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[un]install_multiple_protocol_interfaces() requires but is currently incompatible with feature c_variadic #302
Description
Tracking issue
While implementing the missing protocol handler functions in BootServices, I found that extern "efiapi" is incompatible with Rust's current (unstable) implementation of c variadics. Rustc indicates that C variadics must be extern "C" or extern "cdecl", thus install_multiple_protocol_interfaces() and uninstall_multiple_protocol_interfaces() cannot be implemented at this time.
I'll bring this up on the C variadic tracking issue in the next couple days if it hasn't been already.
Rust tracking issue: rust-lang/rust#44930
EDIT: technically I guess this could be possible with some clever and wildly unsafe asm but I'd much rather wait for Rust to support it.
Activity
timrobertsdev commentedon Oct 20, 2021
Update: Per Josh Triplett on the rustlang zulip,
extern "C"on the "x86_64-unknown-uefi" target should be the proper UEFI ABI, so we can actually implement this, usingextern "C"instead ofextern "efiapi".GabrielMajeri commentedon Dec 11, 2021
@timrobertsdev does this mean we should replace all
extern "efiapi"declarations in the crate withextern "C", and everything will just work correctly? Or is this workaround just for the protocol handler functions?timrobertsdev commentedon Dec 14, 2021
The way I understood it, since we're targeting EFI,
extern "C"will always be the correct calling convention. I've replaced allextern "efiapi"withextern "C"locally and everything still builds and runs/tests correctly.GabrielMajeri commentedon Dec 22, 2021
@timrobertsdev Ah, I've realized there's a little problem which stops us from switching everything over to
extern "C".The reason we've introduced the
efiabicalling convention was to avoid issues like #38 -- if uefi-rs is used as a library in an OS kernel/driver which interacts with UEFI structures but which is built as an ELF binary, then it won't be able to call the methods correctly, unless they're declared withefiabi.I'm not sure what the correct solution would be here.
nicholasbishop commentedon Dec 22, 2021
Perhaps the
[un]install_multiple_protocol_interfacesfunction pointers could be declared withextern "C", and then the wrapper methods that call them could be guarded behind a#[cfg(...)]that only allows them to be called from uefi arches, until such time asefiabisupports variadics.timrobertsdev commentedon Dec 22, 2021
@nicholasbishop That sounds like a good solution to me.
Would an ELF binary be capable of calling these functions (or any boot service functions, I guess)? I had assumed that boot services would be exited by the time a kernel is running.
nicholasbishop commentedon Dec 23, 2021
Fair point, I think Linux at least exits boot services quite early in the EFI stub.
GabrielMajeri commentedon Dec 23, 2021
That's the issue... we can't really guarantee this. It's what a logical, sane UEFI-based application/kernel would do: first load an ELF binary, then exist boot services and jump into it. But if we want to be pedantic and respect Rust's safety guarantees in all cases, something with a
cfgguard like @nicholasbishop suggested would be more appropriate.nicholasbishop commentedon Nov 4, 2022
Quick update here, on current nightly it looks like this is now possible to use
efiabiwith variadics. However, it's currently gated by theextended_varargs_abi_supportfeature so I would recommend we not add it for now to avoid requiring more unstable features.(Also, as far as I can tell,
InstallMultipleProtocolInterfacesis just a thin wrapper around callingInstallProtocolServices. We don't yet have the latter implemented, but we could add that easily I think.)medhefgo commentedon Apr 10, 2023
Just chiming in here to say that getting the calling convention right isn't your only concern if you want to (correctly) support cross ABI calls. If any structs are involved you have to make sure that they are laid out correctly according to the EFI ABI.
In particular, x86 sysv ABI has long long 4-byte aligned if it appears within a struct. The MS/EFI ABI expects it to be 8-byte aligned.
This will cause loaded image protocol (in particular) to have a bad memory layout if the uefi-rs definition were used in ELF ABI context. Whether that is a use-case you want to support or not is a different matter.
nicholasbishop commentedon Apr 11, 2023
At least with the
install_multiple_protocol_interfacesAPI, that shouldn't be a problem. It's only callable while boot services are active, which means you are running a UEFI executable that uses the efiabi calling convention. I think we only need to worry about cross-ABI calls when in runtime-services mode after an OS loads.Soveu commentedon Dec 13, 2024
On nightly, it is no longer gated and should be available on 1.85 stable release next year :)
crawfxrd commentedon Nov 13, 2025
phip1611 commentedon Jan 1, 2026
It was a pain to get varargs to work nicely in Rust or I had a knot in my head. Instead, I came up with a different pragmatic solution #1856
crawfxrd commentedon Sep 9, 2026
phip1611 commentedon Sep 10, 2026
cool, feel free to raise a PR
phip1611 commentedon Sep 22, 2026
this is now unblocked #2098