On Wed, Dec 28, 2016 at 6:53 PM, Carlos O'Donell <carlos@xxxxxxxxxx> wrote: > 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?" Some way to mark a binary as supporting a larger address space. I don't have a precise solution in mind, but an ELF note might be a good way to go here. --Andy -- 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
