On Tue, Aug 08, 2023 at 09:21:03AM +0100, Szabolcs Nagy wrote: > The 08/07/2023 14:00, Mark Brown wrote: > > That's not what the manual page or a quick check of the code suggest > > that mmap() does, they say that the kernel just takes it as a hint and > i should have said that i expect MAP_FIXED_NOREPLACE semantics > (since the x86 code seemed to use that) and then the mapped > address must match exactly thus page aligned. Ah, I see. We do pass MAP_FIXED_NOREPLACE when allocating the stack if an address was specified but currently leave it up to the VM subsystem to figure out what to do with the address. I don't immediately see where mmap() enforces this requirement, but I have to admit I didn't look overly hard. I don't see a problem with enforcing a PAGE_SIZE constraint here. > > > > + if (size == 16 || size % 16) > > > > + return -EINVAL; > > > why %16 and not %8 ? > > I don't think that's needed any more - there was some stuff in an > > earlier version of the code but no longer. > it's kind of important to know the exact logic so the cap token > location can be computed in userspace for arbitrary size. > (this is why i wanted to see the map_shadow_stack man page first > but i was told that comes separately on linux..) Right, I'd already changed it to % 8 in the version I posted yesterday.
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