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> safe code in C You're like a Japanese holdout in the 60s refusing to leave his bunker long after the war is over. C lost. Memory safety is a huge boon for s

> safe code in C You're like a Japanese holdout in the 60s refusing to leave his bunker long after the war is over. C lost. Memory safety is a huge boon for security. Human beings, even the best of them, cannot consistently write correct C code. (Look at OpenBSD.) You can keep fighting the war your side has already lost or you can move on.

uecker 2y ago
Well, memory safety is great but it seems Rust programmers also manage to create memory safety issues just fine: https://rustsec.org/advisories/RUSTSEC-2024-0401.html https://rustsec.org/advisories/RUSTSEC-2024-0401.html https://rustsec.org/advisories/RUSTSEC-2024-0400.html https://rustsec.org/advisories/RUSTSEC-2024-0400.html https://rustsec.org/advisories/RUSTSEC-2024-0377.html https://rustsec.org/advisories/RUSTSEC-2024-0377.html https://rustsec.org/advisories/RUSTSEC-2024-0374.html https://rustsec.org/advisories/RUSTSEC-2024-0374.html etc.
- whytevuhuni 2y ago
  I think the first one, stack overflow, is technically not a memory safety issue, just denial-of-service on resource exhaustion. Stack overflow is well defined as far as I know. The other three are definitely memory safety issues.
  - quotemstr 2y ago
    C++ is a better unsafe language than unsafe Rust, IMHO. The thing about the social dynamic of Rust, though, is that it keeps unsafe code to a minimum.
    - uecker 2y ago
      This may be true, but the minimum unsafe code still seems not that small. Maybe I just had bad luck, but one of the first things I looked at more closely was an implementation of a matrix transpose in Rust (as an example of something relevant to my problem domain) and that directly used unsafe Rust to be reasonably fast and then already had a CVE. This was a revealing experience because was just the same type of bug you might have had in similar C code, but in a language where countless people insist that this "can not happen".
  - ryao 2y ago
    I would consider a stack overflow to be a memory safety issue. The C++ language authors likely would too. C++ famously refused to support variable length stack allocated arrays because of memory safety concerns. In specific, they were worried that code at runtime would make an array so big so big that it would jump the OS guard page, allowing access to unallocated memory that of course is not noticed ahead of time during development. This is probably easy to do unintentionally if you have more stack variables after an enormous stack allocated array and touch them before you touch the array. The alternative is to force developers to use compiler extensions such as alloca(). That makes it easy to pass pointers outside of the stack frame where they are valid and is a definite safety issue. The C++ nitpicking over variable length arrays is silly since it gives us a status quo where C++ developers use alloca() anyway, but it shows that stack overflows are considered a memory safety issue.
    - whytevuhuni 2y ago
      In the general case, I think you might be right, although it's a bit mitigated by the fact that Rust does not have support for variable length arrays, alloca, or anything that uses them, in the standard library. As you said though, it's certainly possible. I was more referring to that specific linked advisory, which is unlikely to use either VLAs or alloca. In that case, where stack overflow would be caused by recursion, a guard frame will always be enough to catch it, and will result in a safe abort [0]. [0] https://github.com/rust-lang/rust/pull/31333 https://github.com/rust-lang/rust/pull/31333
      - ryao 2y ago
        I cited the complaints against VLAs as support for stack overflows being a memory safety issue. I did not mean to imply that Rust supported them.
  - uecker 2y ago
    I agree that one shouldn't have been included. My favorite ones aren't included here anyway, e.g. how a Rust programmer managed to create a safety issue in a matrix transpose or how the messed up str::repeat in their standard library. And don't get me wrong. I think Rust is as safer language in C. Just the idea that C is completely unsafe and it is impossible even for experts to write reasonable safe code while it is completely impossible in Rust to create an issue is just a lot of nonsense. In reality, it is possible to screw up in both languages and people do this, and reality is that safety in Rust is only somewhat better when compared to C with good security practices. But this is not how it is presented. I also think the difference will become even smaller when C safety continues to improve as it did in the last years due to better tooling while Rust is being picked up by average programmers under time pressure who will use "unsafe" just as carelessly as they carelessly hand-roll pointer arithmetic in C today.
ryao 2y ago
Use a sound static analyzer like astree and you can produce memory safe C code: https://www.absint.com/astree/index.htm https://www.absint.com/astree/index.htm Note that the key word here is sound. The more common static analyzers are unsound tools that will miss cases. Sound tools do not, but few people know of them, they are rare and they are typically proprietary and expensive.
- quotemstr 2y ago
  Sure. I'm also a big fan of what Microsoft has done with SAL. And of course you have formally proven C, as used in seL4. I'd say that the contortions you have to go through to write code with these systems takes you out of the domain of "C" and into a domain of a different, safer language merely resembling C. Such a language might be a fine tool! But it's not arbitrary C.
  - uecker 2y ago
    Note that my original comment above was "reasonably safe" and not "perfectly memory safe". You can formally prove something with a lot of effort, but you can also come reasonably close for practical purposes with a lot less effort and more commonly available tools. You are right that "arbitrary C" is not safe while safe Rust is safe, but this is mostly begging the question. The question is what can you do with the language depending on your requirements. If you need safe C this doable with enough effort, if you need reasonably safe C this is even practical in most projects, and all this should be compared to Rust as used in a similar situation which very well may include use of unsafe Rust or C libraries which may also limit the safety.
  - ryao 2y ago
    It is C. It is just written with the aid of formal methods. It would be nice if all software were written that way. That said, if you want another language “resembling C”, there is always WUFFS: https://github.com/google/wuffs https://github.com/google/wuffs The output of the WUFFS compiler certainly resembles C because it is C.
    - 2y ago

