On Fri, Dec 30, 2016 at 06:08:27PM -0800, Andy Lutomirski wrote: > 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. + H.J. There's discussion of proposal of "Program Properties"[1]. It seems fits the purpose. [1] https://sourceware.org/ml/gnu-gabi/2016-q4/msg00000.html -- Kirill A. Shutemov -- 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
