On 26/03/2015 10:43, Ingo Molnar wrote:
>
> * Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote:
>
>> On Wed, 2015-03-25 at 19:36 +0100, Ingo Molnar wrote:
>>> * Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>>>
>>>>> +#define __HAVE_ARCH_REMAP
>>>>> +static inline void arch_remap(struct mm_struct *mm,
>>>>> + unsigned long old_start, unsigned long old_end,
>>>>> + unsigned long new_start, unsigned long new_end)
>>>>> +{
>>>>> + /*
>>>>> + * mremap() doesn't allow moving multiple vmas so we can limit the
>>>>> + * check to old_start == vdso_base.
>>>>> + */
>>>>> + if (old_start == mm->context.vdso_base)
>>>>> + mm->context.vdso_base = new_start;
>>>>> +}
>>>>
>>>> mremap() doesn't allow moving multiple vmas, but it allows the
>>>> movement of multi-page vmas and it also allows partial mremap()s,
>>>> where it will split up a vma.
>>>
>>> I.e. mremap() supports the shrinking (and growing) of vmas. In that
>>> case mremap() will unmap the end of the vma and will shrink the
>>> remaining vDSO vma.
>>>
>>> Doesn't that result in a non-working vDSO that should zero out
>>> vdso_base?
>>
>> Right. Now we can't completely prevent the user from shooting itself
>> in the foot I suppose, though there is a legit usage scenario which
>> is to move the vDSO around which it would be nice to support. I
>> think it's reasonable to put the onus on the user here to do the
>> right thing.
>
> I argue we should use the right condition to clear vdso_base: if the
> vDSO gets at least partially unmapped. Otherwise there's little point
> in the whole patch: either correctly track whether the vDSO is OK, or
> don't ...
That's a good option, but it may be hard to achieve in the case the vDSO
area has been splitted in multiple pieces.
Not sure there is a right way to handle that, here this is a best
effort, allowing a process to unmap its vDSO and having the sigreturn
call done through the stack area (it has to make it executable).
Anyway I'll dig into that, assuming that the vdso_base pointer should be
clear if a part of the vDSO is moved or unmapped. The patch will be
larger since I'll have to get the vDSO size which is private to the
vdso.c file.
> There's also the question of mprotect(): can users mprotect() the vDSO
> on PowerPC?
Yes, mprotect() the vDSO is allowed on PowerPC, as it is on x86, and
certainly all the other architectures.
Furthermore, if it is done on a partial part of the vDSO it is splitting
the vma...
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