uecker 2y ago

Well, memory safety is great but it seems Rust programmers also manage to create memory safety issues just fine: https://rustsec.org/advisories/RUSTSEC-2024-0401.html https://rustsec.org/advisories/RUSTSEC-2024-0401.html https://rustsec.org/advisories/RUSTSEC-2024-0400.html https://rustsec.org/advisories/RUSTSEC-2024-0400.html https://rustsec.org/advisories/RUSTSEC-2024-0377.html https://rustsec.org/advisories/RUSTSEC-2024-0377.html https://rustsec.org/advisories/RUSTSEC-2024-0374.html https://rustsec.org/advisories/RUSTSEC-2024-0374.html etc.

whytevuhuni 2y ago
I think the first one, stack overflow, is technically not a memory safety issue, just denial-of-service on resource exhaustion. Stack overflow is well defined as far as I know. The other three are definitely memory safety issues.
- quotemstr 2y ago
  C++ is a better unsafe language than unsafe Rust, IMHO. The thing about the social dynamic of Rust, though, is that it keeps unsafe code to a minimum.
  - uecker 2y ago
    This may be true, but the minimum unsafe code still seems not that small. Maybe I just had bad luck, but one of the first things I looked at more closely was an implementation of a matrix transpose in Rust (as an example of something relevant to my problem domain) and that directly used unsafe Rust to be reasonably fast and then already had a CVE. This was a revealing experience because was just the same type of bug you might have had in similar C code, but in a language where countless people insist that this "can not happen".
- ryao 2y ago
  I would consider a stack overflow to be a memory safety issue. The C++ language authors likely would too. C++ famously refused to support variable length stack allocated arrays because of memory safety concerns. In specific, they were worried that code at runtime would make an array so big so big that it would jump the OS guard page, allowing access to unallocated memory that of course is not noticed ahead of time during development. This is probably easy to do unintentionally if you have more stack variables after an enormous stack allocated array and touch them before you touch the array. The alternative is to force developers to use compiler extensions such as alloca(). That makes it easy to pass pointers outside of the stack frame where they are valid and is a definite safety issue. The C++ nitpicking over variable length arrays is silly since it gives us a status quo where C++ developers use alloca() anyway, but it shows that stack overflows are considered a memory safety issue.
  - whytevuhuni 2y ago
    In the general case, I think you might be right, although it's a bit mitigated by the fact that Rust does not have support for variable length arrays, alloca, or anything that uses them, in the standard library. As you said though, it's certainly possible. I was more referring to that specific linked advisory, which is unlikely to use either VLAs or alloca. In that case, where stack overflow would be caused by recursion, a guard frame will always be enough to catch it, and will result in a safe abort [0]. [0] https://github.com/rust-lang/rust/pull/31333 https://github.com/rust-lang/rust/pull/31333
    - ryao 2y ago
      I cited the complaints against VLAs as support for stack overflows being a memory safety issue. I did not mean to imply that Rust supported them.
- uecker 2y ago
  I agree that one shouldn't have been included. My favorite ones aren't included here anyway, e.g. how a Rust programmer managed to create a safety issue in a matrix transpose or how the messed up str::repeat in their standard library. And don't get me wrong. I think Rust is as safer language in C. Just the idea that C is completely unsafe and it is impossible even for experts to write reasonable safe code while it is completely impossible in Rust to create an issue is just a lot of nonsense. In reality, it is possible to screw up in both languages and people do this, and reality is that safety in Rust is only somewhat better when compared to C with good security practices. But this is not how it is presented. I also think the difference will become even smaller when C safety continues to improve as it did in the last years due to better tooling while Rust is being picked up by average programmers under time pressure who will use "unsafe" just as carelessly as they carelessly hand-roll pointer arithmetic in C today.

ryao 2y ago

Use a sound static analyzer like astree and you can produce memory safe C code: https://www.absint.com/astree/index.htm https://www.absint.com/astree/index.htm Note that the key word here is sound. The more common static analyzers are unsound tools that will miss cases. Sound tools do not, but few people know of them, they are rare and they are typically proprietary and expensive.

quotemstr 2y ago
Sure. I'm also a big fan of what Microsoft has done with SAL. And of course you have formally proven C, as used in seL4. I'd say that the contortions you have to go through to write code with these systems takes you out of the domain of "C" and into a domain of a different, safer language merely resembling C. Such a language might be a fine tool! But it's not arbitrary C.
- uecker 2y ago
  Note that my original comment above was "reasonably safe" and not "perfectly memory safe". You can formally prove something with a lot of effort, but you can also come reasonably close for practical purposes with a lot less effort and more commonly available tools. You are right that "arbitrary C" is not safe while safe Rust is safe, but this is mostly begging the question. The question is what can you do with the language depending on your requirements. If you need safe C this doable with enough effort, if you need reasonably safe C this is even practical in most projects, and all this should be compared to Rust as used in a similar situation which very well may include use of unsafe Rust or C libraries which may also limit the safety.
- ryao 2y ago
  It is C. It is just written with the aid of formal methods. It would be nice if all software were written that way. That said, if you want another language “resembling C”, there is always WUFFS: https://github.com/google/wuffs https://github.com/google/wuffs The output of the WUFFS compiler certainly resembles C because it is C.
  - 2y ago