portlist: remove unix.Readlink allocs on Linux

name       old time/op    new time/op    delta
    GetList-8    11.2ms ± 5%    11.1ms ± 3%     ~     (p=0.661 n=10+9)

    name       old alloc/op   new alloc/op   delta
    GetList-8    83.3kB ± 1%    67.4kB ± 1%  -19.05%  (p=0.000 n=10+10)

    name       old allocs/op  new allocs/op  delta
    GetList-8     2.89k ± 2%     2.19k ± 1%  -24.24%  (p=0.000 n=10+10)

(real issue is we're calling this code as much as we are, but easy
enough to make it efficient because it'll still need to be called
sometimes in any case)

Updates #5958

Change-Id: I90c20278d73e80315a840aed1397d24faa308d93
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -243,7 +243,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/tunstats                                   from tailscale.com/net/tstun+
         tailscale.com/net/wsconn                                     from tailscale.com/control/controlhttp+
         tailscale.com/paths                                          from tailscale.com/ipn/ipnlocal+
-        tailscale.com/portlist                                       from tailscale.com/ipn/ipnlocal
+     💣 tailscale.com/portlist                                       from tailscale.com/ipn/ipnlocal
         tailscale.com/safesocket                                     from tailscale.com/client/tailscale+
         tailscale.com/smallzstd                                      from tailscale.com/ipn/ipnserver+
   LD 💣 tailscale.com/ssh/tailssh                                    from tailscale.com/cmd/tailscaled

portlist/portlist_linux.go

@@ -18,6 +18,7 @@
 	"sync/atomic"
 	"syscall"
 	"time"
+	"unsafe"
 
 	"go4.org/mem"
 	"golang.org/x/sys/unix"
@@ -215,6 +216,8 @@ func addProcesses(pl []Port) ([]Port, error) {
 		pm[pl[i].inode] = &pl[i]
 	}
 
+	var pathBuf []byte
+
 	err := foreachPID(func(pid string) error {
 		fdPath := fmt.Sprintf("/proc/%s/fd", pid)
 
@@ -251,8 +254,9 @@ func addProcesses(pl []Port) ([]Port, error) {
 				return fmt.Errorf("addProcesses.readDir: %w", err)
 			}
 			for _, fd := range fds {
-				n, err := unix.Readlink(fmt.Sprintf("/proc/%s/fd/%s", pid, fd), targetBuf)
+				pathBuf = fmt.Appendf(pathBuf[:0], "/proc/%s/fd/%s\x00", pid, fd)
-				if err != nil {
+				n, ok := readlink(pathBuf, targetBuf)
+				if !ok {
 					// Not a symlink or no permission.
 					// Skip it.
 					continue
@@ -337,3 +341,22 @@ func fieldIndex(line []byte, n int) int {
 	}
 	return skip
 }
+
+// path must be null terminated.
+func readlink(path, buf []byte) (n int, ok bool) {
+	if len(buf) == 0 || len(path) < 2 || path[len(path)-1] != 0 {
+		return 0, false
+	}
+	var dirfd int = unix.AT_FDCWD
+	r0, _, e1 := unix.Syscall6(unix.SYS_READLINKAT,
+		uintptr(dirfd),
+		uintptr(unsafe.Pointer(&path[0])),
+		uintptr(unsafe.Pointer(&buf[0])),
+		uintptr(len(buf)),
+		0, 0)
+	n = int(r0)
+	if e1 != 0 {
+		return 0, false
+	}
+	return n, true
+}