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. That way setuid works right. -- 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
