Messages in this thread | | | Subject | Re: [v3 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct | From | Yang Shi <> | Date | Tue, 10 Apr 2018 12:33:35 -0700 |
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On 4/10/18 12:17 PM, Cyrill Gorcunov wrote: > On Tue, Apr 10, 2018 at 11:28:13AM -0700, Yang Shi wrote: >>> At the first glance, it looks feasible to me. Will look into deeper >>> later. >> A further look told me this might be *not* feasible. >> >> It looks the new lock will not break check_data_rlimit since in my patch >> both start_brk and brk is protected by mmap_sem. The code flow might look >> like below: >> >> CPU A CPU B >> -------- -------- >> prctl sys_brk >> down_write >> check_data_rlimit check_data_rlimit (need mm->start_brk) >> set brk >> down_write up_write >> set start_brk >> set brk >> up_write >> >> If CPU A gets the mmap_sem first, it will set start_brk and brk, then CPU B >> will check with the new start_brk. And, prctl doesn't care if sys_brk is run >> before it since it gets the new start_brk and brk from parameter. >> >> If we protect start_brk and brk with the new lock, sys_brk might get old >> start_brk, then sys_brk might break rlimit check silently, is that right? >> >> So, it looks using new lock in prctl and keeping mmap_sem in brk path has >> race condition. > I fear so. The check_data_rlimit implies that all elements involved into > validation (brk, start_brk, start_data, end_data) are not changed unpredicably > until written back into mm. In turn if we guard start_brk,brk only (as > it is done in the patch) the check_data_rlimit may pass on wrong data > I think. And as you mentioned the race above exact the example of such > situation. I think for prctl case we can simply left use of mmap_sem > as it were before the patch, after all this syscall is really in cold > path all the time.
The race condition is just valid when protecting start_brk, brk, start_data and end_data with the new lock, but keep using mmap_sem in brk path.
So, we should just need make a little tweak to have mmap_sem protect start_brk, brk, start_data and end_data, then have the new lock protect others so that we still can remove mmap_sem in proc as the patch is aimed to do.
Yang
> > Cyrill
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