On Fri, Dec 30, 2016 at 06:11:05PM -0800, Andy Lutomirski wrote: > On Fri, Dec 30, 2016 at 7:56 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote: > > Keep task's virtual address space size as mm_struct field which > > exists for a long time - it's initialized in setup_new_exec() > > depending on the new task's personality. > > This way TASK_SIZE will always be the same as current->mm->task_size. > > Previously, there could be an issue about different values of > > TASK_SIZE and current->mm->task_size: e.g, a 32-bit process can unset > > ADDR_LIMIT_3GB personality (with personality syscall) and > > so TASK_SIZE will be 4Gb, which is larger than mm->task_size = 3Gb. > > As TASK_SIZE *and* current->mm->task_size are used both in code > > frequently, this difference creates a subtle situations, for example: > > one can mmap addresses > 3Gb, but they will be hidden in > > /proc/pid/pagemap as it checks mm->task_size. > > I've moved initialization of mm->task_size earlier in setup_new_exec() > > as arch_pick_mmap_layout() initializes mmap_legacy_base with > > TASK_UNMAPPED_BASE, which depends on TASK_SIZE. > > I don't like this patch so much because I think that we should figure > out how this will all work in the long run first. I've added some > more people to the thread because other arches have similar issues and > because x86 is about to get considerably more complicated (choices > include 3GB, 4GB, 47-bit, and 56-bit (the latter IIRC)). > > Here are a few of my thoughts on the matter. This isn't all that well > thought out: > > The address space limit, especially if CRIU is in play, isn't really a > hard limit. For example, you could allocate high memory then lower > the limit. Similarly, I see no reason that an x32 program should be > forbidden from mapping some high addresses or, similarly, that an i386 > program can't (if it really wanted to) do a 64-bit mmap() and get a > high address. > > On that note, can we just *delete* the task_size check from pagemap? > It's been there since the very beginning: > > commit 85863e475e59afb027b0113290e3796ee6020b7d > Author: Matt Mackall <mpm@xxxxxxxxxxx> > Date: Mon Feb 4 22:29:04 2008 -0800 > > maps4: add /proc/pid/pagemap interface > > and there's no explanation for why it's needed. > > So maybe we should have a *number* (not a bit) that indicates the > maximum address that mmap() will return unless an override is in use. > Since common practice seems to be to stick this in the personality > field, we may need some fancy encoding. Executing a setuid binary > needs to reset to the default, and personality handles that. If we want to be able to specify arbitrary address as maximum, a fancy encoding would need to claim 51 bits (63 VA - 12 in-page address) on x86 from the persona flag. To me, it's stretching personality interface too far. Maybe it's easier to reset the rlimit for suid binaries? -- 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
