syscall: forkAndExecInChild1: don't reuse pid variable

A named return variable pid is reused in a few places, and while
the code is not wrong, it is somewhat confusing.

This variable used to be called r1 before CL 456516 (which did the right
thing, but slightly added to the confusion).

Now, the code calling SYS_WRITE (initially added by CL 158298) never
checks the number of bytes written, so let's remove the assignment.

In the code that calls SYS_READ it is used, so let's use a different
variable, c, which seems less confusing.

All this hopefully makes the code more readable.

Change-Id: I0d7ec311615100deb7e0aa3f02384eadcc1b47e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/696835
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: kolyshkin

src/syscall/exec_linux.go

@@ -365,11 +365,11 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 		if _, _, err1 = RawSyscall(SYS_CLOSE, uintptr(mapPipe[1]), 0, 0); err1 != 0 {
 			goto childerror
 		}
-		pid, _, err1 = RawSyscall(SYS_READ, uintptr(mapPipe[0]), uintptr(unsafe.Pointer(&err2)), unsafe.Sizeof(err2))
+		c, _, err1 = RawSyscall(SYS_READ, uintptr(mapPipe[0]), uintptr(unsafe.Pointer(&err2)), unsafe.Sizeof(err2))
 		if err1 != 0 {
 			goto childerror
 		}
-		if pid != unsafe.Sizeof(err2) {
+		if c != unsafe.Sizeof(err2) {
 			err1 = EINVAL
 			goto childerror
 		}
@@ -427,7 +427,7 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 			if fd1, _, err1 = RawSyscall6(SYS_OPENAT, uintptr(dirfd), uintptr(unsafe.Pointer(&psetgroups[0])), uintptr(O_WRONLY), 0, 0, 0); err1 != 0 {
 				goto childerror
 			}
-			pid, _, err1 = RawSyscall(SYS_WRITE, fd1, uintptr(unsafe.Pointer(&setgroups[0])), uintptr(len(setgroups)))
+			_, _, err1 = RawSyscall(SYS_WRITE, fd1, uintptr(unsafe.Pointer(&setgroups[0])), uintptr(len(setgroups)))
 			if err1 != 0 {
 				goto childerror
 			}
@@ -438,7 +438,7 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 			if fd1, _, err1 = RawSyscall6(SYS_OPENAT, uintptr(dirfd), uintptr(unsafe.Pointer(&pgid[0])), uintptr(O_WRONLY), 0, 0, 0); err1 != 0 {
 				goto childerror
 			}
-			pid, _, err1 = RawSyscall(SYS_WRITE, fd1, uintptr(unsafe.Pointer(&gidmap[0])), uintptr(len(gidmap)))
+			_, _, err1 = RawSyscall(SYS_WRITE, fd1, uintptr(unsafe.Pointer(&gidmap[0])), uintptr(len(gidmap)))
 			if err1 != 0 {
 				goto childerror
 			}
@@ -452,7 +452,7 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 			if fd1, _, err1 = RawSyscall6(SYS_OPENAT, uintptr(dirfd), uintptr(unsafe.Pointer(&puid[0])), uintptr(O_WRONLY), 0, 0, 0); err1 != 0 {
 				goto childerror
 			}
-			pid, _, err1 = RawSyscall(SYS_WRITE, fd1, uintptr(unsafe.Pointer(&uidmap[0])), uintptr(len(uidmap)))
+			_, _, err1 = RawSyscall(SYS_WRITE, fd1, uintptr(unsafe.Pointer(&uidmap[0])), uintptr(len(uidmap)))
 			if err1 != 0 {
 				goto childerror
 			}