[PREVIEW] [PATCH] staging: erofs: fix race of initializing xattrs of a inode at the same time

[Gao Xiang]

Hi Chao,

On 2019/2/15 15:29, Chao Yu wrote:
> On 2019/2/14 23:34, Gao Xiang wrote:
>> In real scenario, there could be several threads accessing xattrs
>> of the same xattr-uninitialized inode, and init_inode_xattrs()
>> almost at the same time.
> 
> Yeah, nice catch!
> 
>>
>> That's actually an unexpected behavior, this patch closes the race.
>>
>> Fixes: b17500a0fdba ("staging: erofs: introduce xattr & acl support")
>> Cc: <stable at vger.kernel.org> # 4.19+
>> Signed-off-by: Gao Xiang <gaoxiang25 at huawei.com>
>> ---
>>  drivers/staging/erofs/internal.h | 11 ++++++++---
>>  drivers/staging/erofs/xattr.c    | 41 ++++++++++++++++++++++++++++------------
>>  2 files changed, 37 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/staging/erofs/internal.h b/drivers/staging/erofs/internal.h
>> index 3a27c255950b..e3bfde00c7d2 100644
>> --- a/drivers/staging/erofs/internal.h
>> +++ b/drivers/staging/erofs/internal.h
>> @@ -327,12 +327,17 @@ static inline erofs_off_t iloc(struct erofs_sb_info *sbi, erofs_nid_t nid)
>>  	return blknr_to_addr(sbi->meta_blkaddr) + (nid << sbi->islotbits);
>>  }
>>  
>> -#define inode_set_inited_xattr(inode)   (EROFS_V(inode)->flags |= 1)
>> -#define inode_has_inited_xattr(inode)   (EROFS_V(inode)->flags & 1)
>> +/* atomic flag definitions */
>> +#define EROFS_V_EA_INITED_BIT	0
>> +
>> +/* bitlock definitions (arranged in reverse order) */
>> +#define EROFS_V_BL_XATTR_BIT	(BITS_PER_LONG - 1)
>>  
>>  struct erofs_vnode {
>>  	erofs_nid_t nid;
>> -	unsigned int flags;
>> +
>> +	/* atomic flags (including bitlocks) */
>> +	unsigned long flags;
>>  
>>  	unsigned char data_mapping_mode;
>>  	/* inline size in bytes */
>> diff --git a/drivers/staging/erofs/xattr.c b/drivers/staging/erofs/xattr.c
>> index 8c48b0ab31fd..a188aad00bf8 100644
>> --- a/drivers/staging/erofs/xattr.c
>> +++ b/drivers/staging/erofs/xattr.c
>> @@ -44,18 +44,25 @@ static inline void xattr_iter_end_final(struct xattr_iter *it)
>>  
>>  static int init_inode_xattrs(struct inode *inode)
>>  {
>> +	struct erofs_vnode *const vi = EROFS_V(inode);
>>  	struct xattr_iter it;
>>  	unsigned int i;
>>  	struct erofs_xattr_ibody_header *ih;
>>  	struct super_block *sb;
>>  	struct erofs_sb_info *sbi;
>> -	struct erofs_vnode *vi;
>>  	bool atomic_map;
>> +	int ret = 0;
>>  
>> -	if (likely(inode_has_inited_xattr(inode)))
>> +	/* the most case is that xattrs of this inode are initialized. */
>> +	if (likely(test_bit(EROFS_V_EA_INITED_BIT, &vi->flags)))
>>  		return 0;
>>  
>> -	vi = EROFS_V(inode);
>> +	if (wait_on_bit_lock(&vi->flags, EROFS_V_BL_XATTR_BIT, TASK_KILLABLE))
>> +		return -ERESTARTSYS;
>> +
>> +	/* someone has initialized xattrs for us? */
>> +	if (unlikely(test_bit(EROFS_V_EA_INITED_BIT, &vi->flags)))
>> +		goto out_unlock;
>>  
>>  	/*
>>  	 * bypass all xattr operations if ->xattr_isize is not greater than
>> @@ -68,13 +75,16 @@ static int init_inode_xattrs(struct inode *inode)
>>  	if (vi->xattr_isize == sizeof(struct erofs_xattr_ibody_header)) {
>>  		errln("xattr_isize %d of nid %llu is not supported yet",
>>  		      vi->xattr_isize, vi->nid);
>> -		return -ENOTSUPP;
>> +		ret = -ENOTSUPP;
>> +		goto out_unlock;
>>  	} else if (vi->xattr_isize < sizeof(struct erofs_xattr_ibody_header)) {
>>  		if (unlikely(vi->xattr_isize)) {
>>  			DBG_BUGON(1);
>> -			return -EIO;	/* xattr ondisk layout error */
>> +			ret = -EIO;
>> +			goto out_unlock;	/* xattr ondisk layout error */
>>  		}
>> -		return -ENOATTR;
>> +		ret = -ENOATTR;
>> +		goto out_unlock;
>>  	}
>>  
>>  	sb = inode->i_sb;
>> @@ -83,8 +93,10 @@ static int init_inode_xattrs(struct inode *inode)
>>  	it.ofs = erofs_blkoff(iloc(sbi, vi->nid) + vi->inode_isize);
>>  
>>  	it.page = erofs_get_inline_page(inode, it.blkaddr);
>> -	if (IS_ERR(it.page))
>> -		return PTR_ERR(it.page);
>> +	if (IS_ERR(it.page)) {
>> +		ret = PTR_ERR(it.page);
>> +		goto out_unlock;
>> +	}
>>  
>>  	/* read in shared xattr array (non-atomic, see kmalloc below) */
>>  	it.kaddr = kmap(it.page);
>> @@ -97,7 +109,8 @@ static int init_inode_xattrs(struct inode *inode)
>>  						sizeof(uint), GFP_KERNEL);
>>  	if (vi->xattr_shared_xattrs == NULL) {
>>  		xattr_iter_end(&it, atomic_map);
>> -		return -ENOMEM;
>> +		ret = -ENOMEM;
>> +		goto out_unlock;
>>  	}
>>  
>>  	/* let's skip ibody header */
>> @@ -114,7 +127,8 @@ static int init_inode_xattrs(struct inode *inode)
>>  			if (IS_ERR(it.page)) {
>>  				kfree(vi->xattr_shared_xattrs);
>>  				vi->xattr_shared_xattrs = NULL;
>> -				return PTR_ERR(it.page);
>> +				ret = PTR_ERR(it.page);
>> +				goto out_unlock;
>>  			}
>>  
>>  			it.kaddr = kmap_atomic(it.page);
>> @@ -127,8 +141,11 @@ static int init_inode_xattrs(struct inode *inode)
>>  	}
>>  	xattr_iter_end(&it, atomic_map);
>>  
>> -	inode_set_inited_xattr(inode);
>> -	return 0;
>> +	set_bit(EROFS_V_EA_INITED_BIT, &vi->flags);
> 
> Should it be?
> 
> set_bit();
> wake_up_bit();
> return 0;

set_bit(EROFS_V_EA_INITED_BIT, &vi->flags);    <- set xattrs initialized
clear_and_wake_up_bit(EROFS_V_BL_XATTR_BIT, &vi->flags);  <- clear EROFS_V_BL_XATTR_BIT bitlock and wake up waiting tasks...

Thanks,
Gao Xiang

> 
>> +
>> +out_unlock:
>> +	clear_and_wake_up_bit(EROFS_V_BL_XATTR_BIT, &vi->flags);
>> +	return ret;
>>  }
>>  
>>  /*
>>
>