10 ms·

Undefined behavior to access the uninitialized memory. A sanitizer would have flagged that.

jandrese 1y ago
The compiler has no way of knowing that the memory would be undefined, not unless it somehow can verify the data file. The most I think it can do is flag the program for not checking the return value of scanf, but even that is unlikely to be true since the program probably was checking for end of file which is also in the return value. It was failing to check the number of matched parameters. This is the kind of error that is easy to miss given the semantics of scanf.
- andrewmcwatters 1y ago
  Uninitialized variables are a really common case.
  - gmueckl 1y ago
    The pointer to the uninitialized variable is passed to scanf, which writes a value there unless it encounters an error. The compiler cannot understand this contract from the scanf declaration alone.
- nayuki 1y ago
  > The compiler has no way of knowing that the memory would be undefined Yes it would. -fsanitize=address does a bunch of instrumentation - it allocates shadow memory to keep track of what main memory is defined, and it checks every read and write address against the shadow memory. It is a combination of compile-time instrumentation and run-time checking. And yes, it is expensive, so it should be used for debugging and not the final release. https://clang.llvm.org/docs/AddressSanitizer.html https://clang.llvm.org/docs/AddressSanitizer.html , https://learn.microsoft.com/en-us/cpp/sanitizers/asan?view=msvc-170 https://learn.microsoft.com/en-us/cpp/sanitizers/asan?view=m...
  - maccard 1y ago
    This codebase predates ASAN by the best part of a decade.
  - hoten 1y ago
    You both may be right. It could be that ASAN is not instrumenting scanf (or some other random standard lib function). Though since 2015, it certainly has been. https://github.com/google/sanitizers/issues/108 https://github.com/google/sanitizers/issues/108 The simpler policy of "don't allow unintialized locals when declared" would also have caught it with the tools available when the game was made (though a bit ham-fisted).
    - nayuki 1y ago
      The problem is that after calling scanf(), the number of variables that are defined is a variable number. For example: int x, y, z; int n = scanf("%d %d %d", &x, &y, &z); At compile time, you can make no inferences about which of x, y, and z are defined, because that depends on the returned value n. There are many ways to branch out from this. One is to insist on definite assignment - so if we cannot prove all of them are always assigned, then we can treat them as "possibly undefined" and err out. Another way is to avoid passing references and instead allow multiple returns, like Python (this is pseudocode): x, y, z = scanf("%d %d %d") In that case, if the hypothetical `scanf()` returns a tuple that is less than 3 elements or more than 3 elements, then the unpacking will fail at run time and crash exactly at that line. Another way is like Java, which insists that the return value is a scalar, so it can't do what C and Python can do. This can be painful on the programmer, of course.
      - hoten 1y ago
        The idea is that ASAN would replace scanf with a function that does additional book keeping when writing to whatever arbitrary memory location the inputs dictate at runtime. It's probably what the PR resolving the issue I linked to does. Though I didn't check
      - twic 1y ago
andrewmcwatters 1y ago
Yeah, the debugging here is great, but the actual cause is super mild.

The compiler has no way of knowing that the memory would be undefined, not unless it somehow can verify the data file. The most I think it can do is flag the program for not checking the return value of scanf, but even that is unlikely to be true since the program probably was checking for end of file which is also in the return value. It was failing to check the number of matched parameters. This is the kind of error that is easy to miss given the semantics of scanf.

andrewmcwatters 1y ago
Uninitialized variables are a really common case.
- gmueckl 1y ago
  The pointer to the uninitialized variable is passed to scanf, which writes a value there unless it encounters an error. The compiler cannot understand this contract from the scanf declaration alone.
nayuki 1y ago
> The compiler has no way of knowing that the memory would be undefined Yes it would. -fsanitize=address does a bunch of instrumentation - it allocates shadow memory to keep track of what main memory is defined, and it checks every read and write address against the shadow memory. It is a combination of compile-time instrumentation and run-time checking. And yes, it is expensive, so it should be used for debugging and not the final release. https://clang.llvm.org/docs/AddressSanitizer.html https://clang.llvm.org/docs/AddressSanitizer.html , https://learn.microsoft.com/en-us/cpp/sanitizers/asan?view=msvc-170 https://learn.microsoft.com/en-us/cpp/sanitizers/asan?view=m...
- maccard 1y ago
  This codebase predates ASAN by the best part of a decade.
- hoten 1y ago
  You both may be right. It could be that ASAN is not instrumenting scanf (or some other random standard lib function). Though since 2015, it certainly has been. https://github.com/google/sanitizers/issues/108 https://github.com/google/sanitizers/issues/108 The simpler policy of "don't allow unintialized locals when declared" would also have caught it with the tools available when the game was made (though a bit ham-fisted).
  - nayuki 1y ago
    The problem is that after calling scanf(), the number of variables that are defined is a variable number. For example: int x, y, z; int n = scanf("%d %d %d", &x, &y, &z); At compile time, you can make no inferences about which of x, y, and z are defined, because that depends on the returned value n. There are many ways to branch out from this. One is to insist on definite assignment - so if we cannot prove all of them are always assigned, then we can treat them as "possibly undefined" and err out. Another way is to avoid passing references and instead allow multiple returns, like Python (this is pseudocode): x, y, z = scanf("%d %d %d") In that case, if the hypothetical `scanf()` returns a tuple that is less than 3 elements or more than 3 elements, then the unpacking will fail at run time and crash exactly at that line. Another way is like Java, which insists that the return value is a scalar, so it can't do what C and Python can do. This can be painful on the programmer, of course.
    - hoten 1y ago
      The idea is that ASAN would replace scanf with a function that does additional book keeping when writing to whatever arbitrary memory location the inputs dictate at runtime. It's probably what the PR resolving the issue I linked to does. Though I didn't check
    - twic 1y ago

andrewmcwatters 1y ago

Yeah, the debugging here is great, but the actual cause is super mild.