runtime: don't crash if vsyscall and vdso are disabled on x86_64

If vdso is disabled, the goruntime calls gettimeofday from vsyscall.
But if vsyscall is disabled too, all golang binaries crash:

SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0xffffffffff600000} ---
killed by SIGSEGV (core dumped) ++

vsyscall doesn't work as it was designed for a long time due to security
reasons and now vsyscall is a little more expensive than real syscalls:
torvalds/linux@5cec93c216db

This patch reworks the code to call syscalls if the vdso library isn't
available.

Signed-off-by: Andrei Vagin <avagin@google.com>

Author: avagin

src/runtime/sys_linux_amd64.s

@@ -40,6 +40,7 @@
 #define SYS_futex		202
 #define SYS_sched_getaffinity	204
 #define SYS_epoll_create	213
+#define SYS_clock_gettime	228
 #define SYS_exit_group		231
 #define SYS_epoll_ctl		233
 #define SYS_tgkill		234
@@ -241,15 +242,15 @@ noswitch:
 	SUBQ	$16, SP		// Space for results
 	ANDQ	$~15, SP	// Align for C code
 
+	MOVL	$0, DI // CLOCK_REALTIME
+	LEAQ	0(SP), SI
 	MOVQ	runtime·vdsoClockgettimeSym(SB), AX
 	CMPQ	AX, $0
 	JEQ	fallback
-	MOVL	$0, DI // CLOCK_REALTIME
-	LEAQ	0(SP), SI
 	CALL	AX
+ret:
 	MOVQ	0(SP), AX	// sec
 	MOVQ	8(SP), DX	// nsec
-ret:
 	MOVQ	R12, SP		// Restore real SP
 	// Restore vdsoPC, vdsoSP
 	// We don't worry about being signaled between the two stores.
@@ -264,13 +265,8 @@ ret:
 	MOVL	DX, nsec+8(FP)
 	RET
 fallback:
-	LEAQ	0(SP), DI
-	MOVQ	$0, SI
-	MOVQ	runtime·vdsoGettimeofdaySym(SB), AX
-	CALL	AX
-	MOVQ	0(SP), AX	// sec
-	MOVL	8(SP), DX	// usec
-	IMULQ	$1000, DX
+	MOVQ	$SYS_clock_gettime, AX
+	SYSCALL
 	JMP ret
 
 // func nanotime1() int64
@@ -306,15 +302,15 @@ noswitch:
 	SUBQ	$16, SP		// Space for results
 	ANDQ	$~15, SP	// Align for C code
 
+	MOVL	$1, DI // CLOCK_MONOTONIC
+	LEAQ	0(SP), SI
 	MOVQ	runtime·vdsoClockgettimeSym(SB), AX
 	CMPQ	AX, $0
 	JEQ	fallback
-	MOVL	$1, DI // CLOCK_MONOTONIC
-	LEAQ	0(SP), SI
 	CALL	AX
+ret:
 	MOVQ	0(SP), AX	// sec
 	MOVQ	8(SP), DX	// nsec
-ret:
 	MOVQ	R12, SP		// Restore real SP
 	// Restore vdsoPC, vdsoSP
 	// We don't worry about being signaled between the two stores.
@@ -332,13 +328,8 @@ ret:
 	MOVQ	AX, ret+0(FP)
 	RET
 fallback:
-	LEAQ	0(SP), DI
-	MOVQ	$0, SI
-	MOVQ	runtime·vdsoGettimeofdaySym(SB), AX
-	CALL	AX
-	MOVQ	0(SP), AX	// sec
-	MOVL	8(SP), DX	// usec
-	IMULQ	$1000, DX
+	MOVQ	$SYS_clock_gettime, AX
+	SYSCALL
 	JMP	ret
 
 TEXT runtime·rtsigprocmask(SB),NOSPLIT,$0-28

src/runtime/vdso_linux_amd64.go

@@ -17,8 +17,7 @@ var vdsoSymbolKeys = []vdsoSymbolKey{
 	{"__vdso_clock_gettime", 0xd35ec75, 0x6e43a318, &vdsoClockgettimeSym},
 }
 
-// initialize with vsyscall fallbacks
 var (
-	vdsoGettimeofdaySym uintptr = 0xffffffffff600000
-	vdsoClockgettimeSym uintptr = 0
+	vdsoGettimeofdaySym uintptr
+	vdsoClockgettimeSym uintptr
 )

src/syscall/asm_linux_amd64.s

@@ -9,6 +9,8 @@
 // System calls for AMD64, Linux
 //
 
+#define SYS_gettimeofday 96
+
 // func Syscall(trap int64, a1, a2, a3 uintptr) (r1, r2, err uintptr);
 // Trap # in AX, args in DI SI DX R10 R8 R9, return in AX DX
 // Note that this differs from "standard" ABI convention, which
@@ -144,13 +146,19 @@ TEXT ·gettimeofday(SB),NOSPLIT,$0-16
 	MOVQ	tv+0(FP), DI
 	MOVQ	$0, SI
 	MOVQ	runtime·vdsoGettimeofdaySym(SB), AX
+	TESTQ   AX, AX
+	JZ fallback
 	CALL	AX
-
+ret:
 	CMPQ	AX, $0xfffffffffffff001
 	JLS	ok7
 	NEGQ	AX
 	MOVQ	AX, err+8(FP)
 	RET
+fallback:
+	MOVL	$SYS_gettimeofday, AX
+	SYSCALL
+	JMP ret
 ok7:
 	MOVQ	$0, err+8(FP)
 	RET