app: build fake auxv

Some Android systems cannot read from /proc/self/auxv. Work around
this by creating a minimal fake auxv for the Go runtime.

Fixes #9229

Change-Id: Id2d743f43d91be23006aac708a5b5b5cd60a6dca
Reviewed-on: https://go-review.googlesource.com/1284
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

app/android.c

@@ -48,41 +48,18 @@ static void* init_go_runtime(void* unused) {
 	// Defensively heap-allocate argv0, for setenv.
 	char* argv0 = strdup("gojni");
 
-	// Build argv, including the ELF auxiliary vector, which is loaded
-	// from /proc/self/auxv. While there does not appear to be any
-	// spec for this format, there are some notes in
-	//
-	// Phrack, V. 0x0b, Issue 0x3a, P. 0x05.
-	// http://phrack.org/issues/58/5.html
-	//
-	// For our needs, we don't need to know the format beyond the fact
-	// that argv is followed by a meaningless envp, then a series of
-	// NUL terminated bytes that make up auxv.
-
+	// Build argv, including the ELF auxiliary vector.
 	struct {
 		char* argv[2];
 		char* envp[2];
-		char* auxv[1024];
+		uint32_t auxv[64];
 	} x;
 	x.argv[0] = argv0;
 	x.argv[1] = NULL;
 	x.envp[0] = argv0;
 	x.envp[1] = NULL;
 
-	int fd = open("/proc/self/auxv", O_RDONLY, 0);
-	if (fd == -1) {
-		__android_log_print(ANDROID_LOG_FATAL, "Go", "cannot open /proc/self/auxv: %s", strerror(errno));
-	}
-	int n = read(fd, &x.auxv, sizeof x.auxv - 1);
-	if (n < 0) {
-		__android_log_print(ANDROID_LOG_FATAL, "Go", "error reading /proc/self/auxv: %s", strerror(errno));
-	}
-	if (n == sizeof x.auxv - 1) { // x.auxv should be more than plenty.
-		__android_log_print(ANDROID_LOG_FATAL, "Go", "/proc/self/auxv too big");
-	}
-	close(fd);
-	x.auxv[n / sizeof(char*)] = NULL;
-
+	build_auxv(x.auxv, sizeof(x.auxv)/sizeof(uint32_t));
 	int32_t argc = 1;
 	_rt0_arm_linux1(argc, x.argv);
 	return NULL;

app/android.go

@@ -33,6 +33,21 @@ int go_started;
 // OpenJDK there is JNI_GetCreatedJavaVMs, but this is not available
 // on android.
 JavaVM* current_vm;
+
+// build_auxv builds an ELF auxiliary vector for initializing the Go
+// runtime. While there does not appear to be any spec for this
+// format, there are some notes in
+//
+// Phrack, V. 0x0b, Issue 0x3a, P. 0x05.
+// http://phrack.org/issues/58/5.html
+//
+// Much of the time on linux the real auxv can be read from the file
+// /proc/self/auxv, however there are several conditions under which
+// Android apps cannot read this file (see a note to this effect in
+// sources/android/cpufeatures/cpu-features.c). So we construct a
+// fake one, working backwards from what the Go runtime wants to see
+// as defined by the code in src/runtime/os_linux_GOARCH.c.
+void build_auxv(uint32_t *auxv, size_t len);
 */
 import "C"
 import (

app/android_arm.c

@@ -0,0 +1,31 @@
+// 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.
+
+// +build android,arm
+
+#include <android/log.h>
+#include <stdint.h>
+#include <string.h>
+#include "_cgo_export.h"
+
+#define AT_PLATFORM  15
+#define AT_HWCAP     16
+#define HWCAP_VFP    (1 << 6)
+#define HWCAP_VFPv3  (1 << 13)
+
+void build_auxv(uint32_t *auxv, size_t len) {
+	// Minimum auxv required by runtime/os_linux_arm.go.
+	int i;
+	if (len < 5) {
+		__android_log_print(ANDROID_LOG_FATAL, "Go", "auxv len %d too small", len);
+	}
+	auxv[0] = AT_PLATFORM;
+	*(char**)&auxv[1] = strdup("v7l");
+
+	auxv[2] = AT_HWCAP;
+	auxv[3] = HWCAP_VFP | HWCAP_VFPv3;
+	for (i = 4; i < len; i++) {
+		auxv[i] = 0;
+	}
+}

app/android_x86.c

@@ -0,0 +1,38 @@
+// 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.
+
+// +build android,x86
+
+#include <android/log.h>
+#include <dlfcn.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+#include "_cgo_export.h"
+
+void build_auxv(uint32_t *xauxv, size_t xauxv_len) {
+	char* auxv = (char*)xauxv;
+	size_t auxv_len = xauxv_len*sizeof(uint32_t);
+
+	// TODO(crawshaw): determine if we can read /proc/self/auxv on
+	//                 x86 android release builds.
+	int fd = open("/proc/self/auxv", O_RDONLY, 0);
+	if (fd == -1) {
+		__android_log_print(ANDROID_LOG_FATAL, "Go", "cannot open /proc/self/auxv: %s", strerror(errno));
+	}
+	int n = read(fd, &auxv, auxv_len);
+	if (n < 0) {
+		__android_log_print(ANDROID_LOG_FATAL, "Go", "error reading /proc/self/auxv: %s", strerror(errno));
+	}
+	if (n == auxv_len) { // auxv should be more than plenty.
+		__android_log_print(ANDROID_LOG_FATAL, "Go", "/proc/self/auxv too big");
+	}
+	close(fd);
+
+	for (; n < auxv_len; n++) {
+		auxv[n] = 0;
+	}
+}