[libc] -Wcast-function-type in libc/src/signal/linux/signal_utils.h

[nickdesaulniers]

[270/361] Building CXX object projects/libc/src/signal/linux/CMakeFiles/libc.src.signal.linux.raise.dir/raise.cpp.o
In file included from /android0/llvm-project/libc/src/signal/linux/raise.cpp:10:
/android0/llvm-project/libc/src/signal/linux/signal_utils.h: In member function ‘__llvm_libc_18_0_0_git::KernelSigaction& __llvm_libc_18_0_0_git::KernelSigaction::operator=(const sigaction&)’:
/android0/llvm-project/libc/src/signal/linux/signal_utils.h:38:20: warning: cast between incompatible function types from ‘void (*)(int, siginfo_t*, void*)’ to ‘void (*)(int)’ [-Wcast-function-type]
   38 |       sa_handler = reinterpret_cast<HandlerType *>(sa.sa_sigaction);
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/android0/llvm-project/libc/src/signal/linux/signal_utils.h: In member function ‘__llvm_libc_18_0_0_git::KernelSigaction::operator sigaction() const’:
/android0/llvm-project/libc/src/signal/linux/signal_utils.h:51:25: warning: cast between incompatible function types from ‘void (*)(int)’ to ‘void (*)(int, siginfo_t*, void*)’ [-Wcast-function-type]
   51 |       sa.sa_sigaction = reinterpret_cast<SiginfoHandlerType *>(sa_handler);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I'm trying to get -Werror re-enabled in #74506; building with GCC flags the above warning.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

``` [270/361] Building CXX object projects/libc/src/signal/linux/CMakeFiles/libc.src.signal.linux.raise.dir/raise.cpp.o In file included from /android0/llvm-project/libc/src/signal/linux/raise.cpp:10: /android0/llvm-project/libc/src/signal/linux/signal_utils.h: In member function ‘__llvm_libc_18_0_0_git::KernelSigaction& __llvm_libc_18_0_0_git::KernelSigaction::operator=(const sigaction&)’: /android0/llvm-project/libc/src/signal/linux/signal_utils.h:38:20: warning: cast between incompatible function types from ‘void (*)(int, siginfo_t*, void*)’ to ‘void (*)(int)’ [-Wcast-function-type] 38 | sa_handler = reinterpret_cast<HandlerType *>(sa.sa_sigaction); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /android0/llvm-project/libc/src/signal/linux/signal_utils.h: In member function ‘__llvm_libc_18_0_0_git::KernelSigaction::operator sigaction() const’: /android0/llvm-project/libc/src/signal/linux/signal_utils.h:51:25: warning: cast between incompatible function types from ‘void (*)(int)’ to ‘void (*)(int, siginfo_t*, void*)’ [-Wcast-function-type] 51 | sa.sa_sigaction = reinterpret_cast<SiginfoHandlerType *>(sa_handler); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ``` I'm trying to get -Werror re-enabled in https://github.com//pull/74506; building with GCC flags the above warning.

[nickdesaulniers]

https://android.googlesource.com/platform/bionic/+/refs/heads/main/libc/include/bits/signal_types.h?autodive=0%2F%2F#60

alludes to the issue that is mentioned near the warning.

llvm-project/libc/src/signal/linux/signal_utils.h

Lines 21 to 22 in 1f283a6

	// The POSIX definition of struct sigaction and the sigaction data structure
	// expected by the rt_sigaction syscall differ in their definition. So, we

[SchrodingerZhu]

Casting to an intermediate value should suppress the warning. See https://godbolt.org/z/PncMfvfz7

using T0 = void (*)(int, void*, void*);
using T1 = void (*)(int);

T0 test(T1 f) {
    return reinterpret_cast<T0>(f);
}

T0 test1(T1 f) {
    void * ptr = reinterpret_cast<void *>(f);
    return reinterpret_cast<T0>(ptr);
}

[nickdesaulniers]

Sure, we can probably get by with laundering the type through a void*. Though the comment in bionic makes me think that our current implementation is wrong; or might need to be adjusted further in the future.

[SchrodingerZhu]

Would HandlerType = void (*) (int, ...) work then?

Casting around

void (*a)(int, ...);
void (*b)((int, void*, void*);

will just work without warning. Per psABI's view, it seems to be safe enough.

However, I suppose you are talking about separating them based on implementations rather than figuring out an generic type that just works.