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Comment by charleslmunger

6 months ago

Out of curiosity, does this idiom work in fil-c?

https://github.com/protocolbuffers/protobuf/blob/cb873c8987d...

      // This somewhat silly looking add-and-subtract behavior provides provenance
      // from the original input buffer's pointer. After optimization it produces
      // the same assembly as just casting `(uintptr_t)ptr+input_delta`
      // https://godbolt.org/z/zosG88oPn
      size_t position =
      (uintptr_t)ptr + e->input_delta - (uintptr_t)e->buffer_start;
      return e->buffer_start + position;

It does use the implementation defined behavior that a char pointer + 1 casted to uintptr is the same as casting to uintptr then adding 1.

5 comments

charleslmunger

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pizlonator  6 months ago

Yeah that should just work

Code that strives to preserve provenance works in Fil-C

  • charleslmunger  6 months ago

    Very cool. Hardware asan did not catch the pointer provenance bug in the previous implementation of that code because it relies on tag bits, and the produced pointer was bit-identical to the intended one. It sounds like fil-c would have caught it because the pointer capabilities are stored elsewhere.