From 3697609aaa1f18b918080f1557b65022e94564be Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Sat, 22 Oct 2022 11:21:10 -0700
Subject: [PATCH] portlist: remove unix.Readlink allocs on Linux
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    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 |  2 +-
 portlist/portlist_linux.go  | 27 +++++++++++++++++++++++++--
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/cmd/tailscaled/depaware.txt b/cmd/tailscaled/depaware.txt
index d9f7bd5e6..6dfd22671 100644
--- a/cmd/tailscaled/depaware.txt
+++ b/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
diff --git a/portlist/portlist_linux.go b/portlist/portlist_linux.go
index a40fb9909..4d5faabbf 100644
--- a/portlist/portlist_linux.go
+++ b/portlist/portlist_linux.go
@@ -18,6 +18,7 @@ import (
 	"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)
-				if err != nil {
+				pathBuf = fmt.Appendf(pathBuf[:0], "/proc/%s/fd/%s\x00", pid, fd)
+				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
+}