Switch macOS to using mach ports for trap handling #2632
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alexcrichton
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This commit moves macOS to using mach ports instead of signals for handling traps. The motivation for this is listed in bytecodealliance#2456, namely that once mach ports are used in a process that means traditional UNIX signal handlers won't get used. This means that if Wasmtime is integrated with Breakpad, for example, then Wasmtime's trap handler never fires and traps don't work. The `traphandlers` module is refactored as part of this commit to split the platform-specific bits into their own files (it was growing quite a lot for one inline `cfg_if!`). The `unix.rs` and `windows.rs` files remain the same as they were before with a few minor tweaks for some refactored interfaces. The `macos.rs` file is brand new and lifts almost its entire implementation from SpiderMonkey, adapted for Wasmtime though. The main gotcha with mach ports is that a separate thread is what services the exception. Some unsafe magic allows this separate thread to read non-`Send` and temporary state from other threads, but is hoped to be safe in this context. The unfortunate downside is that calling wasm on macOS now involves taking a global lock and modifying a global hash map twice-per-call. I'm not entirely sure how to get out of this cost for now, but hopefully for any embeddings on macOS it's not the end of the world. Closes bytecodealliance#2456
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One thing worth pointing out is that I believe Wasmtime currently complies for arm64 macOS (although it doesn't work AFAIK), but this PR would break that. Structures for the thread state would need to be copied over from various headers to get that working. |
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We actually run aarch64 mac CI on our project that uses wasmtime, so I think we can just add a PR that builds off of this one for the aarch64 specific things that would need to change. 🙂 |
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Oh wait, I was just reminded that we might not be able to verify this PR on the aarch64 mac though, due to #2552, we actually had to disable the tests we had that purposefully paniced inside the wasm module for aarch64 mac due to it actually crashing our host program. |
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Ah ok, I didn't realize it already worked well enough to at least have some testing! I've pushed up d07d29b although it is entirely untested on my end (I don't have arm64 hardware). It should compile but I suspect at least some tests fail. |
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Great, thanks! |
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This commit fixes an issue discovered in the wasmtime-go bindings when the Go runtime was crashing on macOS only when running wasm code that trapped. It turns out that our switch to `siglongjmp` from `longjmp` actually broke macOS! This breakage happens because all subsequent signals after the first signal are all delivered on the main stack, not the sigaltstack, even if the sigaltstack is configured. This causes the Go runtime to crash since it expects to run on the sigaltstack. The fix in this commit is to actually return from the signal handler to trigger the kernel's updating of the sigaltstack no longer being in use. Before we return, however, we configure the register context to return to to call some custom code which immediately does the unwind we would otherwise have done. This works around the issue on macOS hopefully without adding too many portability problems. Ideally this will all go away as well with bytecodealliance#2632 as well.
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* Fix preservation of the sigaltstack on macOS This commit fixes an issue discovered in the wasmtime-go bindings when the Go runtime was crashing on macOS only when running wasm code that trapped. It turns out that our switch to `siglongjmp` from `longjmp` actually broke macOS! This breakage happens because all subsequent signals after the first signal are all delivered on the main stack, not the sigaltstack, even if the sigaltstack is configured. This causes the Go runtime to crash since it expects to run on the sigaltstack. The fix in this commit is to actually return from the signal handler to trigger the kernel's updating of the sigaltstack no longer being in use. Before we return, however, we configure the register context to return to to call some custom code which immediately does the unwind we would otherwise have done. This works around the issue on macOS hopefully without adding too many portability problems. Ideally this will all go away as well with #2632 as well. * Fix compile warning
This was referenced Mar 12, 2021
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I've based my work in #2723 from this PR, which has been rebased and tweaked. Thanks a lot @alexcrichton for all the initial work on this! |
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This commit moves macOS to using mach ports instead of signals for
handling traps. The motivation for this is listed in #2456, namely that
once mach ports are used in a process that means traditional UNIX signal
handlers won't get used. This means that if Wasmtime is integrated with
Breakpad, for example, then Wasmtime's trap handler never fires and
traps don't work.
The
traphandlersmodule is refactored as part of this commit to splitthe platform-specific bits into their own files (it was growing quite a
lot for one inline
cfg_if!). Theunix.rsandwindows.rsfilesremain the same as they were before with a few minor tweaks for some
refactored interfaces. The
macos.rsfile is brand new and lifts almostits entire implementation from SpiderMonkey, adapted for Wasmtime
though.
The main gotcha with mach ports is that a separate thread is what
services the exception. Some unsafe magic allows this separate thread to
read non-
Sendand temporary state from other threads, but is hoped tobe safe in this context. The unfortunate downside is that calling wasm
on macOS now involves taking a global lock and modifying a global hash
map twice-per-call. I'm not entirely sure how to get out of this cost
for now, but hopefully for any embeddings on macOS it's not the end of
the world.
Closes #2456