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mm/mmap.c: expand_downwards: don't require the gap if !vm_prev 

expand_stack(vma) fails if address < stack_guard_gap even if there is no
vma->vm_prev.  I don't think this makes sense, and we didn't do this
before the recent commit 1be7107fbe ("mm: larger stack guard gap,
between vmas").

We do not need a gap in this case, any address is fine as long as
security_mmap_addr() doesn't object.

This also simplifies the code, we know that address >= prev->vm_end and
thus underflow is not possible.

Link: http://lkml.kernel.org/r/20170628175258.GA24881@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Larry Woodman <lwoodman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2017-07-10 15:49:54 -07:00 committed by Linus Torvalds
parent 561b5e0709
commit 32e4e6d5cb
1 changed files with 3 additions and 7 deletions
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10
mm/mmap.c
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@ -2316,7 +2316,6 @@ int expand_downwards(struct vm_area_struct *vma,
{
struct mm_struct *mm = vma->vm_mm;
struct vm_area_struct *prev;
unsigned long gap_addr;
int error;
address &= PAGE_MASK;
@ -2325,15 +2324,12 @@ int expand_downwards(struct vm_area_struct *vma,
return error;
/* Enforce stack_guard_gap */
gap_addr = address - stack_guard_gap;
if (gap_addr > address)
return -ENOMEM;
prev = vma->vm_prev;
if (prev && prev->vm_end > gap_addr &&
/* Check that both stack segments have the same anon_vma? */
if (prev && !(prev->vm_flags & VM_GROWSDOWN) &&
(prev->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) {
if (!(prev->vm_flags & VM_GROWSDOWN))
if (address - prev->vm_end < stack_guard_gap)
return -ENOMEM;
/* Check that both stack segments have the same anon_vma? */
}
/* We must make sure the anon_vma is allocated. */