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libct: speedup process.Env handling
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The current implementation sets all the environment variables passed in
Process.Env in the current process, one by one, then uses os.Environ to
read those back.

As pointed out in [1], this is slow, as runc calls os.Setenv for every
variable, and there may be a few thousands of those. Looking into how
os.Setenv is implemented, it is indeed slow, especially when cgo is
enabled.

Looking into why it was implemented, I found commit 9744d72 and traced
it to [2], which discusses the actual reasons. At the time were:

 - HOME is not passed into container as it is set in setupUser by
   os.Setenv and has no effect on config.Env;
 - there is no deduplication of environment variables.

Yet it was decided to not go ahead with this patch, but later [3] was
merged with the carry of this patch.

Now, from what I see:

1. Passing environment to exec is way faster than using os.Setenv and
   os.Environ (tests show ~20x faster in simple Go test, and 2x faster
   in real-world test, see below).
2. Setting environment variables in the runc context can result is ugly
   side effects (think GODEBUG, LD_PRELOAD, or _LIBCONTAINER_*).
3. Nothing in runtime spec says that the environment needs to be
   deduplicated, or the order of preference (whether the first or the
   last value of a variable with the same name is to be used). We should
   stick to what we have in order to maintain backward compatibility.

This patch:
 - switches to passing env directly to exec;
 - adds deduplication mechanism to retain backward compatibility;
 - sets PATH from process.Env in the current process;
 - adds HOME to process.Env if not set;

The benchmark added by the previous commit shows ~3x improvement:

	                │   before    │                after                 │
	                │   sec/op    │    sec/op     vs base                │
	ExecInBigEnv-20   61.53m ± 1%   21.87m ± 16%  -64.46% (p=0.000 n=10)

[1]: #1983
[2]: docker-archive/libcontainer#418
[3]: docker-archive/libcontainer#432

Signed-off-by: Kir Kolyshkin <[email protected]>
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kolyshkin committed Nov 1, 2024
1 parent d71b39c commit 3b7b613
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Showing 5 changed files with 115 additions and 42 deletions.
53 changes: 53 additions & 0 deletions libcontainer/env.go
Original file line number Diff line number Diff line change
@@ -0,0 +1,53 @@
package libcontainer

import (
"errors"
"fmt"
"os"
"slices"
"strings"
)

// prepareEnv checks supplied environment variables for validity, removes
// duplicates (leaving the last value only), and sets PATH from env, if found.
// Returns the deduplicated environment, and a flag telling if HOME is found.
func prepareEnv(env []string) ([]string, bool, error) {
if env == nil {
return nil, false, nil
}
// Deduplication code based on dedupEnv from Go 1.22 os/exec.

// Construct the output in reverse order, to preserve the
// last occurrence of each key.
out := make([]string, 0, len(env))
saw := make(map[string]bool, len(env))
for n := len(env); n > 0; n-- {
kv := env[n-1]
i := strings.IndexByte(kv, '=')
if i == -1 {
return nil, false, errors.New("invalid environment variable: missing '='")
}
if i == 0 {
return nil, false, errors.New("invalid environment variable: name cannot be empty")
}
key := kv[:i]
if saw[key] { // Duplicate.
continue
}
saw[key] = true
if strings.IndexByte(kv, 0) >= 0 {
return nil, false, fmt.Errorf("invalid environment variable %q: contains nul byte (\\x00)", key)
}
if key == "PATH" {
// Needs to be set as it is used for binary lookup.
if err := os.Setenv("PATH", kv[i+1:]); err != nil {
return nil, false, err
}
}
out = append(out, kv)
}
// Restore the original order.
slices.Reverse(out)

return out, saw["HOME"], nil
}
40 changes: 40 additions & 0 deletions libcontainer/env_test.go
Original file line number Diff line number Diff line change
@@ -0,0 +1,40 @@
package libcontainer

import (
"slices"
"testing"
)

func TestPrepareEnvDedup(t *testing.T) {
tests := []struct {
env, wantEnv []string
}{
{
env: []string{},
wantEnv: []string{},
},
{
env: []string{"HOME=/root", "FOO=bar"},
wantEnv: []string{"HOME=/root", "FOO=bar"},
},
{
env: []string{"A=a", "A=b", "A=c"},
wantEnv: []string{"A=c"},
},
{
env: []string{"TERM=vt100", "HOME=/home/one", "HOME=/home/two", "TERM=xterm", "HOME=/home/three", "FOO=bar"},
wantEnv: []string{"TERM=xterm", "HOME=/home/three", "FOO=bar"},
},
}

for _, tc := range tests {
env, _, err := prepareEnv(tc.env)
if err != nil {
t.Error(err)
continue
}
if !slices.Equal(env, tc.wantEnv) {
t.Errorf("want %v, got %v", tc.wantEnv, env)
}
}
}
54 changes: 16 additions & 38 deletions libcontainer/init_linux.go
Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,6 @@ import (
"runtime"
"runtime/debug"
"strconv"
"strings"
"syscall"

"github.com/containerd/console"
Expand Down Expand Up @@ -185,8 +184,8 @@ func startInitialization() (retErr error) {
defer pidfdSocket.Close()
}

// clear the current process's environment to clean any libcontainer
// specific env vars.
// From here on, we don't need current process environment. It is not
// used directly anywhere below this point, but let's clear it anyway.
os.Clearenv()

defer func() {
Expand All @@ -209,9 +208,11 @@ func startInitialization() (retErr error) {
}

func containerInit(t initType, config *initConfig, pipe *syncSocket, consoleSocket, pidfdSocket, fifoFile, logPipe *os.File) error {
if err := populateProcessEnvironment(config.Env); err != nil {
env, homeSet, err := prepareEnv(config.Env)
if err != nil {
return err
}
config.Env = env

