syscall: simplify execve handling on libc platforms

Rather than providing three different execve variables for different
platforms, use a single variable. Provide a small wrapper that handles
conversion to uintptr for the AIX/Solaris case.

Note that this removes special handling for openbsd/mips64, which is
now a dead port.

Updates #61546

Change-Id: I3d6387c31669f64bfb61639536803e595f478647
Reviewed-on: https://go-review.googlesource.com/c/go/+/693880
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>

src/syscall/exec_libc.go

@@ -61,7 +61,13 @@ func write1(fd uintptr, buf uintptr, nbyte uintptr) (n uintptr, err Errno)
 
 // syscall defines this global on our behalf to avoid a build dependency on other platforms
 func init() {
-	execveLibc = execve
+	execveLibc = execveLibcWrapper
+}
+
+func execveLibcWrapper(path *byte, argv **byte, envp **byte) error {
+	return execve(uintptr(unsafe.Pointer(path)),
+		uintptr(unsafe.Pointer(argv)),
+		uintptr(unsafe.Pointer(envp)))
 }
 
 // Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.

src/syscall/exec_unix.go

@@ -265,9 +265,7 @@ func runtime_AfterExec()
 
 // execveLibc is non-nil on OS using libc syscall, set to execve in exec_libc.go; this
 // avoids a build dependency for other platforms.
-var execveLibc func(path uintptr, argv uintptr, envp uintptr) Errno
-var execveDarwin func(path *byte, argv **byte, envp **byte) error
-var execveOpenBSD func(path *byte, argv **byte, envp **byte) error
+var execveLibc func(path *byte, argv **byte, envp **byte) error
 
 // Exec invokes the execve(2) system call.
 func Exec(argv0 string, argv []string, envv []string) (err error) {
@@ -291,19 +289,12 @@ func Exec(argv0 string, argv []string, envv []string) (err error) {
 	}
 
 	var err1 error
-	if runtime.GOOS == "solaris" || runtime.GOOS == "illumos" || runtime.GOOS == "aix" {
-		// RawSyscall should never be used on Solaris, illumos, or AIX.
-		err1 = execveLibc(
-			uintptr(unsafe.Pointer(argv0p)),
-			uintptr(unsafe.Pointer(&argvp[0])),
-			uintptr(unsafe.Pointer(&envvp[0])))
-	} else if runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
-		// Similarly on Darwin.
-		err1 = execveDarwin(argv0p, &argvp[0], &envvp[0])
-	} else if runtime.GOOS == "openbsd" && runtime.GOARCH != "mips64" {
-		// Similarly on OpenBSD.
-		err1 = execveOpenBSD(argv0p, &argvp[0], &envvp[0])
-	} else {
+	switch runtime.GOOS {
+	case "aix", "darwin", "illumos", "ios", "openbsd", "solaris":
+		// RawSyscall should never be used on these platforms.
+		err1 = execveLibc(argv0p, &argvp[0], &envvp[0])
+
+	default:
 		_, _, err1 = RawSyscall(SYS_EXECVE,
 			uintptr(unsafe.Pointer(argv0p)),
 			uintptr(unsafe.Pointer(&argvp[0])),

src/syscall/syscall_darwin.go

@@ -228,7 +228,7 @@ func Kill(pid int, signum Signal) (err error) { return kill(pid, int(signum), 1)
 //sys	getcwd(buf []byte) (n int, err error)
 
 func init() {
-	execveDarwin = execve
+	execveLibc = execve
 }
 
 func fdopendir(fd int) (dir uintptr, err error) {

src/syscall/syscall_openbsd_libc.go

@@ -13,7 +13,7 @@ import (
 var dupTrampoline = abi.FuncPCABI0(libc_dup3_trampoline)
 
 func init() {
-	execveOpenBSD = execve
+	execveLibc = execve
 }
 
 func syscallInternal(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno) {