Re: [RFC] [PATCH] hugetlbfs :shmget with SHM_HUGETLB only works as root

[Ciju Rajan K <ciju@linux.vnet.ibm.com>]

Hi Wli,
   I tested your patch. But that is not solving the problem.
   If the code change to user_shm_lock() is not a good solution, could 
you please suggest a method so that the normal user is able to allocate 
the huge pages, if his gid is added to /proc/sys/vm/hugetlb_shm_group

Thanks
Ciju

William Lee Irwin III wrote:
> On Wed, Nov 14, 2007 at 09:31:41AM -0600, aglitke wrote:
>> ... if the user's locked limit (ulimit -l) is set to unlimited, allowed
>> (above) is set to 1.  In that case, the second part of that if() is
>> bypassed, and the function grants permission.  Therefore, the easy
>> solution is to make sure your user's lock_limit is RLIM_INFINITY.
> 
> This function deserves a minor cleanup and a bit more commenting.
> 
> Reading user->locked_shm within shmlock_user_lock would be nice, too.
> 
> Maybe something like this (untested, uncompiled) would do.
> 
> 
> -- wli
> 
> 
> diff --git a/mm/mlock.c b/mm/mlock.c
> index 7b26560..5f51792 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -234,6 +234,12 @@ asmlinkage long sys_munlockall(void)
>  /*
>   * Objects with different lifetime than processes (SHM_LOCK and SHM_HUGETLB
>   * shm segments) get accounted against the user_struct instead.
> + * First, user_shm_lock() checks that the user has permission to lock
> + * enough memory; then if so, the locked shm is accounted to the user's
> + * system-wide state. shmlock_user_lock protects the per-user field
> + * tracking how much locked_shm is in use within the struct user_struct.
> + * shmlock_user_lock is taken early to guard the read-only check that
> + * user->locked_shm is in-bounds against updates to user->locked_shm.
>   */
>  static DEFINE_SPINLOCK(shmlock_user_lock);
> 
> @@ -242,19 +248,22 @@ int user_shm_lock(size_t size, struct user_struct *user)
>  	unsigned long lock_limit, locked;
>  	int allowed = 0;
> 
> +	spin_lock(&shmlock_user_lock);
>  	locked = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
>  	lock_limit = current->signal->rlim[RLIMIT_MEMLOCK].rlim_cur;
>  	if (lock_limit == RLIM_INFINITY)
>  		allowed = 1;
> -	lock_limit >>= PAGE_SHIFT;
> -	spin_lock(&shmlock_user_lock);
> -	if (!allowed &&
> -	    locked + user->locked_shm > lock_limit && !capable(CAP_IPC_LOCK))
> -		goto out;
> -	get_uid(user);
> -	user->locked_shm += locked;
> -	allowed = 1;
> -out:
> +	else {
> +		lock_limit >>= PAGE_SHIFT;
> +		if (locked + user->locked_shm <= lock_limit)
> +			allowed = 1;
> +		else if (capable(CAP_IPC_LOCK))
> +			allowed = 1;
> +	}
> +	if (allowed) {
> +		get_uid(user);
> +		user->locked_shm += locked;
> +	}
>  	spin_unlock(&shmlock_user_lock);
>  	return allowed;
>  }