> 
>> >	EBUSY	O_EXCL was specified in flags and pathname refers
>> >		to a block device that is in use by the system
>> >		(e.g., it is mounted).
>> 
>> ENOSPC is closer to what applications expect in this situation.
>> 
>> Note that the normal range of simple directory offset values is
>> 2..2^63, so hitting this error is going to be rare to impossible.
>> 
>> Fixes: 6faddda69f62 ("libfs: Add directory operations for stable offsets")
>> Cc: <stable@vger.kernel.org> # v6.9+
>> Reviewed-by: Jeff Layton <jlayton@kernel.org>
>> Reviewed-by: Yang Erkun <yangerkun@huawei.com>
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>> ---
>>  fs/libfs.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/libfs.c b/fs/libfs.c
>> index 748ac5923154..f6d04c69f195 100644
>> --- a/fs/libfs.c
>> +++ b/fs/libfs.c
>> @@ -292,7 +292,9 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
>> 
>>  	ret = mtree_alloc_cyclic(&octx->mt, &offset, dentry, DIR_OFFSET_MIN,
>>  				 LONG_MAX, &octx->next_offset, GFP_KERNEL);
>> -	if (ret < 0)
>> +	if (unlikely(ret == -EBUSY))
>> +		return -ENOSPC;
>> +	if (unlikely(ret < 0))
>>  		return ret;
>
> You've just added an extra comparison to a hot path.
> Doing:
> 	if (ret < 0)
> 		return ret == -EBUSY ? -ENOSPC : ret;
> would be better.

This also has two comparisons: one for ret < 0 and another for ret ==
-EBUSY. So I don't see a difference. I was curious to see if compilers
can somehow optimize one or the other, so I ran the two on godbolt and I
see no real difference between the two: https://godbolt.org/z/9Gav6b6Mf
Chuck Lever Dec. 16, 2024, 1:51 p.m. UTC | #3 
On 12/16/24 8:39 AM, Pratyush Yadav wrote:
> On Sun, Dec 15 2024, David Laight wrote:
> 
>> From: cel@kernel.org
>>> Sent: 15 December 2024 18:58
>>>
>>> From: Chuck Lever <chuck.lever@oracle.com>
>>>
>>> Testing shows that the EBUSY error return from mtree_alloc_cyclic()
>>> leaks into user space. The ERRORS section of "man creat(2)" says:
>>>
>>>> 	EBUSY	O_EXCL was specified in flags and pathname refers
>>>> 		to a block device that is in use by the system
>>>> 		(e.g., it is mounted).
>>>
>>> ENOSPC is closer to what applications expect in this situation.
>>>
>>> Note that the normal range of simple directory offset values is
>>> 2..2^63, so hitting this error is going to be rare to impossible.
>>>
>>> Fixes: 6faddda69f62 ("libfs: Add directory operations for stable offsets")
>>> Cc: <stable@vger.kernel.org> # v6.9+
>>> Reviewed-by: Jeff Layton <jlayton@kernel.org>
>>> Reviewed-by: Yang Erkun <yangerkun@huawei.com>
>>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>>> ---
>>>   fs/libfs.c | 4 +++-
>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/libfs.c b/fs/libfs.c
>>> index 748ac5923154..f6d04c69f195 100644
>>> --- a/fs/libfs.c
>>> +++ b/fs/libfs.c
>>> @@ -292,7 +292,9 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
>>>
>>>   	ret = mtree_alloc_cyclic(&octx->mt, &offset, dentry, DIR_OFFSET_MIN,
>>>   				 LONG_MAX, &octx->next_offset, GFP_KERNEL);
>>> -	if (ret < 0)
>>> +	if (unlikely(ret == -EBUSY))
>>> +		return -ENOSPC;
>>> +	if (unlikely(ret < 0))
>>>   		return ret;
>>
>> You've just added an extra comparison to a hot path.
>> Doing:
>> 	if (ret < 0)
>> 		return ret == -EBUSY ? -ENOSPC : ret;
>> would be better.
> 
> This also has two comparisons: one for ret < 0 and another for ret ==
> -EBUSY. So I don't see a difference. I was curious to see if compilers
> can somehow optimize one or the other, so I ran the two on godbolt and I
> see no real difference between the two: https://godbolt.org/z/9Gav6b6Mf

In my version, both comparisons are done every time through this flow.
David's version changes it so that only one comparison is done unless
@ret is less than zero (which is rare).

I've updated simple_offset_add() in my tree to use David's version.
diff mbox series

Patch

diff --git a/fs/libfs.c b/fs/libfs.c
index 748ac5923154..f6d04c69f195 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -292,7 +292,9 @@  int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
 
 	ret = mtree_alloc_cyclic(&octx->mt, &offset, dentry, DIR_OFFSET_MIN,
 				 LONG_MAX, &octx->next_offset, GFP_KERNEL);
-	if (ret < 0)
+	if (unlikely(ret == -EBUSY))
+		return -ENOSPC;
+	if (unlikely(ret < 0))
 		return ret;
 
 	offset_set(dentry, offset);