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A valid offset by C rules, but to the non-allocated byte one past the end, and UB to actually dereference. You need at least a 2^31+2 byte allocation for 0x80000001 to be a valid offset of an allocated byte.

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Neither GCC or LLVM supports ptrdiff+1 size allocations. Both of them delegate responsibility to the malloc and mmap implementations in libc to report an error for overflow large allocations. Some malloc implementations handle this and others (glibc) are broken with GCC/LLVM.

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Rust explicitly defines isize::MAX as the maximum size object that's permitted. Unsafe code needs to uphold this by making sure not to do it. Some malloc implementations like musl & jemalloc have this check internally but Rust has to check itself with an unknown/generic malloc.

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See gcc.gnu.org/bugzilla/show_ for a thread about this. ptrdiff_t+1 or larger objects are just not permitted with LLVM and GCC. It's not a C standard violation but rather they require that all standard ways of allocating objects (C, POSIX) have checks preventing larger objects.

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I helped fix multiple malloc and libc implementations but there are still common ones like glibc that are broken with GCC/LLVM. It's a bad idea to use objects larger than PTRDIFF_MAX even with a compiler supporting it since x-y will be undefined if it overflows.

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Just to be crystal clear... by "compiler supporting PTRDIFF_MAX + 1", you mean "the compiler assumes an object of PTRDIFF_MAX + 1 can possibly exist" (and gcc/llvm doesn't)?

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Yes, that's what I mean. GCC and LLVM don't support objects larger than PTRDIFF_MAX. It's not seen as a bug but it could be seen as a missing feature. It doesn't appear there's any interest in implementing it though. So, you need a compiler supporting that *and* it's still hard.

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(Honestly, playing with 8088 so much, that gives me some comfort that even on flat addrspace archs, the whole addrspace can't be used for a single object in high-level langs :P)

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twitter.com/DanielMicay/st I guess my follow up is: what do you mean by it's unrealistic to avoid "x - y" when it overflows? The sentence immediately after describes how gcc/llvm do it... it prevents your program from continuing :)!

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Daniel Micay

@DanielMicay

Jul 6, 2021

Replying to @DanielMicay @brouhaha and @iximeow

Even with the C standard semantics, it's unrealistic to avoid x-y when it would overflow. GCC and LLVM don't give you the opportunity to try to use it correctly. It just isn't supported. It's one of many rules they don't really bother to document. It's how they intend it to work.

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x - y overflow for pointers is undefined behavior but it's incredibly common to do it with arbitrary slices, etc. in both C and Rust. You can do things in a tricky way with uintptr_t casts but it has an impact on optimization / code generation.

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It's simply unrealistic to require that anything doing x-y either enforces that the object is PTRDIFF_MAX or performs the operation indirectly rather than via normal pointer arithmetic.

9:36 PM · Jul 6, 2021Twitter Web App

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The only realistic approach is limiting object size to PTRDIFF_MAX, which is required by GCC/LLVM anyway since larger objects are broken in other assorted cases beyond what the C standard specifies as UB. What they do isn't broken, they just require a runtime/stdlib enforcing it.