5 ms·

I'm curious, have you used CBMC? The machine model tracks the array with its size and compares this to any index. This is loaded into an SMT solver. This is no

I'm curious, have you used CBMC? The machine model tracks the array with its size and compares this to any index. This is loaded into an SMT solver. This is not a standard static analyzer. It's compiling the code to an abstract machine model and then running through this model with an SMT solver. CBMC isn't perfect, but the strategies you are talking about are meant to defeat a linter or a shallow analyzer in a compiler. CBMC isn't a linter.

WalterBright 3y ago
I have never heard of CBMC before this thread. SMT cannot solve cases where it simply cannot know what the length of an array is. For example, when array is allocated in code not available to the solver, or is set by the environment. I remember trying out Spark a few years ago, which advertised its compile time checking abilities. I tried using an integer equation that used an OR, and it gave up on it. If you need to add in range checking code to help the solver along, then the range check itself is a source of bugs, and the range limit has to be made available somewhere and be correct. In my proposal, the array length is set at the time of creation of the array, and the length is carried along with the pointer. The solver's role then becomes one of eliminating the bounds check in the cases where it can prove the index is within range. The user doesn't have to do anything. We've been using it in the implementation of the D compiler for a very long time now, and problems with buffer overflows are a faded memory. P.S. I also added manual integer overflow checks when passing the size to malloc(), no matter how unlikely an overflow would be :-)
- nanolith 3y ago
  > SMT cannot solve cases where it simply cannot know what the length of an array is. Sure it can. It sets the array length to a non-deterministic unsigned integer, and then finds a counter-example where this array length is invalid. CBMC will also make the array pointer itself non-deterministic if you haven't refined it. So, the pointer could be NULL, could be pointing to invalid memory, etc. Don't think of values as fixed numbers, but rather as functions shaping a non-deterministic value. The range check made upstream becomes part of this function. The goal of the SMT solver is to find a counter-example that crashes the machine model. If you want a rather leaky abstraction, imagine that by transforming the source code into an SMT equation, it's effectively turning it inside out and transforming it into a logic equation with modulo math. I agree with you that it would be nice if bounds were included in C arrays. But, we have to work with what we have. Unfortunately, especially in firmware that requires a commercial C compiler, we are stuck with C. In those cases, if we want safer software, we need tooling like this.
  - WalterBright 3y ago
    Thank you for the explanation. I agree that if one isn't going to enhance C, one is going to have to resort to these tools. C gets new features now and then. Why not add something incredibly useful, like the slice proposal? Instead, C23 got enhanced with the crazy Unicode identifiers. Richard Cattermole has been adding them to D's C support, requiring 6000 lines of code!! https://github.com/dlang/dmd/pull/15307 https://github.com/dlang/dmd/pull/15307 The entire C parser is 6000 lines of code: https://github.com/dlang/dmd/blob/master/compiler/src/dmd/cparse.d https://github.com/dlang/dmd/blob/master/compiler/src/dmd/cp...
    - nanolith 3y ago
      Yeah... I'm not too happy with some of the choices made in C18 and C23. One of the reasons why I chose to standardize my C SAX-like parser on C18 for now was to avoid the Unicode complexity. There's also a widening feature gap between modern C and modern C++ that makes interop harder.
    - lifthrasiir 3y ago
      > Richard Cattermole has been adding them to D's C support, requiring 6000 lines of code!! I wondered if you were implementing a normalization (which is complicated), but that doesn't seem the case? I have read the PR and it's clear that it solves multiple issues at once: no start-continue distinction (bug), a code generator, the table itself (which is just an inefficiently formatted block of data). 6000 lines of code just for Unicode identifiers are simply an exaggeration.
      - WalterBright 3y ago
        There are different ways to count lines. I included in the C parser count the comment lines. 6000 is also a round number, I did that deliberately to imply the number was not exact.
        lifthrasiir 3y ago
        A new generated code alone is 4000 lines long [1]. The actual code added is just 2000 lines, and some are used to pay debts, I mean, to make a proper code generator. It is debatable whether that generator should count in the number of lines, since you can always write a generator in a simpler scripting language (for example, Rust uses Python to generate the `unicode-xid` crate). In any case however it is far less complex and, in my opinion, much smaller than the entire C parser proper. [1] https://github.com/dlang/dmd/pull/15307/files#diff-3677bcc896b4eacdde46b6df9ddfd0be58e388de66016b53d2f4fcbc5cefab84 https://github.com/dlang/dmd/pull/15307/files#diff-3677bcc89...

