Document the vDSO and add a reference parser

It turns out that parsing the vDSO is nontrivial if you don't already
have an ELF dynamic loader around.  So document it in Documentation/ABI
and add a reference CC0-licenced parser.

This code is dedicated to Go issue 1933:
http://code.google.com/p/go/issues/detail?id=1933

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/a315a9514cd71bcf29436cc31e35aada21a5ff21.1310563276.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 111 insertions, 0 deletions

diff --git a/Documentation/vDSO/vdso_test.c b/Documentation/vDSO/vdso_test.c
new file mode 100644
index 000000000000..fff633432dff
--- /dev/null
+++ b/Documentation/vDSO/vdso_test.c
@@ -0,0 +1,111 @@
+/*
+ * vdso_test.c: Sample code to test parse_vdso.c on x86_64
+ * Copyright (c) 2011 Andy Lutomirski
+ * Subject to the GNU General Public License, version 2
+ *
+ * You can amuse yourself by compiling with:
+ * gcc -std=gnu99 -nostdlib
+ *     -Os -fno-asynchronous-unwind-tables -flto
+ *      vdso_test.c parse_vdso.c -o vdso_test
+ * to generate a small binary with no dependencies at all.
+ */
+
+#include <sys/syscall.h>
+#include <sys/time.h>
+#include <unistd.h>
+#include <stdint.h>
+
+extern void *vdso_sym(const char *version, const char *name);
+extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
+extern void vdso_init_from_auxv(void *auxv);
+
+/* We need a libc functions... */
+int strcmp(const char *a, const char *b)
+{
+	/* This implementation is buggy: it never returns -1. */
+	while (*a || *b) {
+		if (*a != *b)
+			return 1;
+		if (*a == 0 || *b == 0)
+			return 1;
+		a++;
+		b++;
+	}
+
+	return 0;
+}
+
+/* ...and two syscalls.  This is x86_64-specific. */
+static inline long linux_write(int fd, const void *data, size_t len)
+{
+
+	long ret;
+	asm volatile ("syscall" : "=a" (ret) : "a" (__NR_write),
+		      "D" (fd), "S" (data), "d" (len) :
+		      "cc", "memory", "rcx",
+		      "r8", "r9", "r10", "r11" );
+	return ret;
+}
+
+static inline void linux_exit(int code)
+{
+	asm volatile ("syscall" : : "a" (__NR_exit), "D" (code));
+}
+
+void to_base10(char *lastdig, uint64_t n)
+{
+	while (n) {
+		*lastdig = (n % 10) + '0';
+		n /= 10;
+		lastdig--;
+	}
+}
+
+__attribute__((externally_visible)) void c_main(void **stack)
+{
+	/* Parse the stack */
+	long argc = (long)*stack;
+	stack += argc + 2;
+
+	/* Now we're pointing at the environment.  Skip it. */
+	while(*stack)
+		stack++;
+	stack++;
+
+	/* Now we're pointing at auxv.  Initialize the vDSO parser. */
+	vdso_init_from_auxv((void *)stack);
+
+	/* Find gettimeofday. */
+	typedef long (*gtod_t)(struct timeval *tv, struct timezone *tz);
+	gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
+
+	if (!gtod)
+		linux_exit(1);
+
+	struct timeval tv;
+	long ret = gtod(&tv, 0);
+
+	if (ret == 0) {
+		char buf[] = "The time is                     .000000\n";
+		to_base10(buf + 31, tv.tv_sec);
+		to_base10(buf + 38, tv.tv_usec);
+		linux_write(1, buf, sizeof(buf) - 1);
+	} else {
+		linux_exit(ret);
+	}
+
+	linux_exit(0);
+}
+
+/*
+ * This is the real entry point.  It passes the initial stack into
+ * the C entry point.
+ */
+asm (
+	".text\n"
+	".global _start\n"
+        ".type _start,@function\n"
+        "_start:\n\t"
+        "mov %rsp,%rdi\n\t"
+        "jmp c_main"
+	);