lkml.org 
[lkml]   [2017]   [Jun]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
Subject[PATCH 0/1] expand_downwards: don't require the gap if !vm_prev
See the patch, but actually I have another question...

Now that the stack-guard-page has gone, why do we need to allow to grow
into the previous VM_GROWSDOWN vma? IOW, why we can not simply remove
the VM_GROWSDOWN check in expand_downwards() ?

Yes, this is what the kernel did before the recent changes. But afaics
only because the kernel could not know if the vma->vm_start page is
actually guard or not.

IOW, iiuc before the recent change it was not simple to _disallow_ this,
and that is why it worked. Just for example, suppose an application does

addr = mmap(MAP_GROWSDOWN);
mprotect(addr, PAGE_SIZE, PROT_NONE);
*(addr + PAGE_SIZE) = 0;

and of course this should not fail.

But the the kernel could not know if vm_start == addr + PAGE_SIZE is the
"valid" address, or this vma was expanded before and vm_start is the stack
guard.

Yes, we can probably check anon_vma's as the comment suggests, but imo we
we can just remove the VM_GROWSDOWN case unconditionally.

Oleg.

\
 
 \ /
  Last update: 2017-06-28 19:53    [W:0.136 / U:0.248 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site