Hello,
execve() system call should support argument length of
MAX_ARG_STRLEN (PAGE_SIZE * 32). On 64K page size systems, we
are not able to pass 32 * PAGE_SIZE arguments into the execve()
system call because of the following reasons.
* struct linux_binprm's vma starts with a size of PAGE_SIZE
vma->vm_end = STACK_TOP_MAX;
vma->vm_start = vma->vm_end - PAGE_SIZE;
* The VMA expands as much depending upon the argument size. So
for 32 * PAGE_SIZE argument, it becomes 33 * PAGE_SIZE.
* 33 * PAGE_SIZE with 64K pages fails the following test in
get_arg_page() function. 33 * PAGE_SIZE is more than 2MB
(8 MB /4) with 64K page size.
if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
* Right now RLIMIT_STACK is hard coded 8MB which does not take
PAGE_SIZE into account.
Wondering what should be the solution for this problem ?
* Change the default stack size from 8MB ?
* Change the ratio test from 1/4th ?
Thoughts ?
Regards
Anshuman
On Fri 30-06-17 11:59:37, Anshuman Khandual wrote:
> Hello,
>
> execve() system call should support argument length of
> MAX_ARG_STRLEN (PAGE_SIZE * 32). On 64K page size systems, we
> are not able to pass 32 * PAGE_SIZE arguments into the execve()
> system call because of the following reasons.
>
> * struct linux_binprm's vma starts with a size of PAGE_SIZE
>
> vma->vm_end = STACK_TOP_MAX;
> vma->vm_start = vma->vm_end - PAGE_SIZE;
>
> * The VMA expands as much depending upon the argument size. So
> for 32 * PAGE_SIZE argument, it becomes 33 * PAGE_SIZE.
>
> * 33 * PAGE_SIZE with 64K pages fails the following test in
> get_arg_page() function. 33 * PAGE_SIZE is more than 2MB
> (8 MB /4) with 64K page size.
>
> if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
>
> * Right now RLIMIT_STACK is hard coded 8MB which does not take
> PAGE_SIZE into account.
>
> Wondering what should be the solution for this problem ?
>
> * Change the default stack size from 8MB ?
just increase the ulimit if you want to use such a large arguments.
> * Change the ratio test from 1/4th ?
--
Michal Hocko
SUSE Labs
On 06/30/2017 07:52 PM, Michal Hocko wrote:
> On Fri 30-06-17 11:59:37, Anshuman Khandual wrote:
>> Hello,
>>
>> execve() system call should support argument length of
>> MAX_ARG_STRLEN (PAGE_SIZE * 32). On 64K page size systems, we
>> are not able to pass 32 * PAGE_SIZE arguments into the execve()
>> system call because of the following reasons.
>>
>> * struct linux_binprm's vma starts with a size of PAGE_SIZE
>>
>> vma->vm_end = STACK_TOP_MAX;
>> vma->vm_start = vma->vm_end - PAGE_SIZE;
>>
>> * The VMA expands as much depending upon the argument size. So
>> for 32 * PAGE_SIZE argument, it becomes 33 * PAGE_SIZE.
>>
>> * 33 * PAGE_SIZE with 64K pages fails the following test in
>> get_arg_page() function. 33 * PAGE_SIZE is more than 2MB
>> (8 MB /4) with 64K page size.
>>
>> if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
>>
>> * Right now RLIMIT_STACK is hard coded 8MB which does not take
>> PAGE_SIZE into account.
>>
>> Wondering what should be the solution for this problem ?
>>
>> * Change the default stack size from 8MB ?
> just increase the ulimit if you want to use such a large arguments.
>
Yeah that is possible but it does not still offset the fact that
the calculation is broken on the page size of 64K. I mean, yeah
its not practical to have such a large argument. But the point
is whether we would want to support the MAX_ARG_STRLEN semantic
for execve system call or not. At present its broken for 64K
and I am asking whether we will be willing to revisit the
'1/4th of the stack' condition.
On Mon 03-07-17 13:58:59, Anshuman Khandual wrote:
> On 06/30/2017 07:52 PM, Michal Hocko wrote:
> > On Fri 30-06-17 11:59:37, Anshuman Khandual wrote:
> >> Hello,
> >>
> >> execve() system call should support argument length of
> >> MAX_ARG_STRLEN (PAGE_SIZE * 32). On 64K page size systems, we
> >> are not able to pass 32 * PAGE_SIZE arguments into the execve()
> >> system call because of the following reasons.
