Messages in this thread | | | Date | Sat, 20 Jul 2013 17:22:46 +0200 | From | Oleg Nesterov <> | Subject | [PATCH v2 0/1] mm: shift VM_GROWS* check from mmap_region() to do_mmap_pgoff() |
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Andrew,
This patch replaces the buggy
mm-shift-vm_grows-check-from-mmap_region-to-do_mmap_pgoff.patch
I do not send the *-fix.patch because I'd like to update the changelog.
However it needs the explicit ack from Hugh.
On 07/19, Hugh Dickins wrote: > > On Tue, 16 Jul 2013, Andrew Morton wrote: > > On Sun, 14 Jul 2013 18:54:51 +0200 Oleg Nesterov <oleg@redhat.com> wrote: > > > > > mmap() doesn't allow the non-anonymous mappings with VM_GROWS* bit set. > > > In particular this means that mmap_region()->vma_merge(file, vm_flags) > > > must always fail if vm_flags & VM_GROWS. > > I didn't understand that sentence: if file is non-NULL perhaps?
Yes, this looks confusing, sorry.
I meant, vma_merge() must fail if "vm_flags & VM_GROWS" is set incorrectly. is_mergeable_vma() compares both vm_file and vm_flags.
Even if file == NULL (at this stage), "VM_SHARED | VM_GROWS" is not correct too, and vma_merge() can't succeed.
And, to clarify, I only mentioned this because I tried to convince myself that this change (if correct) can't make any difference except "avoid the not-really-correct do_munmap".
> > > So it does not make sense to > > > check VM_GROWS* after we already allocated the new vma, the only caller, > > > do_mmap_pgoff(), which can pass this flag can do the check itself. > > > > > > And this looks a bit more correct, mmap_region() already unmapped the > > > old mapping at this stage. But if mmap() is going to fail, it should > > > avoid do_munmap() if possible. > > I agree with the sentiment, but the patch looks wrong to me.
Heh. You are right of course.
> It did need staring, yes, but it looks NOK to me: this change permits > mmap(addr, len, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_GROWSDOWN, fd, off) > where fd is for a real file:
And note that "Only MAP_PRIVATE|MAP_ANONYMOUS can use MAP_GROWS" even tries to document that "MAP_PRIVATE && file" is not allowed too.
I have no idea how I managed to forget that MAP_PRIVATE never sets VM_MAYSHARE.
Thanks a lot Hugh.
Oleg.
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