FFI `Incompatible types when assigning to type 'void(*)()' from type 'void(*)()'`

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Description

It appears we're unable to assign a function pointer via FFI:

$ cat ffibug.c 
#include <stdio.h>

void func() {
    printf("hello\n");
}

void (*func_ptr)() = NULL;

void call_func() {
    if (func_ptr != NULL) func_ptr();
}
$ gcc -shared -fPIC -o libffibug.so ffibug.c
$ php -v
PHP 8.2.0-dev (cli) (built: Jul 19 2022 20:25:19) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.2.0-dev, Copyright (c) Zend Technologies
$ php -a
Interactive shell

php > $ffi = FFI::cdef('void func(); void (*func_ptr)(); void call_func();', __DIR__ . '/libffibug.so');
php > $ffi->call_func();
php > $ffi->func_ptr = $ffi->func;

Warning: Uncaught FFI\Exception: Incompatible types when assigning to type 'void(*)()' from type 'void(*)()' in php shell code:1
Stack trace:
#0 {main}
  thrown in php shell code on line 1
php > 

The following patch seems to fix:

diff --git a/ext/ffi/ffi.c b/ext/ffi/ffi.c
index cb82c3a3c3..3cd568826f 100644
--- a/ext/ffi/ffi.c
+++ b/ext/ffi/ffi.c
@@ -267,7 +267,8 @@ static bool zend_ffi_is_compatible_type(zend_ffi_type *dst_type, zend_ffi_type *
 				dst_type = ZEND_FFI_TYPE(dst_type->pointer.type);
 				src_type = ZEND_FFI_TYPE(src_type->pointer.type);
 				if (dst_type->kind == ZEND_FFI_TYPE_VOID ||
-				    src_type->kind == ZEND_FFI_TYPE_VOID) {
+				    src_type->kind == ZEND_FFI_TYPE_VOID ||
+				    src_type->kind == dst_type->kind) {
 				    return 1;
 				}
 			} else if (dst_type->kind == ZEND_FFI_TYPE_ARRAY &&

$ php -a
Interactive shell

php > $ffi = FFI::cdef('void func(); void (*func_ptr)(); void call_func();', __DIR__ . '/libffibug.so');
php > $ffi->call_func();
php > $ffi->func_ptr = $ffi->func;
php > $ffi->call_func();
hello
php > 
$ 

I'm not familiar with FFI internals so could use a review. FFI tests continue to pass at least. If it looks ok I can follow up with a PR + phpt.

PHP Version

HEAD

Operating System

Debian testing

[cmb69]

Hmm, I think we could do that, but that would allow to assign any function pointer to any other. So we would not only ignore the return and parameter types, but also the calling convention. That would seriously undermine the type safety of FFI. To wit:

$ cat ffibug.c 
#include <stdio.h>

void func(char *str) {
    printf("hello %s\n");
}

void (*func_ptr)() = NULL;

void call_func() {
    if (func_ptr != NULL) func_ptr();
}
$ gcc -shared -fPIC -o libffibug.so ffibug.c
$ php -v
PHP 8.2.0-dev (cli) (built: Jul 19 2022 20:25:19) (NTS DEBUG)
Copyright (c) The PHP Group
Zend Engine v4.2.0-dev, Copyright (c) Zend Technologies
$ php -a
Interactive shell

php > $ffi = FFI::cdef('void func(char *); void (*func_ptr)(); void call_func();', __DIR__ . '/libffibug.so');
php > $ffi->func_ptr = $ffi->func;
php > $ffi->call_func();

Segmentation fault (core dumped)

If we want to support function pointer compatibility (what looks sensible), we should also check the details (src_type->func and dst_type->func).

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Yes good idea @cmb69. I'll submit a PR incorporating that.

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#9107