> >>
> >> * struct linux_binprm's vma starts with a size of PAGE_SIZE
> >>
> >> vma->vm_end = STACK_TOP_MAX;
> >> vma->vm_start = vma->vm_end - PAGE_SIZE;
> >>
> >> * The VMA expands as much depending upon the argument size. So
> >> for 32 * PAGE_SIZE argument, it becomes 33 * PAGE_SIZE.
> >>
> >> * 33 * PAGE_SIZE with 64K pages fails the following test in
> >> get_arg_page() function. 33 * PAGE_SIZE is more than 2MB
> >> (8 MB /4) with 64K page size.
> >>
> >> if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
> >>
> >> * Right now RLIMIT_STACK is hard coded 8MB which does not take
> >> PAGE_SIZE into account.
> >>
> >> Wondering what should be the solution for this problem ?
> >>
> >> * Change the default stack size from 8MB ?
> > just increase the ulimit if you want to use such a large arguments.
> >
>
> Yeah that is possible but it does not still offset the fact that
> the calculation is broken on the page size of 64K. I mean, yeah
> its not practical to have such a large argument. But the point
> is whether we would want to support the MAX_ARG_STRLEN semantic
> for execve system call or not. At present its broken for 64K
> and I am asking whether we will be willing to revisit the
> '1/4th of the stack' condition.
I dunno. We have this 1/4 of RLIMIT semantic for years and it doesn't
seem there were any bug reports. Yes, MAX_ARG_STRLEN being PAGE_SIZE
dependent is unfortunate because it makes an arch independent default
ulimit hard to get right but I am not sure we actually have to lose
sleep over this.
Or do you have any specific proposal how to "fix" this limitation which
wouldn't break other userspace?
--
Michal Hocko
SUSE Labs
On 07/03/2017 02:51 PM, Michal Hocko wrote:
> On Mon 03-07-17 13:58:59, Anshuman Khandual wrote:
>> On 06/30/2017 07:52 PM, Michal Hocko wrote:
>>> On Fri 30-06-17 11:59:37, Anshuman Khandual wrote:
>>>> Hello,
>>>>
>>>> execve() system call should support argument length of
>>>> MAX_ARG_STRLEN (PAGE_SIZE * 32). On 64K page size systems, we
>>>> are not able to pass 32 * PAGE_SIZE arguments into the execve()
>>>> system call because of the following reasons.
>>>>
>>>> * struct linux_binprm's vma starts with a size of PAGE_SIZE
>>>>
>>>> vma->vm_end = STACK_TOP_MAX;
>>>> vma->vm_start = vma->vm_end - PAGE_SIZE;
>>>>
>>>> * The VMA expands as much depending upon the argument size. So
>>>> for 32 * PAGE_SIZE argument, it becomes 33 * PAGE_SIZE.
>>>>
>>>> * 33 * PAGE_SIZE with 64K pages fails the following test in
>>>> get_arg_page() function. 33 * PAGE_SIZE is more than 2MB
>>>> (8 MB /4) with 64K page size.
>>>>
>>>> if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
>>>>
>>>> * Right now RLIMIT_STACK is hard coded 8MB which does not take
>>>> PAGE_SIZE into account.
>>>>
>>>> Wondering what should be the solution for this problem ?
>>>>
>>>> * Change the default stack size from 8MB ?
>>> just increase the ulimit if you want to use such a large arguments.
>>>
>>
>> Yeah that is possible but it does not still offset the fact that
>> the calculation is broken on the page size of 64K. I mean, yeah
>> its not practical to have such a large argument. But the point
>> is whether we would want to support the MAX_ARG_STRLEN semantic
>> for execve system call or not. At present its broken for 64K
>> and I am asking whether we will be willing to revisit the
>> '1/4th of the stack' condition.
>
> I dunno. We have this 1/4 of RLIMIT semantic for years and it doesn't
> seem there were any bug reports. Yes, MAX_ARG_STRLEN being PAGE_SIZE
> dependent is unfortunate because it makes an arch independent default
> ulimit hard to get right but I am not sure we actually have to lose
> sleep over this.
I understand your point.
>
> Or do you have any specific proposal how to "fix" this limitation which
> wouldn't break other userspace?
There are three variables here MAX_ARG_STRLEN, RLIMIT_STACK and the 25%
condition. Execve() is supporting MAX_ARG_STRLEN for a long time, hence
it cannot be changed now. That leaves us to change either the default
RLIMIT_STACK value or the 25% condition. Both are kernel internal
implementation. But I am not sure how changing them might affect any
other userspace behavior, hence asking for suggestions. I just wanted
to explore the possibilities of a fix here.