// Clean the RLIMIT_NOFILE cache in go runtime.
// Issue: https://github.com/opencontainers/runc/issues/4195
Expand All @@ -225,6 +226,7 @@ func containerInit(t initType, config *initConfig, pipe *syncSocket, consoleSock
pidfdSocket: pidfdSocket,
config: config,
logPipe: logPipe,
addHome: !homeSet,
}
return i.Init()
case initStandard:
Expand All @@ -236,36 +238,13 @@ func containerInit(t initType, config *initConfig, pipe *syncSocket, consoleSock
config: config,
fifoFile: fifoFile,
logPipe: logPipe,
addHome: !homeSet,
}
return i.Init()
}
return fmt.Errorf("unknown init type %q", t)
}

// populateProcessEnvironment loads the provided environment variables into the
// current processes's environment.
func populateProcessEnvironment(env []string) error {
for _, pair := range env {
name, val, ok := strings.Cut(pair, "=")
if !ok {
return errors.New("invalid environment variable: missing '='")
}
if name == "" {
return errors.New("invalid environment variable: name cannot be empty")
}
if strings.IndexByte(name, 0) >= 0 {
return fmt.Errorf("invalid environment variable %q: name contains nul byte (\\x00)", name)
}
if strings.IndexByte(val, 0) >= 0 {
return fmt.Errorf("invalid environment variable %q: value contains nul byte (\\x00)", name)
}
if err := os.Setenv(name, val); err != nil {
return err
}
}
return nil
}

// verifyCwd ensures that the current directory is actually inside the mount
// namespace root of the current process.
func verifyCwd() error {
Expand Down Expand Up @@ -294,8 +273,8 @@ func verifyCwd() error {

// finalizeNamespace drops the caps, sets the correct user
// and working dir, and closes any leaked file descriptors
// before executing the command inside the namespace
func finalizeNamespace(config *initConfig) error {
// before executing the command inside the namespace.
func finalizeNamespace(config *initConfig, addHome bool) error {
// Ensure that all unwanted fds we may have accidentally
// inherited are marked close-on-exec so they stay out of the
// container
Expand Down Expand Up @@ -341,7 +320,7 @@ func finalizeNamespace(config *initConfig) error {
if err := system.SetKeepCaps(); err != nil {
return fmt.Errorf("unable to set keep caps: %w", err)
}
if err := setupUser(config); err != nil {
if err := setupUser(config, addHome); err != nil {
return fmt.Errorf("unable to setup user: %w", err)
}
// Change working directory AFTER the user has been set up, if we haven't done it yet.
Expand Down Expand Up @@ -459,8 +438,9 @@ func syncParentSeccomp(pipe *syncSocket, seccompFd int) error {
return readSync(pipe, procSeccompDone)
}

// setupUser changes the groups, gid, and uid for the user inside the container
func setupUser(config *initConfig) error {
// setupUser changes the groups, gid, and uid for the user inside the container,
// and appends user's HOME to config.Env if addHome is true.
func setupUser(config *initConfig, addHome bool) error {
// Set up defaults.
defaultExecUser := user.ExecUser{
Uid: 0,
Expand Down Expand Up @@ -541,11 +521,9 @@ func setupUser(config *initConfig) error {
return err
}

// if we didn't get HOME already, set it based on the user's HOME
if envHome := os.Getenv("HOME"); envHome == "" {
if err := os.Setenv("HOME", execUser.Home); err != nil {
return err
}
// If we didn't get HOME already, set it based on the user's HOME.
if addHome {
config.Env = append(config.Env, "HOME="+execUser.Home)
}
return nil
}
Expand Down
5 changes: 3 additions & 2 deletions libcontainer/setns_init_linux.go
Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@ type linuxSetnsInit struct {
pidfdSocket *os.File
config *initConfig
logPipe *os.File
addHome bool
}

func (l *linuxSetnsInit) getSessionRingName() string {
Expand Down Expand Up @@ -100,7 +101,7 @@ func (l *linuxSetnsInit) Init() error {
return err
}
}
if err := finalizeNamespace(l.config); err != nil {
if err := finalizeNamespace(l.config, l.addHome); err != nil {
return err
}
if err := apparmor.ApplyProfile(l.config.AppArmorProfile); err != nil {
Expand Down Expand Up @@ -153,5 +154,5 @@ func (l *linuxSetnsInit) Init() error {
if err := utils.UnsafeCloseFrom(l.config.PassedFilesCount + 3); err != nil {
return err
}
return system.Exec(name, l.config.Args, os.Environ())
return system.Exec(name, l.config.Args, l.config.Env)
}
5 changes: 3 additions & 2 deletions libcontainer/standard_init_linux.go
Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,7 @@ type linuxStandardInit struct {
fifoFile *os.File
logPipe *os.File
config *initConfig
addHome bool
}

func (l *linuxStandardInit) getSessionRingParams() (string, uint32, uint32) {
Expand Down Expand Up @@ -189,7 +190,7 @@ func (l *linuxStandardInit) Init() error {
return err
}
}
if err := finalizeNamespace(l.config); err != nil {
if err := finalizeNamespace(l.config, l.addHome); err != nil {
return err
}
// finalizeNamespace can change user/group which clears the parent death
Expand Down Expand Up @@ -287,5 +288,5 @@ func (l *linuxStandardInit) Init() error {
if err := utils.UnsafeCloseFrom(l.config.PassedFilesCount + 3); err != nil {
return err
}
return system.Exec(name, l.config.Args, os.Environ())
return system.Exec(name, l.config.Args, l.config.Env)
}

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