perf: Steam's web helper runs its GPU, audio and zygote work inside one process (#566)

tools/linuxfs/preload/net.c

@@ -262,6 +262,7 @@ __attribute__((visibility("hidden"))) void bl_binfmt_before(const char *path, ch
 __attribute__((visibility("hidden"))) int bl_binfmt_retry(const char *path, char *const argv[], char *const envp[], int error);
 __attribute__((visibility("hidden"))) void bl_binfmt_before_search(const char *file, char *const argv[], char *const envp[]);
 __attribute__((visibility("hidden"))) int bl_binfmt_after_search(const char *file, char *const argv[], char *const envp[], int error);
+__attribute__((visibility("hidden"))) char *const *bl_webhelper_argv(const char *path, char *const argv[]);
 __attribute__((visibility("hidden"))) int bl_binfmt_spawn(pid_t *pid, const char *path, const posix_spawn_file_actions_t *fa,
                                                           const posix_spawnattr_t *attr, char *const argv[], char *const envp[],
                                                           spawn_fn real);
@@ -308,7 +309,8 @@ static int maybe_answer_lsof(const char *path, char *const argv[]) {
   _exit(0);
 }
 
-int execve(const char *path, char *const argv[], char *const envp[]) {
+int execve(const char *path, char *const argv_in[], char *const envp[]) {
+  char *const *argv = bl_webhelper_argv(path, argv_in);
   static int (*real)(const char *, char *const[], char *const[]);
   if (!real) real = (int (*)(const char *, char *const[], char *const[]))dlsym(RTLD_NEXT, "execve");
   maybe_answer_lsof(path, argv);
@@ -332,7 +334,8 @@ static int answer_lsof_via_spawn(const char *path, char *const argv[], pid_t *pi
   return 0;
 }
 
-int execvp(const char *file, char *const argv[]) {
+int execvp(const char *file, char *const argv_in[]) {
+  char *const *argv = bl_webhelper_argv(file, argv_in);
   static int (*real)(const char *, char *const[]);
   if (!real) real = (int (*)(const char *, char *const[]))dlsym(RTLD_NEXT, "execvp");
   maybe_answer_lsof(file, argv);
@@ -341,7 +344,8 @@ int execvp(const char *file, char *const argv[]) {
   return bl_binfmt_after_search(file, argv, environ, errno);
 }
 
-int execv(const char *path, char *const argv[]) {
+int execv(const char *path, char *const argv_in[]) {
+  char *const *argv = bl_webhelper_argv(path, argv_in);
   static int (*real)(const char *, char *const[]);
   if (!real) real = (int (*)(const char *, char *const[]))dlsym(RTLD_NEXT, "execv");
   maybe_answer_lsof(path, argv);

tools/linuxfs/preload/webhelper.c

@@ -0,0 +1,73 @@
+#define _GNU_SOURCE
+#include <stdlib.h>
+#include <string.h>
+
+#ifndef WEBHELPER_DEFAULT
+#define WEBHELPER_DEFAULT "--no-zygote --in-process-gpu -AudioServiceOutOfProcess"
+#endif
+
+static int is_webhelper_browser(const char *path, char *const argv[]) {
+  if (!path || !argv) return 0;
+  const char *base = strrchr(path, '/');
+  base = base ? base + 1 : path;
+  if (strcmp(base, "steamwebhelper")) return 0;
+  for (int i = 1; argv[i]; i++)
+    if (!strncmp(argv[i], "--type=", 7)) return 0;
+  return 1;
+}
+
+static char *join_features(const char *have, const char *add) {
+  size_t a = have ? strlen(have) : 0, b = strlen(add);
+  char *out = malloc(a + b + 2);
+  if (!out) return NULL;
+  out[0] = 0;
+  if (a) strcat(strcat(out, have), ",");
+  return strcat(out, add);
+}
+
+__attribute__((visibility("hidden"))) char *const *bl_webhelper_argv(const char *path, char *const argv[]) {
+  if (!is_webhelper_browser(path, argv)) return argv;
+  const char *spec = getenv("BL_WEBHELPER_ARGS");
+  if (!spec || !*spec) spec = WEBHELPER_DEFAULT;
+  if (!strcmp(spec, "stock")) return argv;
+
+  int argc = 0;
+  while (argv[argc]) argc++;
+  char *tokens = strdup(spec);
+  if (!tokens) return argv;
+  int extra = 0;
+  for (char *c = tokens; *c; c++)
+    if (*c == ' ') extra++;
+  char **out = calloc(argc + extra + 4, sizeof *out);
+  if (!out) return argv;
+
+  const char *enable = NULL, *disable = NULL;
+  int n = 0;
+  for (int i = 0; i < argc; i++) {
+    if (i && !strncmp(argv[i], "--enable-features=", 18)) enable = argv[i] + 18;
+    else if (i && !strncmp(argv[i], "--disable-features=", 19)) disable = argv[i] + 19;
+    else out[n++] = argv[i];
+  }
+  char *add_enable = NULL, *add_disable = NULL, *save = NULL;
+  for (char *t = strtok_r(tokens, " ", &save); t; t = strtok_r(NULL, " ", &save)) {
+    if (t[0] == '+' && t[1]) {
+      char *joined = join_features(add_enable, t + 1);
+      free(add_enable);
+      add_enable = joined;
+    } else if (t[0] == '-' && t[1] && t[1] != '-') {
+      char *joined = join_features(add_disable, t + 1);
+      free(add_disable);
+      add_disable = joined;
+    } else if (!strncmp(t, "--", 2)) {
+      out[n++] = t;
+    }
+  }
+  char *merged;
+  if (add_enable && (merged = join_features(enable, add_enable))) enable = merged;
+  if (add_disable && (merged = join_features(disable, add_disable))) disable = merged;
+  char *flag;
+  if (enable && (flag = malloc(strlen(enable) + 19))) out[n++] = strcat(strcpy(flag, "--enable-features="), enable);
+  if (disable && (flag = malloc(strlen(disable) + 20))) out[n++] = strcat(strcpy(flag, "--disable-features="), disable);
+  out[n] = NULL;
+  return out;
+}

