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SubjectRe: [PATCH] ipc,shm: disable shmmax and shmall by default
On 04/11/2014 10:27 PM, Davidlohr Bueso wrote:
> On Fri, 2014-04-11 at 20:28 +0200, Manfred Spraul wrote:
>> Hi Davidlohr,
>>
>> On 04/03/2014 02:20 AM, Davidlohr Bueso wrote:
>>> The default size for shmmax is, and always has been, 32Mb.
>>> Today, in the XXI century, it seems that this value is rather small,
>>> making users have to increase it via sysctl, which can cause
>>> unnecessary work and userspace application workarounds[1].
>>>
>>> [snip]
>>> Running this patch through LTP, everything passes, except the following,
>>> which, due to the nature of this change, is quite expected:
>>>
>>> shmget02 1 TFAIL : call succeeded unexpectedly
>> Why is this TFAIL expected?
> So looking at shmget02.c, this is the case that fails:
>
> for (i = 0; i < TST_TOTAL; i++) {
> /*
> * Look for a failure ...
> */
>
> TEST(shmget(*(TC[i].skey), TC[i].size, TC[i].flags));
>
> if (TEST_RETURN != -1) {
> tst_resm(TFAIL, "call succeeded unexpectedly");
> continue;
> }
>
> Where TC[0] is:
> struct test_case_t {
> int *skey;
> int size;
> int flags;
> int error;
> } TC[] = {
> /* EINVAL - size is 0 */
> {
> &shmkey2, 0, IPC_CREAT | IPC_EXCL | SHM_RW, EINVAL},
>
> So it's expected because now 0 is actually valid. And before:
>
> EINVAL A new segment was to be created and size < SHMMIN or size > SHMMAX
>
>>> diff --git a/ipc/shm.c b/ipc/shm.c
>>> index 7645961..ae01ffa 100644
>>> --- a/ipc/shm.c
>>> +++ b/ipc/shm.c
>>> @@ -490,10 +490,12 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
>>> int id;
>>> vm_flags_t acctflag = 0;
>>>
>>> - if (size < SHMMIN || size > ns->shm_ctlmax)
>>> + if (ns->shm_ctlmax &&
>>> + (size < SHMMIN || size > ns->shm_ctlmax))
>>> return -EINVAL;
>>>
>>> - if (ns->shm_tot + numpages > ns->shm_ctlall)
>>> + if (ns->shm_ctlall &&
>>> + ns->shm_tot + numpages > ns->shm_ctlall)
>>> return -ENOSPC;
>>>
>>> shp = ipc_rcu_alloc(sizeof(*shp));
>> Ok, I understand it:
>> Your patch disables checking shmmax, shmall *AND* checking for SHMMIN.
> Right, if shmmax is 0, then there's no point checking for shmmin,
> otherwise we'd always end up returning EINVAL.
>
>> a) Have you double checked that 0-sized shm segments work properly?
>> Does the swap code handle it properly, ...? EINVAL A new segment was to be created and size < SHMMIN or size > SHMMAX
> Hmm so I've been using this patch just fine on my laptop since I sent
> it. So far I haven't seen any issues. Are you refering to something in
> particular? I'd be happy to run any cases you're concerned with.
I'm thinking about malicious applications.
Create 0-sized segments and then map them. Does find_vma_intersection
handle that case?
The same for all other functions that are called by the shm code.

You can't replace code review by "runs for a month"
>> b) It's that yet another risk for user space incompatibility?
> Sorry, I don't follow here.
Applications expect that shmget(,0,) fails.

--
Manfred


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