syscall: fix a few Linux system calls

These functions claimed to return error (an interface)
and be implemented entirely in assembly, but it's not
possible to create an interface from assembly
(at least not easily).

In reality the functions were written to return an errno uintptr
despite the Go prototype saying error.
When the errno was 0, they coincidentally filled out a nil error
by writing the 0 to the type word of the interface.
If the errno was ever non-zero, the functions would
create a non-nil error that would crash when trying to
call err.Error().

Luckily these functions (Seek, Time, Gettimeofday) pretty
much never fail, so it was all kind of working.

Found by go vet.

LGTM=bradfitz, r
R=golang-codereviews, bradfitz, r
CC=golang-codereviews
https://golang.org/cl/99320043

src/pkg/syscall/asm_linux_386.s

@@ -162,7 +162,7 @@ oksock1:
 // taking the address of the return value newoffset.
 // Underlying system call is
 //	llseek(int fd, int offhi, int offlo, int64 *result, int whence)
-TEXT ·Seek(SB),NOSPLIT,$0-32
+TEXT ·seek(SB),NOSPLIT,$0-28
 	CALL	runtime·entersyscall(SB)
 	MOVL	$SYS__LLSEEK, AX	// syscall entry
 	MOVL	4(SP), BX	// fd

src/pkg/syscall/asm_linux_amd64.s

@@ -110,7 +110,7 @@ ok2:
 	MOVQ	$0, 80(SP)	// errno
 	RET
 
-TEXT ·Gettimeofday(SB),NOSPLIT,$0-24
+TEXT ·gettimeofday(SB),NOSPLIT,$0-16
 	MOVQ	8(SP), DI
 	MOVQ	$0, SI
 	MOVQ	runtime·__vdso_gettimeofday_sym(SB), AX
@@ -124,11 +124,3 @@ TEXT ·Gettimeofday(SB),NOSPLIT,$0-24
 ok7:
 	MOVQ	$0, 16(SP)  // errno
 	RET
-
-TEXT ·Time(SB),NOSPLIT,$0-32
-	MOVQ	8(SP), DI
-	MOVQ	runtime·__vdso_time_sym(SB), AX
-	CALL	AX
-	MOVQ	AX, 16(SP)  // tt
-	MOVQ	$0, 24(SP)  // errno
-	RET

src/pkg/syscall/asm_linux_arm.s

@@ -98,7 +98,7 @@ ok2:
 	RET
 
 #define SYS__LLSEEK 140  /* from zsysnum_linux_arm.go */
-// func Seek(fd int, offset int64, whence int) (newoffset int64, errno int)
+// func seek(fd int, offset int64, whence int) (newoffset int64, errno int)
 // Implemented in assembly to avoid allocation when
 // taking the address of the return value newoffset.
 // Underlying system call is

src/pkg/syscall/syscall_linux_386.go

@@ -135,7 +135,15 @@ func Setrlimit(resource int, rlim *Rlimit) (err error) {
 
 // Underlying system call writes to newoffset via pointer.
 // Implemented in assembly to avoid allocation.
-func Seek(fd int, offset int64, whence int) (newoffset int64, err error)
+func seek(fd int, offset int64, whence int) (newoffset int64, err Errno)
+
+func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
+	newoffset, errno := seek(fd, offset, whence)
+	if errno != 0 {
+		return 0, errno
+	}
+	return newoffset, nil
+}
 
 // Vsyscalls on amd64.
 //sysnb	Gettimeofday(tv *Timeval) (err error)

src/pkg/syscall/syscall_linux_amd64.go

@@ -58,8 +58,28 @@ package syscall
 
 func Getpagesize() int { return 4096 }
 
-func Gettimeofday(tv *Timeval) (err error)
-func Time(t *Time_t) (tt Time_t, err error)
+//go:noescape
+func gettimeofday(tv *Timeval) (err Errno)
+
+func Gettimeofday(tv *Timeval) (err error) {
+	errno := gettimeofday(tv)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}
+
+func Time(t *Time_t) (tt Time_t, err error) {
+	var tv Timeval
+	errno := gettimeofday(&tv)
+	if errno != 0 {
+		return errno
+	}
+	if t != nil {
+		*t = tv.Sec
+	}
+	return Time_t(tv.Sec), nil
+}
 
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }
 

src/pkg/syscall/syscall_linux_arm.go

@@ -23,9 +23,17 @@ func NsecToTimeval(nsec int64) (tv Timeval) {
 	return
 }
 
-// Seek is defined in assembly.
+// Underlying system call writes to newoffset via pointer.
+// Implemented in assembly to avoid allocation.
+func seek(fd int, offset int64, whence int) (newoffset int64, err Errno)
 
-func Seek(fd int, offset int64, whence int) (newoffset int64, err error)
+func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
+	newoffset, errno := seek(fd, offset, whence)
+	if errno != 0 {
+		return 0, errno
+	}
+	return newoffset, nil
+}
 
 //sys	accept(s int, rsa *RawSockaddrAny, addrlen *_Socklen) (fd int, err error)
 //sys	accept4(s int, rsa *RawSockaddrAny, addrlen *_Socklen, flags int) (fd int, err error)

src/pkg/syscall/syscall_unix_test.go

@@ -300,3 +300,15 @@ func TestRlimit(t *testing.T) {
 		t.Fatalf("Setrlimit: restore failed: %#v %v", rlimit, err)
 	}
 }
+
+func TestSeekFailure(t *testing.T) {
+	_, err := syscall.Seek(-1, 0, 0)
+	if err == nil {
+		t.Fatalf("Seek(-1, 0, 0) did not fail")
+	}
+	str := err.Error() // used to crash on Linux
+	t.Logf("Seek: %v", str)
+	if str == "" {
+		t.Fatalf("Seek(-1, 0, 0) return error with empty message")
+	}
+}