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

7 files changed, 57 insertions, 17 deletions

diff --git a/src/pkg/syscall/asm_linux_386.s b/src/pkg/syscall/asm_linux_386.s
index 668fc2bee5..30b22073df 100644
--- a/src/pkg/syscall/asm_linux_386.s
+++ b/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
diff --git a/src/pkg/syscall/asm_linux_amd64.s b/src/pkg/syscall/asm_linux_amd64.s
index d75096dec0..995b60ecd0 100644
--- a/src/pkg/syscall/asm_linux_amd64.s
+++ b/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
diff --git a/src/pkg/syscall/asm_linux_arm.s b/src/pkg/syscall/asm_linux_arm.s
index bf54b4fe6d..8499a73704 100644
--- a/src/pkg/syscall/asm_linux_arm.s
+++ b/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
diff --git a/src/pkg/syscall/syscall_linux_386.go b/src/pkg/syscall/syscall_linux_386.go
index 4487848f73..c491a286ce 100644
--- a/src/pkg/syscall/syscall_linux_386.go
+++ b/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)
diff --git a/src/pkg/syscall/syscall_linux_amd64.go b/src/pkg/syscall/syscall_linux_amd64.go
index 9e8663d014..1e330dd99c 100644
--- a/src/pkg/syscall/syscall_linux_amd64.go
+++ b/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) }
 
diff --git a/src/pkg/syscall/syscall_linux_arm.go b/src/pkg/syscall/syscall_linux_arm.go
index 68c796b448..9fe80232a8 100644
--- a/src/pkg/syscall/syscall_linux_arm.go
+++ b/src/pkg/syscall/syscall_linux_arm.go
@@ -23,9 +23,17 @@ func NsecToTimeval(nsec int64) (tv Timeval) {
 	return
 }
 
-// Seek is defined in assembly.
-
-func Seek(fd int, offset int64, whence int) (newoffset int64, 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 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
+}
 
 //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)
diff --git a/src/pkg/syscall/syscall_unix_test.go b/src/pkg/syscall/syscall_unix_test.go
index 32d7b9f2e4..a0afb91fcf 100644
--- a/src/pkg/syscall/syscall_unix_test.go
+++ b/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")
+	}
+}