src/runtime/cgo/gcc_android.c

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

#include <pthread.h>
#include <dlfcn.h>
#include "libcgo.h"

// Truncated to a different magic value on 32-bit; that's ok.
#define magic1 (0x23581321345589ULL)

// From https://android.googlesource.com/platform/bionic/+/refs/heads/android10-tests-release/libc/private/bionic_asm_tls.h#69.
#define TLS_SLOT_APP 2

// inittls allocates a thread-local storage slot for g.
//
// It finds the first available slot using pthread_key_create and uses
// it as the offset value for runtime.tls_g.
static void
inittls(void **tlsg, void **tlsbase)
{
	pthread_key_t k;
	int i, err;
	void *handle, *get_ver, *off;

	// Check for Android Q where we can use the free TLS_SLOT_APP slot.
	handle = dlopen("libc.so", RTLD_LAZY);
	if (handle == NULL) {
		fatalf("inittls: failed to dlopen main program");
		return;
	}
	// android_get_device_api_level is introduced in Android Q, so its mere presence
	// is enough.
	get_ver = dlsym(handle, "android_get_device_api_level");
	dlclose(handle);
	if (get_ver != NULL) {
		off = (void *)(TLS_SLOT_APP*sizeof(void *));
		// tlsg is initialized to Q's free TLS slot. Verify it while we're here.
		if (*tlsg != off) {
			fatalf("tlsg offset wrong, got %ld want %ld\n", *tlsg, off);
		}
		return;
	}

	err = pthread_key_create(&k, nil);
	if(err != 0) {
		fatalf("pthread_key_create failed: %d", err);
	}
	pthread_setspecific(k, (void*)magic1);
	// If thread local slots are laid out as we expect, our magic word will
	// be located at some low offset from tlsbase. However, just in case something went
	// wrong, the search is limited to sensible offsets. PTHREAD_KEYS_MAX was the
	// original limit, but issue 19472 made a higher limit necessary.
	for (i=0; i<384; i++) {
		if (*(tlsbase+i) == (void*)magic1) {
			*tlsg = (void*)(i*sizeof(void *));
			pthread_setspecific(k, 0);
			return;
		}
	}
	fatalf("inittls: could not find pthread key");
}

void (*x_cgo_inittls)(void **tlsg, void **tlsbase) = inittls;