On Thu, Jan 5, 2017 at 11:39 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > On 01/05/2017 11:29 AM, Kirill A. Shutemov wrote: >> On Thu, Jan 05, 2017 at 11:13:57AM -0800, Dave Hansen wrote: >>> On 12/26/2016 05:54 PM, Kirill A. Shutemov wrote: >>>> MM would use min(RLIMIT_VADDR, TASK_SIZE) as upper limit of virtual >>>> address available to map by userspace. >>> >>> What happens to existing mappings above the limit when this upper limit >>> is dropped? >> >> Nothing: we only prevent creating new mappings. All existing are not >> affected. >> >> The semantics here the same as with other resource limits. >> >>> Similarly, why do we do with an application running with something >>> incompatible with the larger address space that tries to raise the >>> limit? Say, legacy MPX. >> >> It has to know what it does. Yes, it can change limit to the point where >> application is unusable. But you can to the same with other limits. > > I'm not sure I'm comfortable with this. Do other rlimit changes cause > silent data corruption? I'm pretty sure doing this to MPX would. > What actually goes wrong in this case? That is, what combination of MPX setup of subsequent allocations will cause a problem, and is the problem worse than just a segfault? IMO it would be really nice to keep the messy case confined to MPX. FWIW, this problem is kind of generic. If you run code in a process, MPX or otherwise, that assumes something about pointer values and then create a pointer that violates its assumptions, you will cause problems. For example, some VMs use high bits to store metadata. If you feed a pointer that's too big to such code, boom. This is exactly why high addresses need to be opt-in. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
