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net/netutil: only check Linux sysctls w/ procfs, assume absent means false

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Fixes #7217

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Brad Fitzpatrick
2023-02-08 20:11:25 +00:00
committed by Brad Fitzpatrick
parent 05adf22383
commit 2477fc4952
2 changed files with 27 additions and 13 deletions
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26
net/netutil/ip_forward.go
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@@ -9,7 +9,6 @@ import (
"fmt"
"net/netip"
"os"
"os/exec"
"path/filepath"
"runtime"
"strconv"
@@ -194,27 +193,28 @@ const (
// given interface.
// The iface param determines which interface to check against, "" means to check
// global config.
// It tries to lookup the value directly from `/proc/sys`, and falls back to
// using `sysctl` on failure.
// This is Linux-specific: it only reads from /proc/sys and doesn't shell out to
// sysctl (which on Linux just reads from /proc/sys anyway).
func ipForwardingEnabledLinux(p protocol, iface string) (bool, error) {
k := ipForwardSysctlKey(slashFormat, p, iface)
bs, err := os.ReadFile(filepath.Join("/proc/sys", k))
if err != nil {
// Fallback to using sysctl.
// Sysctl accepts `/` as separator.
bs, err = exec.Command("sysctl", "-n", k).Output()
if err != nil {
// But in case it doesn't.
k := ipForwardSysctlKey(dotFormat, p, iface)
bs, err = exec.Command("sysctl", "-n", k).Output()
if err != nil {
return false, fmt.Errorf("couldn't check %s (%v)", k, err)
if os.IsNotExist(err) {
// If IPv6 is disabled, sysctl keys like "net.ipv6.conf.all.forwarding" just don't
// exist on disk. But first diagnose whether procfs is even mounted before assuming
// absence means false.
if fi, err := os.Stat("/proc/sys"); err != nil {
return false, fmt.Errorf("failed to check sysctl %v; no procfs? %w", k, err)
} else if !fi.IsDir() {
return false, fmt.Errorf("failed to check sysctl %v; /proc/sys isn't a directory, is %v", k, fi.Mode())
}
return false, nil
}
return false, err
}
on, err := strconv.ParseBool(string(bytes.TrimSpace(bs)))
if err != nil {
return false, fmt.Errorf("couldn't parse %s (%v)", k, err)
return false, fmt.Errorf("couldn't parse %s: %w", k, err)
}
return on, nil
}