Re: [PATCH] Memory shortage can result in inconsistent flocks state

[Chuck Ebbert]

On 09/11/2007 08:38 AM, Pavel Emelyanov wrote:
> When the flock_lock_file() is called to change the flock
> from F_RDLCK to F_WRLCK or vice versa the existing flock
> can be removed without appropriate warning.
> 
> Look:
>         for_each_lock(inode, before) {
>                 struct file_lock *fl = *before;
>                 if (IS_POSIX(fl))
>                         break;
>                 if (IS_LEASE(fl))
>                         continue;
>                 if (filp != fl->fl_file)
>                         continue;
>                 if (request->fl_type == fl->fl_type)
>                         goto out;
>                 found = 1;
>                 locks_delete_lock(before); <<<<<< !
>                 break;
>         }
> 
> if after this point the subsequent locks_alloc_lock() will
> fail the return code will be -ENOMEM, but the existing lock
> is already removed.
> 
> This is a known feature that such "re-locking" is not atomic,
> but in the racy case the file should stay locked (although by
> some other process), but in this case the file will be unlocked.
> 
> The proposal is to prepare the lock in advance keeping no chance
> to fail in the future code.
> 
> Found during making the flocks pid-namespaces aware.
> 
> Signed-off-by: Pavel Emelyanov <xemul@xxxxxxxxxx>
> 
> ---
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index 0db1a14..f59d066 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -732,6 +732,14 @@ static int flock_lock_file(struct file *
>  	lock_kernel();
>  	if (request->fl_flags & FL_ACCESS)
>  		goto find_conflict;
> +
> +	if (request->fl_type != F_UNLCK) {
> +		error = -ENOMEM;
> +		new_fl = locks_alloc_lock();
> +		if (new_fl == NULL)
> +			goto out;
> +	}
> +
>  	for_each_lock(inode, before) {
>  		struct file_lock *fl = *before;
>  		if (IS_POSIX(fl))
> @@ -753,10 +761,6 @@ static int flock_lock_file(struct file *
>  		goto out;
>  	}
>  
> -	error = -ENOMEM;
> -	new_fl = locks_alloc_lock();
> -	if (new_fl == NULL)
> -		goto out;
>  	/*
>  	 * If a higher-priority process was blocked on the old file lock,
>  	 * give it the opportunity to lock the file.

Doesn't that create a leak in some cases?

>         for_each_lock(inode, before) {
>                 struct file_lock *fl = *before;
>                 if (IS_POSIX(fl))
>                         break;
>                 if (IS_LEASE(fl))
>                         continue;
>                 if (filp != fl->fl_file)
>                         continue;
>                 if (request->fl_type == fl->fl_type)
>                         goto out;  <<<<<<<<<<<<<<<< LEAK?
>                 found = 1;
>                 locks_delete_lock(before);
>                 break;
>         }
-
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