Re: Can't build lang/openjdk{17,21} on NetBSD/aarch64 (Apple M3)

[PHO <pho%cielonegro.org@localhost>]

My previous mail about Apple Silicon's W^X enforcement turned out to be 
wrong. I wrote this code to see if W+X works on the very chip, and 
surprisingly, it worked:

#include <err.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>

const uint32_t insn_ret = 0xD65F03C0;
const uint32_t insn_nop = 0xD503201F;

int main() {
	printf("Allocating a page with W+X. This would fail unless PaX MPROTECT 
is enabled.\n");
	void* page = mmap(NULL, sysconf(_SC_PAGESIZE), PROT_EXEC|PROT_WRITE, 
MAP_ANON, -1, 0);
	if (page == MAP_FAILED)
		err(1, "mmap failed. Did you forget to paxctl +m the executable?");

	printf("Successfully mapped a page. Now writing some instructions on 
it...\n");
	uint32_t* insn_p = page;
	*(insn_p++) = insn_ret;
	*(insn_p++) = insn_nop;
	__builtin___clear_cache(page, page + sysconf(_SC_PAGESIZE));

	printf("Done. Now calling it as a function.\n");
	typedef void(*thunk_t)();
	thunk_t thunk = page;
	thunk();

	printf("We survived and returned to main(). The call was successful.\n");
	printf("What if we rewrite instructions and run it again? Does that 
work?\n");
	insn_p = page;
	*(insn_p++) = 0xD2800550; // mov x16, #42
	*(insn_p++) = insn_ret;
	*(insn_p++) = insn_nop;
	__builtin___clear_cache(page, page + sysconf(_SC_PAGESIZE));

	register uint64_t x16 __asm__("x16") = 0;
	thunk();
	// Tell the compiler that we just clobbered x16.
	__asm__ __volatile__ ("": "=r"(x16));

	if (x16 == 42)
		printf("It worked!\n");
	else
		errx(1, "We tried to write 42 to x16 but x16 now contains something we 
didn't expect: %lu\n", x16);
	return 0;
}

So it's likely that the problem is about clearing instruction cache, 
which OpenJDK 17 does, but somehow isn't working on this chip. But 
omitting __builtin_clear_cache() from the code above did make it execute 
wrong instructions so the builtin itself is working.