syscall: clean up variable declarations in forkAndExecInChild

The various forkAndExecInChild implementations have comments
explaining that they pre-declare variables to force allocations
to occur before forking, but then later use ":=" declarations
for additional variables.

To make it clearer that those ":=" declarations do not allocate,
we move their declarations up to the predeclared blocks.

For #57208.

Change-Id: Ie8cb577fa7180b51b64d6dc398169053fdf8ea97
Reviewed-on: https://go-review.googlesource.com/c/go/+/456516
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 13 insertions, 10 deletions

diff --git a/src/syscall/exec_libc2.go b/src/syscall/exec_libc2.go
index 41bc79a721..9b04f96c81 100644
--- a/src/syscall/exec_libc2.go
+++ b/src/syscall/exec_libc2.go
@@ -52,14 +52,17 @@ func runtime_AfterForkInChild()
 // functions that do not grow the stack.
 //
 //go:norace
-func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
+func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err1 Errno) {
 	// Declare all variables at top in case any
 	// declarations require heap allocation (e.g., err1).
 	var (
-		r1     uintptr
-		err1   Errno
-		nextfd int
-		i      int
+		r1              uintptr
+		nextfd          int
+		i               int
+		err             error
+		pgrp            _C_int
+		cred            *Credential
+		ngroups, groups uintptr
 	)
 
 	// guard against side effects of shuffling fds below.
@@ -94,7 +97,7 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 
 	// Enable tracing if requested.
 	if sys.Ptrace {
-		if err := ptrace(PTRACE_TRACEME, 0, 0, 0); err != nil {
+		if err = ptrace(PTRACE_TRACEME, 0, 0, 0); err != nil {
 			err1 = err.(Errno)
 			goto childerror
 		}
@@ -120,7 +123,7 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 	if sys.Foreground {
 		// This should really be pid_t, however _C_int (aka int32) is
 		// generally equivalent.
-		pgrp := _C_int(sys.Pgid)
+		pgrp = _C_int(sys.Pgid)
 		if pgrp == 0 {
 			r1, _, err1 = rawSyscall(abi.FuncPCABI0(libc_getpid_trampoline), 0, 0, 0)
 			if err1 != 0 {
@@ -149,9 +152,9 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 	}
 
 	// User and groups
-	if cred := sys.Credential; cred != nil {
-		ngroups := uintptr(len(cred.Groups))
-		groups := uintptr(0)
+	if cred = sys.Credential; cred != nil {
+		ngroups = uintptr(len(cred.Groups))
+		groups = uintptr(0)
 		if ngroups > 0 {
 			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
 		}