7 ms·

> Using cxx does not, in fact, remove any `unsafe` blocks- thus it is not "lying," nor is it "the ultimate evil." (Such hyperbole makes productive discussion in

> Using cxx does not, in fact, remove any `unsafe` blocks- thus it is not "lying," nor is it "the ultimate evil." (Such hyperbole makes productive discussion in this space rather difficult...) If I can write an unsound Rust program using autocxx or cxx safe Rust APIs, then these APIs are unsound. Whether these APIs are macros or function calls does not matter. There is no distinction at the API level about whether incorrect unsafe code is expanded, or called into. The API is safe, and for these crates, the currently-safe API introduces undefined behavior. cxx will be fixed to require unsafe, but AFAICT autocxx usecase is to satisfy Chromiums requirements which require completely avoiding unsafe.

Rusky 6y ago
There is no such thing as a "safe macro" or an "unsafe macro," so you can't make the usual argument there. Macros (and other forms of code generation) all exist before and outside the "safety system," and so must be inspected in an audit regardless. Putting "unsafe" in the name or input of cxx/autocxx/etc. is a great tweak that improves local readability, but it's not a fundamental change. The actual generated wrapper functions would still be safe, after all. If you're generating `unsafe` blocks, good tooling will point them out to you just the same as if they were hand-written. Nobody is trying to sweep that responsibility under the rug- not cxx, not Chromium, not autocxx. That is, Chromium's requirement is notably not "completely avoiding unsafe." (This is another piece of hyperbole that derails productive discussion.) Their actual requirement is perfectly reasonable: restrict manual per-function unsafe to exceptional scenarios that deserve extra attention, above and beyond the usual check that a function is always safe to call with arguments that match its type. Updating autocxx to require surface-level `unsafe` in the same way as cxx does not conflict with this requirement. However, it's also relevant that autocxx is a) rather new, and b) not (yet?) an official Google project: https://github.com/google/autocxx#license-and-usage-notes https://github.com/google/autocxx#license-and-usage-notes. It would be unwise to blindly infer any sort of Chromium policy from it at this point, especially when they have just published a rather detailed document that contradicts your inference. Overall, the problem we should be trying to solve here is not simply "how do we prevent people from generating unsound code," but "how can we enable people to correctly bind to large C++ APIs?"
- fluffything 6y ago
  > There is no such thing as a "safe macro" or an "unsafe macro," so you can't make the usual argument there From all the soundness issues that have been filled on macros that were not safe in the past, the impression I have is that all exported Rust macros must be safe. > but "how can we enable people to correctly bind to large C++ APIs?" This is a solved problem: rust-bindgen, it correctly generates unsafe Rust FFI wrappers for all C++ code. What the Chromium devs actually want is what I pointed out in my root comment: to automatically generate correct soundness proofs for their C++ code / APIs, so that they can automatically generate safe Rust FFI wrappers for them. This is an unsolved problem, which is probably impossible to solve. So I think this is an absurd requirement from the part of the Chromium project, that deserves the absurd solution proposed by the `autocxx` crate, which is to just assume that these proofs exist. The disclaimer "You can only use `autocxx` on those APIs for which those proofs actually exist" isn't very reassuring TBH. They don't appear interested in finding out whether those proofs actually exist, so what's the point really. They will just start using Rust, continue getting CVEs because "oh, I guess we shouldn't have blidnly wrapped those thousands of APIs", and conclude that Rust isn't more secure than C++.
  - Rusky 6y ago
    "Just use bindgen" is a cop-out, not a solution. It is technically correct, merely by marking all APIs unsafe. But as the Chromium document describes, this introduces a lot of noise that can mask the more involved proofs. What Chromium wants is, again, not a way to generate soundness proofs, but a way to consolidate large numbers of similar proofs into a single place (the invocation of the binding macro), so that the remaining proofs can stand out. Please, do us all a favor and stop misattributing absurdities to Chromium and autocxx. This is a useful part of the design space, ill-served by bindgen alone. Chromium is not the first project to hit it, nor will it be the last. For example, here is a Servo/Gecko dev's take on the subject: https://www.reddit.com/r/rust/comments/ielvxu/the_cxx_debate/g2jurb3/ https://www.reddit.com/r/rust/comments/ielvxu/the_cxx_debate...
    - fluffything 6y ago