WalterBright 3y ago

I have never heard of CBMC before this thread. SMT cannot solve cases where it simply cannot know what the length of an array is. For example, when array is allocated in code not available to the solver, or is set by the environment. I remember trying out Spark a few years ago, which advertised its compile time checking abilities. I tried using an integer equation that used an OR, and it gave up on it. If you need to add in range checking code to help the solver along, then the range check itself is a source of bugs, and the range limit has to be made available somewhere and be correct. In my proposal, the array length is set at the time of creation of the array, and the length is carried along with the pointer. The solver's role then becomes one of eliminating the bounds check in the cases where it can prove the index is within range. The user doesn't have to do anything. We've been using it in the implementation of the D compiler for a very long time now, and problems with buffer overflows are a faded memory. P.S. I also added manual integer overflow checks when passing the size to malloc(), no matter how unlikely an overflow would be :-)

nanolith 3y ago
> SMT cannot solve cases where it simply cannot know what the length of an array is. Sure it can. It sets the array length to a non-deterministic unsigned integer, and then finds a counter-example where this array length is invalid. CBMC will also make the array pointer itself non-deterministic if you haven't refined it. So, the pointer could be NULL, could be pointing to invalid memory, etc. Don't think of values as fixed numbers, but rather as functions shaping a non-deterministic value. The range check made upstream becomes part of this function. The goal of the SMT solver is to find a counter-example that crashes the machine model. If you want a rather leaky abstraction, imagine that by transforming the source code into an SMT equation, it's effectively turning it inside out and transforming it into a logic equation with modulo math. I agree with you that it would be nice if bounds were included in C arrays. But, we have to work with what we have. Unfortunately, especially in firmware that requires a commercial C compiler, we are stuck with C. In those cases, if we want safer software, we need tooling like this.
- WalterBright 3y ago
  Thank you for the explanation. I agree that if one isn't going to enhance C, one is going to have to resort to these tools. C gets new features now and then. Why not add something incredibly useful, like the slice proposal? Instead, C23 got enhanced with the crazy Unicode identifiers. Richard Cattermole has been adding them to D's C support, requiring 6000 lines of code!! https://github.com/dlang/dmd/pull/15307 https://github.com/dlang/dmd/pull/15307 The entire C parser is 6000 lines of code: https://github.com/dlang/dmd/blob/master/compiler/src/dmd/cparse.d https://github.com/dlang/dmd/blob/master/compiler/src/dmd/cp...
  - nanolith 3y ago
    Yeah... I'm not too happy with some of the choices made in C18 and C23. One of the reasons why I chose to standardize my C SAX-like parser on C18 for now was to avoid the Unicode complexity. There's also a widening feature gap between modern C and modern C++ that makes interop harder.
  - lifthrasiir 3y ago
    > Richard Cattermole has been adding them to D's C support, requiring 6000 lines of code!! I wondered if you were implementing a normalization (which is complicated), but that doesn't seem the case? I have read the PR and it's clear that it solves multiple issues at once: no start-continue distinction (bug), a code generator, the table itself (which is just an inefficiently formatted block of data). 6000 lines of code just for Unicode identifiers are simply an exaggeration.
    - WalterBright 3y ago
      There are different ways to count lines. I included in the C parser count the comment lines. 6000 is also a round number, I did that deliberately to imply the number was not exact.
      - lifthrasiir 3y ago
        A new generated code alone is 4000 lines long [1]. The actual code added is just 2000 lines, and some are used to pay debts, I mean, to make a proper code generator. It is debatable whether that generator should count in the number of lines, since you can always write a generator in a simpler scripting language (for example, Rust uses Python to generate the `unicode-xid` crate). In any case however it is far less complex and, in my opinion, much smaller than the entire C parser proper. [1] https://github.com/dlang/dmd/pull/15307/files#diff-3677bcc896b4eacdde46b6df9ddfd0be58e388de66016b53d2f4fcbc5cefab84 https://github.com/dlang/dmd/pull/15307/files#diff-3677bcc89...