tools/tests/test_binfmt_preload.py

@@ -60,7 +60,7 @@ class BinfmtPreloadTest(unittest.TestCase):
         cls.library = cls.tmp / "libbinfmt.so"
         subprocess.run(["gcc", "-shared", "-fPIC", "-O2", "-Wall", "-Wextra", "-Werror", "-DBINFMT_TEST",
                         '-DBINFMT_HANDLER="%s"' % cls.handler, "-o", str(cls.library),
-                        "-pthread"] + [str(PRELOAD / name) for name in ("binfmt.c", "net.c", "udevmon.c", "pathcache.c")] + ["-ldl"],
+                        "-pthread"] + [str(PRELOAD / name) for name in ("binfmt.c", "net.c", "udevmon.c", "pathcache.c", "webhelper.c")] + ["-ldl"],
                        check=True)
         source = cls.tmp / "program.c"
         source.write_text(PROGRAM)

tools/tests/test_cdp_guard.py

@@ -16,7 +16,7 @@ class CdpGuardTest(unittest.TestCase):
     def setUpClass(cls):
         cls.build = tempfile.TemporaryDirectory()
         cls.library = Path(cls.build.name) / "net.so"
-        subprocess.run(["cc", "-shared", "-fPIC", "-O2", str(SOURCE), str(SOURCE.parent / "binfmt.c"), "-x", "c", "-", "-ldl",
+        subprocess.run(["cc", "-shared", "-fPIC", "-O2", str(SOURCE), str(SOURCE.parent / "binfmt.c"), str(SOURCE.parent / "webhelper.c"), "-x", "c", "-", "-ldl",
                         "-o", str(cls.library)],
                        input=b"int bl_udevmon_stand_in(int fd) { return 0; }", check=True, capture_output=True)
 

tools/tests/test_webhelper_preload.py

@@ -0,0 +1,69 @@
+import os
+from pathlib import Path
+import shutil
+import subprocess
+import tempfile
+import unittest
+
+PRELOAD = Path(__file__).resolve().parents[1] / "linuxfs/preload"
+
+PROGRAM = r"""
+#include <stdio.h>
+char *const *bl_webhelper_argv(const char *path, char *const argv[]);
+int main(int argc, char **argv) {
+  char *const *out = bl_webhelper_argv(argv[1], argv + 1);
+  for (int i = 0; out[i]; i++) printf("%s\n", out[i]);
+  return 0;
+}
+"""
+
+WEBHELPER = "/root/.local/share/Steam/steamrtarm64/steamwebhelper"
+
+
+@unittest.skipUnless(shutil.which("cc"), "needs cc")
+class WebhelperPreloadTest(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.tmp = Path(tempfile.mkdtemp())
+        source = cls.tmp / "program.c"
+        source.write_text(PROGRAM)
+        cls.program = cls.tmp / "program"
+        subprocess.run(["cc", "-O2", "-Wall", "-Werror", "-o", str(cls.program), str(source),
+                        str(PRELOAD / "webhelper.c")], check=True)
+
+    @classmethod
+    def tearDownClass(cls):
+        shutil.rmtree(cls.tmp, ignore_errors=True)
+
+    def argv(self, *args, spec=None):
+        env = {k: v for k, v in os.environ.items() if k != "BL_WEBHELPER_ARGS"}
+        if spec is not None:
+            env["BL_WEBHELPER_ARGS"] = spec
+        result = subprocess.run([str(self.program), *args], env=env, capture_output=True, text=True, check=True)
+        return result.stdout.splitlines()
+
+    def test_the_browser_process_keeps_its_helpers_in_process(self):
+        out = self.argv(WEBHELPER, "-lang=en_US", "--enable-features=A", "--no-sandbox", "--disable-features=B,C")
+        self.assertEqual(out, [WEBHELPER, "-lang=en_US", "--no-sandbox", "--no-zygote", "--in-process-gpu",
+                               "--enable-features=A", "--disable-features=B,C,AudioServiceOutOfProcess"])
+
+    def test_feature_lists_are_added_when_steam_passes_none(self):
+        out = self.argv(WEBHELPER, spec="+X -Y")
+        self.assertEqual(out, [WEBHELPER, "--enable-features=X", "--disable-features=Y"])
+
+    def test_child_processes_and_other_programs_are_left_alone(self):
+        child = [WEBHELPER, "--type=renderer", "--enable-features=A"]
+        self.assertEqual(self.argv(*child), child)
+        other = ["/usr/bin/steam", "--enable-features=A"]
+        self.assertEqual(self.argv(*other), other)
+
+    def test_stock_turns_it_off(self):
+        args = [WEBHELPER, "--enable-features=A"]
+        self.assertEqual(self.argv(*args, spec="stock"), args)
+
+    def test_an_empty_setting_means_the_default(self):
+        self.assertIn("--no-zygote", self.argv(WEBHELPER, spec=""))
+
+
+if __name__ == "__main__":
+    unittest.main()