On Mon, Dec 26, 2016 at 06:06:01PM -0800, Andy Lutomirski wrote: > On Mon, Dec 26, 2016 at 5:54 PM, Kirill A. Shutemov > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > > This patch introduces new rlimit resource to manage maximum virtual > > address available to userspace to map. > > > > On x86, 5-level paging enables 56-bit userspace virtual address space. > > Not all user space is ready to handle wide addresses. It's known that > > at least some JIT compilers use high bit in pointers to encode their > > information. It collides with valid pointers with 5-level paging and > > leads to crashes. > > > > The patch aims to address this compatibility issue. > > > > MM would use min(RLIMIT_VADDR, TASK_SIZE) as upper limit of virtual > > address available to map by userspace. > > > > The default hard limit will be RLIM_INFINITY, which basically means that > > TASK_SIZE limits available address space. > > > > The soft limit will also be RLIM_INFINITY everywhere, but the machine > > with 5-level paging enabled. In this case, soft limit would be > > (1UL << 47) - PAGE_SIZE. It’s current x86-64 TASK_SIZE_MAX with 4-level > > paging which known to be safe > > > > New rlimit resource would follow usual semantics with regards to > > inheritance: preserved on fork(2) and exec(2). This has potential to > > break application if limits set too wide or too narrow, but this is not > > uncommon for other resources (consider RLIMIT_DATA or RLIMIT_AS). > > > > As with other resources you can set the limit lower than current usage. > > It would affect only future virtual address space allocations. > > > > Use-cases for new rlimit: > > > > - Bumping the soft limit to RLIM_INFINITY, allows current process all > > its children to use addresses above 47-bits. > > > > - Bumping the soft limit to RLIM_INFINITY after fork(2), but before > > exec(2) allows the child to use addresses above 47-bits. > > > > - Lowering the hard limit to 47-bits would prevent current process all > > its children to use addresses above 47-bits, unless a process has > > CAP_SYS_RESOURCES. > > > > - It’s also can be handy to lower hard or soft limit to arbitrary > > address. User-mode emulation in QEMU may lower the limit to 32-bit > > to emulate 32-bit machine on 64-bit host. > > I tend to think that this should be a personality or an ELF flag, not > an rlimit. My plan was to implement ELF flag on top. Basically, ELF flag would mean that we bump soft limit to hard limit on exec. > That way setuid works right. Um.. I probably miss background here. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>