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Re: [PATCH v2] Fix PR64242


Jakub Jelinek wrote:
On Fri, Dec 07, 2018 at 04:19:22PM +0000, Wilco Dijkstra wrote:

>> The test case doesn't need an aligned object to fail, so why did you add it?
> It needed it on i686, because otherwise it happened to see the value it
> wanted in the caller's stack frame.

Right, so I fixed that by increasing the size of the frame in broken_setjmp to be
larger than the frame in main, so it's now extremely unlikely to accidentally read
from a random stack location and end up with a valid stack pointer.

> >> +  /* Compute expected next alloca offset - some targets don't align properly
> >> +     and allocate too much.  */
> >> +  p = q + (q - p);
> >
> > This is UB, pointer difference is only defined within the same object.
> > So, you can only do such subtraction in some integral type rather than as
> > pointer subtraction. 
> __builtin_setjmp is already undefined behaviour, and the stack corruption is
> even more undefined - trying to avoid harmless theoretical undefined behaviour
> wouldn't be helpful.

> No, __builtin_setjmp is a GNU extension, not undefined behavior.  

Well the evidence is that it's undocumented, unspecified and causes undefined

> And 
> something that is UB and might be harmless today might be harmful tomorrow,
> gcc optimizes heavily on the assumption that UB doesn't happen in the
> program, so might optimize that subtraction to 0 or 42 or whatever else.
>> > And I'm not sure you have a guarantee that every zero sized alloca is at the
>> > same offset from the previous one.
>> The above pointer adjustment handles the case where alloca overallocates.
>> It passes on x86-64 which always adds 8 unnecessary bytes.
> What guarantee is there that it overallocates each time the same though?

How could it not be? It could only vary if it was reading an uninitialized register or
adding a random extra amount as a form of ASLR. But there is no point in trying
to support future unknown features/bugs since it will give false negatives today.


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