> If FilPizlonator determined that the inline assembly is not safe, then it'll replace it with a Fil-C panic. That panic will provide diagnostics about why the assembly was rejected.
Most stupid thing I ever heard. If a safety violation is known at compile-time, you error at compile-time. You might never catch it in a test, and there you have the panic at the customer. He will be pleased.
I would guess for the use-case of "I have a C project and I want to run it in Fil-C" the ability for this to be a warning + run-time panic is very helpful for quickly getting started. Reminds me of GHC's -fdefer-type-errors.
I agree that I wouldn't want to deploy a program where those panics are reachable*, but it's still handy for local development and/or maybe the developer knows they aren't reachable.
I haven't checked, but I'd guess there's a warning and a -Werror -style flag to opt-in to having a hard error for unsafe assembly?
* Obviously a panic is better than not. But guaranteed safeness is better than either of those.
You are not thinking straight if you are making out of bounds errors in inline asm.
Inline asm should take 10x or more effort compared to writing the surrounding c++ code and should be tested with protected pages at the edges if possible. It should always have assertions before/after that check invariants too.
Also there are at a lot of cases that this won’t work. One example is implementing strlen using avx512 where you want to align the address down to a multiple of 64 and run until the end of the page, so you can do simd while avoiding segfault.
Another example is just handling loop remainders with masking in avx512.
Also it is pretty naive to think an LLM got this right
Overall it seems like a huge waste of time.
If you are writing inline asm and want to make it better, just get as many LLMs or, even better, humans to review it. LLMs are really good at finding mistakes in inline asm, with a high false positive rate though, so you have to understand the concept.
For example one bug I had was about not consuming the inputs before writing to the outputs. Compiler can assign the same register to input and outputs unless outputs are marked with & (or something like that). It was super frustrating to debug this until I asked an LLM and it found the problem.
> While reviewing folks' C and C++ code, I've found the following reasons for inline assembly, where 1 is most common:
3a. rdpru (similar issues to cpuid) and rdpmc perhaps surrounded with lfence or cpuid inside the same assembly chunk
For obvious reasons, this is somewhat niche and may not even make it into production code, but it’s also important when you do need it. It’s also memory safe. I guess in such cases you’d use fast C rather than Fil-C though.
4a. rseq
Probably even less feasible than atomics TBH, as such blocks will usually also contain control flow (at least that implied by to the nature of rseqs).
> Before the advent of AI, writing a parser for x86_64 assembly would have been such an annoying task that I might have never gotten around to implementing support for memory safe inline assembly [...].
It is annoying, but even before the advent of AI that didn’t stop the developers of TCC for instance.
What is more frightening about this than safe C assembly is that this level of implementation is achievable not with a SOTA model, but with a cost effective model like KIMI. There was human judgment involved in the middle, but reading the article, My reading of the process is as follows:
1.A developer identified the necessity of inline assembly.
2.Defined the safety boundaries for 'memory-safe' inline assembly.
3.Established strict policies for memory access.
4.Curated an allowlist of permissible instructions.
5.Set rigorous test criteria and 'done' conditions.
In short, with the overall guardrails in place, a sub agent loop was run, and this level of code was produced. This raises a number of interesting points about how we should use AI. I haven't looked at all the code, but the idea of passing assembly through safe zones without memory access, and using that as a foundation to achieve this level of implementation through AI, is quite impressive
I find it charming that to distinguish Fil-C from the K&R language they use the term 'Yolo-C'. I have never used inline asm before, I actually didn't realise how much behaviour it's specifying! (When I've needed asm it was on non-gcc compilers)
Edit to add: If I'm understanding this correctly we should be able to run this against projects and detect asm violations, I feel like this would be very valuable to be able to feed these back to maintainers
I wonder if an adversarial user could bypass the checks and achieve memory corruption / code execution. Maybe not a practical attack in most situations but a fun exercise.
> This includes things like asm volatile("" : : : "memory"), which is an old-school way of saying atomic_signal_fence(memory_order_seq_cst).
Not quite. AIUI, the first is just a barrier for the compiler, while the second is also a CPU memory barrier. Godbolt seems to confirm that.
I'm confused; did you reply on the wrong thread, or am I misunderstanding something? I was merely suggesting it'd be good to mention Boost on your website and how well it works with Fil-C. This suggestion was not specifically regarding anything called assembly; it was just a general comment. I merely mentioned it because you were here.
Zero, since I made those programs work back when all inline asm was an error.
So currently most of those still have the hacks to go down the no-inlineasm path when building with Fil-C
For the few where I reinstated the inline assembly, there were no bugs found.
It would be a good experiment to try to reinstate the inlineasm paths in all of the programs that had them. I suspect there’s a low chance of finding a bug if it’s in inline assembly that’s on the critical path.