Rusky 6y ago

There is no such thing as a "safe macro" or an "unsafe macro," so you can't make the usual argument there. Macros (and other forms of code generation) all exist before and outside the "safety system," and so must be inspected in an audit regardless. Putting "unsafe" in the name or input of cxx/autocxx/etc. is a great tweak that improves local readability, but it's not a fundamental change. The actual generated wrapper functions would still be safe, after all. If you're generating `unsafe` blocks, good tooling will point them out to you just the same as if they were hand-written. Nobody is trying to sweep that responsibility under the rug- not cxx, not Chromium, not autocxx. That is, Chromium's requirement is notably not "completely avoiding unsafe." (This is another piece of hyperbole that derails productive discussion.) Their actual requirement is perfectly reasonable: restrict manual per-function unsafe to exceptional scenarios that deserve extra attention, above and beyond the usual check that a function is always safe to call with arguments that match its type. Updating autocxx to require surface-level `unsafe` in the same way as cxx does not conflict with this requirement. However, it's also relevant that autocxx is a) rather new, and b) not (yet?) an official Google project: https://github.com/google/autocxx#license-and-usage-notes https://github.com/google/autocxx#license-and-usage-notes. It would be unwise to blindly infer any sort of Chromium policy from it at this point, especially when they have just published a rather detailed document that contradicts your inference. Overall, the problem we should be trying to solve here is not simply "how do we prevent people from generating unsound code," but "how can we enable people to correctly bind to large C++ APIs?"

fluffything 6y ago
> There is no such thing as a "safe macro" or an "unsafe macro," so you can't make the usual argument there From all the soundness issues that have been filled on macros that were not safe in the past, the impression I have is that all exported Rust macros must be safe. > but "how can we enable people to correctly bind to large C++ APIs?" This is a solved problem: rust-bindgen, it correctly generates unsafe Rust FFI wrappers for all C++ code. What the Chromium devs actually want is what I pointed out in my root comment: to automatically generate correct soundness proofs for their C++ code / APIs, so that they can automatically generate safe Rust FFI wrappers for them. This is an unsolved problem, which is probably impossible to solve. So I think this is an absurd requirement from the part of the Chromium project, that deserves the absurd solution proposed by the `autocxx` crate, which is to just assume that these proofs exist. The disclaimer "You can only use `autocxx` on those APIs for which those proofs actually exist" isn't very reassuring TBH. They don't appear interested in finding out whether those proofs actually exist, so what's the point really. They will just start using Rust, continue getting CVEs because "oh, I guess we shouldn't have blidnly wrapped those thousands of APIs", and conclude that Rust isn't more secure than C++.
- Rusky 6y ago
  "Just use bindgen" is a cop-out, not a solution. It is technically correct, merely by marking all APIs unsafe. But as the Chromium document describes, this introduces a lot of noise that can mask the more involved proofs. What Chromium wants is, again, not a way to generate soundness proofs, but a way to consolidate large numbers of similar proofs into a single place (the invocation of the binding macro), so that the remaining proofs can stand out. Please, do us all a favor and stop misattributing absurdities to Chromium and autocxx. This is a useful part of the design space, ill-served by bindgen alone. Chromium is not the first project to hit it, nor will it be the last. For example, here is a Servo/Gecko dev's take on the subject: https://www.reddit.com/r/rust/comments/ielvxu/the_cxx_debate/g2jurb3/ https://www.reddit.com/r/rust/comments/ielvxu/the_cxx_debate...
  - fluffything 6y ago