On Mon, Jan 2, 2017 at 12:35 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > 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 There is another proposal: https://fedoraproject.org/wiki/Toolchain/Watermark#Markup_for_ELF_objects which covers much more than mine. -- H.J. -- 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
