3 ms·

With a safe runtime, a pointer is really something like a `struct { u32 allocation_base, allocation_offset;}`. (it may be worth doing fancy variable-bit-width m

by o11c 2y ago

With a safe runtime, a pointer is really something like a `struct { u32 allocation_base, allocation_offset;}`. (it may be worth doing fancy variable-bit-width math to allow many small allocations but only a few large ones; it's also likely worth it to have a dedicated "leaf" section of memory that is intended not to contain any pointers) An implementation of `sort` would start with: `assert (begin.allocation_base == end.allocation_base)`. Most likely, this would be implicit when `end - begin` or `begin < end` is called (but not `!=`, which is well-defined between unrelated pointers). If we ignore the uninitialized data (which is not the same kind of UB, and usually not interesting), the `strlen` loop would assert when, after not encountering a NUL, `s.allocation_offset` exceeds `100` (which is known in the allocation metadata).

tsimionescu 2y ago
That makes sense, thank you! I'd bet that there are corners of non-UB vlaid programs that would be sensitive to the fact that pointers are no longer simple numbers, but maybe that's wrong or at least could be worked around. I would add that to make this memory safe for multi-threaded programs, you also need to implcitly synchronize on fat pointer accesses, to prevent corrupted pointers from forming.
badmintonbaseba 2y ago
Tracking allocations is necessary, but not sufficient. struct S { int arr1[100]; std::string s; int arr2[100]; }; void foo(S& s) { //arr1 and arr2 are in the same complete object //so they are in the same allocation std::sort(std::begin(arr1), std::end(arr2)); } To make it sound you really need to track arrays specifically (including implicit single-element ones), not just allocations. It's surely somewhat feasible, as the constexpr interpreters in compilers do track it, but something like that would probably be way inefficient at runtime.
- o11c 2y ago
  (if `sizeof(char *)` != `sizeof(int)` it will be detected as an illegal memory access, since at a minimum every word is tagged whether it's a pointer or not. Otherwise ...) That's really an aliasing problem, not specific to arrays. The tricky part about tagging memory with a full type is that a lot of code, even in standards, relies on aliasing that's supposed to be forbidden. Still, even if the `sort` isn't detected (due to the user insisting on permissiveness rather than adding annotations), it would still detect any attempt to use `s` afterward. As for a limited array-specific solution ... I can't think of one that would handle all variants of `sort((int *)&s[0], (int *)&s[1])`. And do we want to forbid `sort(&s.arr[0], (int *)&s.s)`?
gpderetta 2y ago
one day we will bring segments and far pointers back.
- zozbot234 2y ago
  CHERI is pretty much there already

by o11c 2y ago

tsimionescu 2y ago

That makes sense, thank you! I'd bet that there are corners of non-UB vlaid programs that would be sensitive to the fact that pointers are no longer simple numbers, but maybe that's wrong or at least could be worked around. I would add that to make this memory safe for multi-threaded programs, you also need to implcitly synchronize on fat pointer accesses, to prevent corrupted pointers from forming.

badmintonbaseba 2y ago

Tracking allocations is necessary, but not sufficient. struct S { int arr1[100]; std::string s; int arr2[100]; }; void foo(S& s) { //arr1 and arr2 are in the same complete object //so they are in the same allocation std::sort(std::begin(arr1), std::end(arr2)); } To make it sound you really need to track arrays specifically (including implicit single-element ones), not just allocations. It's surely somewhat feasible, as the constexpr interpreters in compilers do track it, but something like that would probably be way inefficient at runtime.

o11c 2y ago
(if `sizeof(char *)` != `sizeof(int)` it will be detected as an illegal memory access, since at a minimum every word is tagged whether it's a pointer or not. Otherwise ...) That's really an aliasing problem, not specific to arrays. The tricky part about tagging memory with a full type is that a lot of code, even in standards, relies on aliasing that's supposed to be forbidden. Still, even if the `sort` isn't detected (due to the user insisting on permissiveness rather than adding annotations), it would still detect any attempt to use `s` afterward. As for a limited array-specific solution ... I can't think of one that would handle all variants of `sort((int *)&s[0], (int *)&s[1])`. And do we want to forbid `sort(&s.arr[0], (int *)&s.s)`?

gpderetta 2y ago

one day we will bring segments and far pointers back.

zozbot234 2y ago
CHERI is pretty much there already