While we're at it, does anyone else want something like a good LSP but for assembly?
I mean one that infers as much context as possible and tries to help as much as possible.
This has to be assembler specific of course. For example, I use fasm which has higher level macros. An LSP could suggest struct fields and other stuff.
Most stupid thing I ever heard. If a safety violation is known at compile-time, you error at compile-time. You might never catch it in a test, and there you have the panic at the customer. He will be pleased.
I would guess for the use-case of "I have a C project and I want to run it in Fil-C" the ability for this to be a warning + run-time panic is very helpful for quickly getting started. Reminds me of GHC's -fdefer-type-errors.
I agree that I wouldn't want to deploy a program where those panics are reachable*, but it's still handy for local development and/or maybe the developer knows they aren't reachable.
I haven't checked, but I'd guess there's a warning and a -Werror -style flag to opt-in to having a hard error for unsafe assembly?
* Obviously a panic is better than not. But guaranteed safeness is better than either of those.
Inline asm should take 10x or more effort compared to writing the surrounding c++ code and should be tested with protected pages at the edges if possible. It should always have assertions before/after that check invariants too.
Also there are at a lot of cases that this won’t work. One example is implementing strlen using avx512 where you want to align the address down to a multiple of 64 and run until the end of the page, so you can do simd while avoiding segfault.
Another example is just handling loop remainders with masking in avx512.
Also it is pretty naive to think an LLM got this right
Overall it seems like a huge waste of time.
If you are writing inline asm and want to make it better, just get as many LLMs or, even better, humans to review it. LLMs are really good at finding mistakes in inline asm, with a high false positive rate though, so you have to understand the concept.
For example one bug I had was about not consuming the inputs before writing to the outputs. Compiler can assign the same register to input and outputs unless outputs are marked with & (or something like that). It was super frustrating to debug this until I asked an LLM and it found the problem.
3a. rdpru (similar issues to cpuid) and rdpmc perhaps surrounded with lfence or cpuid inside the same assembly chunk
For obvious reasons, this is somewhat niche and may not even make it into production code, but it’s also important when you do need it. It’s also memory safe. I guess in such cases you’d use fast C rather than Fil-C though.
4a. rseq
Probably even less feasible than atomics TBH, as such blocks will usually also contain control flow (at least that implied by to the nature of rseqs).
> Before the advent of AI, writing a parser for x86_64 assembly would have been such an annoying task that I might have never gotten around to implementing support for memory safe inline assembly [...].
It is annoying, but even before the advent of AI that didn’t stop the developers of TCC for instance.
1.A developer identified the necessity of inline assembly.
2.Defined the safety boundaries for 'memory-safe' inline assembly.
3.Established strict policies for memory access.
4.Curated an allowlist of permissible instructions.
5.Set rigorous test criteria and 'done' conditions.
In short, with the overall guardrails in place, a sub agent loop was run, and this level of code was produced. This raises a number of interesting points about how we should use AI. I haven't looked at all the code, but the idea of passing assembly through safe zones without memory access, and using that as a foundation to achieve this level of implementation through AI, is quite impressive
Anyway, this is also very useful for humans to use, so it's mostly a lovely coincidence this level of safety arrived with useful chatbots.
Edit to add: If I'm understanding this correctly we should be able to run this against projects and detect asm violations, I feel like this would be very valuable to be able to feed these back to maintainers
> This includes things like asm volatile("" : : : "memory"), which is an old-school way of saying atomic_signal_fence(memory_order_seq_cst).
Not quite. AIUI, the first is just a barrier for the compiler, while the second is also a CPU memory barrier. Godbolt seems to confirm that.
https://godbolt.org/z/a844zKej8
The quote uses atomic_signal_fence.
If you find a way to bypass my checks, file a bug. I tried very hard to break it. My agent loops tried even harder
What happens if you ask to find the strings that will erroneously return True from validateSafeInlineAsm for disallowed asm? :)
Example of a bug found most recently was that sahf was allowed without a cc constraint.
Anyway, if you find bugs, file them. Would be fun to see if there’s a case me and my agents missed
I might give it a try when I have a chance, I'll let you know if anything comes of it.
There was some debugging thing where it embeds debug info using module level assembly that you have to disable.
[1] https://fil-c.org/programs_that_work
It’s module assembly
They’re different
[0] https://fil-c.org/programs_that_work
So currently most of those still have the hacks to go down the no-inlineasm path when building with Fil-C
For the few where I reinstated the inline assembly, there were no bugs found.
It would be a good experiment to try to reinstate the inlineasm paths in all of the programs that had them. I suspect there’s a low chance of finding a bug if it’s in inline assembly that’s on the critical path.
I mean one that infers as much context as possible and tries to help as much as possible.
This has to be assembler specific of course. For example, I use fasm which has higher level macros. An LSP could suggest struct fields and other stuff.