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
