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

Hi  Adam,

If this condition check is not included, the root user have to use the function setrlimit() to set the lock_limit of a normal user to RLIM_INFINITY. I think the /proc interface 'hugetlb_shm_group' is introduced to avoid these difficulties.

Please correct me, if I am wrong.

Regarding the problem with the 'if' condition, I feel that even in the case of

user's lock_limit is set to unlimited, he could use unlimited hugepages and
normal page shm segments. So what is the advantage in this scenario.

I tried to avoid the #ifdef statements. But the variable sysctl_hugetlb_shm_group is defined in fs/hugetlbfs/inode.c, this segment is enabled only when the config parameter CONFIG_HUGETLBFS is set to yes. If the hugetlbfs is not selected while configuring,

there would be a compilation error.

Is there any better way so that the root user can configure the gid in 'hugetlb_shm_group'

and the user is able to access the huge pages using shmget().

Thanks
Ciju

aglitke wrote:

Hi Ciju:

I am still not exactly sure why this patch is needed.  As I read
user_shm_lock():

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;

... 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.

On Wed, 2007-11-14 at 19:45 +0530, Ciju Rajan K wrote:
<snip>

@@ -248,8 +249,14 @@ int user_shm_lock(size_t size, struct us
         allowed = 1;
     lock_limit >>= PAGE_SHIFT;
     spin_lock(&shmlock_user_lock);
+#ifdef CONFIG_HUGETLB_PAGE
+    if (!allowed &&
+        locked + user->locked_shm > lock_limit &&
+        (!(capable(CAP_IPC_LOCK) || in_group_p(sysctl_hugetlb_shm_group))))

This will allow any user in hugetlb_shm_group to make unlimited use of
huge page shm segments _and_ normal page shm segments.  Definitely not
what you want.

+#else
     if (!allowed &&
         locked + user->locked_shm > lock_limit && !capable(CAP_IPC_LOCK))
+#endif
         goto out;
     get_uid(user);
     user->locked_shm += locked;

Please don't add new #ifdefs into .c files, headers only.

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• References:
  • [RFC] [PATCH] hugetlbfs :shmget with SHM_HUGETLB only works as root
    • From: Ciju Rajan K <[email protected]>
  • Re: [RFC] [PATCH] hugetlbfs :shmget with SHM_HUGETLB only works as root
    • From: aglitke <[email protected]>

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