add a real explicit_bzero implementation

Clang, GCC and other compilers special-case standard C functions like
memset. Calls to memset will be optimized out.

OpenBSD provides explicit_bzero to work around this but Android simply
defines it as memset so nothing prevents it from being optimized away.

This implementation uses a memory read constraint via empty inline
assembly rather than something that may be broken via link-time
optimization in the future.

libc/Android.bp

@@ -1028,6 +1028,7 @@ cc_library_static {
         "bionic/error.cpp",
         "bionic/eventfd.cpp",
         "bionic/exec.cpp",
+        "bionic/explicit_bzero.cpp",
         "bionic/faccessat.cpp",
         "bionic/fchmod.cpp",
         "bionic/fchmodat.cpp",

libc/bionic/explicit_bzero.cpp

@@ -0,0 +1,7 @@
+#include <string.h>
+
+void* explicit_bzero(void* s, size_t n) {
+  void *ptr = memset(s, 0, n);
+  __asm__ __volatile__("" : : "r"(ptr) : "memory");
+  return ptr;
+}

libc/include/string.h

@@ -56,6 +56,7 @@ void* mempcpy(void* __dst, const void* __src, size_t __n) __INTRODUCED_IN(23);
 #endif
 void* memmove(void* __dst, const void* __src, size_t __n);
 void* memset(void* __dst, int __ch, size_t __n);
+void* explicit_bzero(void *s, size_t n);
 void* memmem(const void* __haystack, size_t __haystack_size, const void* __needle, size_t __needle_size) __attribute_pure__;
 
 char* strchr(const char* __s, int __ch) __attribute_pure__;

libc/libc.map.txt

@@ -332,6 +332,7 @@ LIBC {
     execvp;
     execvpe; # introduced=21
     exit;
+    explicit_bzero; # introduced=30
     faccessat;
     fallocate; # introduced=21
     fallocate64; # introduced=21

libc/upstream-openbsd/android/include/openbsd-compat.h

@@ -53,8 +53,6 @@ extern const char* __progname;
 /* OpenBSD has this, but we can't really implement it correctly on Linux. */
 #define issetugid() 0
 
-#define explicit_bzero(p, s) memset(p, 0, s)
-
 /* OpenBSD has these in <sys/param.h>, but "ALIGN" isn't something we want to reserve. */
 #define ALIGNBYTES (sizeof(uintptr_t) - 1)
 #define ALIGN(p) (((uintptr_t)(p) + ALIGNBYTES) &~ ALIGNBYTES)