On Fri, Dec 30, 2016 at 7:56 AM, Dmitry Safonov [off-list ref] 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 [off-list ref]
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.
We should also probably come up with a reasonable set of getters and
setters for CRIU and otherwise. I can think of the following
questions that might be asked:
- What is the highest currently mapped VA? (/proc can already answer
this. If needed, a prctl could be added, too. Using /proc is a bit42 more lines