On 12/26/2016 09:24 PM, Kirill A. Shutemov wrote: > 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. Could you clarify what you mean by an "ELF flag?" -- Cheers, Carlos. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
