[PATCH 0/1] expand_downwards: don't require the gap if !vm_prev
From: Oleg Nesterov
Date: Wed Jun 28 2017 - 13:52:51 EST
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.