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.
