x86-64 client: redirect the web helper's exec instead of editing Steam

The client checksums its files as it starts (a CRC pass on its own
restart, not only sizes), so the size-padded steamwebhelper.sh can be
caught: "bad CRC on steamrt64/steamwebhelper.sh", then a 2-3 minute
reinstall on every start (seen in Bannerlator's port). The session
shim's exec hook (preload/net.c) now turns steamrt64's
`exec ./steamwebhelper` into the arm64 web helper's launcher when
BL_X64_ARM_WEBHELPER names it (set by bannerlator-steam-x64 for the
client); nothing of Steam's is edited, and the arm64 helper's own path
(steamrtarm64) never matches.

tools/linuxfs/overlay/usr/local/bin/bannerlator-steam-x64

@@ -557,33 +557,14 @@ export BL_WEBHELPER_HIDE_TRACER=1
 x64_exec() {
   x64_env
   cd "$STEAM_X64"
-  # The client's own web helper launcher, pointed at the arm64 client's helper (FEX hands that arm64
-  # program to the CPU). The client checks its files' sizes at every start and reinstalls itself
-  # over one that differs (device: install loop, "steamwebhelper.sh is 176 bytes, expected 84"), so
-  # the replacement is padded to the size of the client's own copy. It is pointed again whenever it
-  # differs, for as long as this client runs: a full check puts the client's copy back.
-  case "${1:-}" in
-    -shutdown|steam://*) ;;
-    *) if [ "${BL_X64_X86_WEBHELPER:-0}" != 1 ] && [ -x /usr/local/bin/bannerlator-x64-arm-webhelper ] &&
-          [ -x "$ARM_STEAM/steamrtarm64/steamwebhelper" ]; then
-         (
-           client=$$ wh=$STEAM_X64/steamrt64/steamwebhelper.sh
-           base=$(printf '#!/bin/sh\nexec /usr/local/bin/bannerlator-x64-arm-webhelper "$@"\n#')
-           size= want=
-           while kill -0 "$client" 2>/dev/null; do
-             if [ -f "$wh" ] && ! grep -q bannerlator-x64-arm-webhelper "$wh" 2>/dev/null; then
-               size=$(stat -c %s "$wh") want=
-               pad=$((size - ${#base} - 1))
-               [ "$pad" -ge 0 ] && want=$base$(printf '%*s' "$pad" '' | tr ' ' '-')
-             fi
-             if [ -n "$want" ] && [ "$(cat "$wh" 2>/dev/null)" != "$want" ]; then
-               printf '%s\n' "$want" > "$wh.new" && chmod 755 "$wh.new" && mv -f "$wh.new" "$wh"
-             fi
-             sleep 1
-           done
-         ) &
-       fi ;;
-  esac
+  # The client's web helper is the arm64 client's (bannerlator-x64-arm-webhelper; FEX hands that arm64
+  # program to the CPU): the session shim turns steamrt64/steamwebhelper.sh's exec of ./steamwebhelper
+  # into it (preload/net.c). Nothing of the client's is edited - it checksums its files as it starts
+  # and reinstalled itself over an edited launcher (2-3 minutes) on every start.
+  if [ "${BL_X64_X86_WEBHELPER:-0}" != 1 ] && [ -x /usr/local/bin/bannerlator-x64-arm-webhelper ] &&
+     [ -x "$ARM_STEAM/steamrtarm64/steamwebhelper" ]; then
+    export BL_X64_ARM_WEBHELPER=/usr/local/bin/bannerlator-x64-arm-webhelper
+  fi
   exec "${BL_FEX:-/usr/local/bin/FEX}" "$STEAM_X64/steamrt64/steam" "$@"
 }
 

tools/linuxfs/overlay/usr/local/bin/bannerlator-x64-arm-webhelper

@@ -1,9 +1,10 @@
 #!/bin/bash
 # The x86-64 client's web helper (its whole UI) is the arm64 client's, run natively: both clients are
-# the same Steam build, and steamrt64/steamwebhelper.sh is pointed here (bannerlator-steam-x64) through
-# an arm64 binary, which FEX hands to the CPU. Under FEX the x86 helper could not have the GPU (its GPU
-# process needs libgbm, which FEX does not thunk) and drew on SwiftShader; this one draws with ANGLE on
-# Turnip and takes the session's preloads (KGSL poll fix, fast path), as from the arm64 client.
+# the same Steam build, and the session shim turns the x86 client's exec of its own helper into this
+# (preload/net.c, BL_X64_ARM_WEBHELPER); FEX hands the arm64 program here to the CPU. Under FEX the
+# x86 helper could not have the GPU (its GPU process needs libgbm, which FEX does not thunk) and drew
+# on SwiftShader; this one draws with ANGLE on Turnip and takes the session's preloads (KGSL poll fix,
+# fast path), as from the arm64 client.
 x64_home=$HOME
 arm=/root/.local/share/Steam/steamrtarm64
 
@@ -11,7 +12,7 @@ arm=/root/.local/share/Steam/steamrtarm64
 for v in $(compgen -e); do
   case $v in FEX_*) unset "$v" ;; esac
 done
-unset LD_PRELOAD LIBGL_ALWAYS_SOFTWARE LIBGL_DRI3_DISABLE BL_WEBHELPER_HIDE_TRACER
+unset LD_PRELOAD LIBGL_ALWAYS_SOFTWARE LIBGL_DRI3_DISABLE BL_WEBHELPER_HIDE_TRACER BL_X64_ARM_WEBHELPER
 # The arm64 session's GPU settings (saved by bannerlator-steam-x64 at start).
 if [ -f "$x64_home/arm-session.env" ]; then
   eval "$(grep -E '^declare -x (VK_ICD_FILENAMES|VK_DRIVER_FILES|GALLIUM_DRIVER|MESA_LOADER_DRIVER_OVERRIDE|LIBGL_KOPPER_DRI2|TU_DEBUG|ZINK_DEBUG|ZINK_DESCRIPTORS|MESA_NO_ERROR|MESA_DISK_CACHE_DATABASE)=' "$x64_home/arm-session.env")"

tools/linuxfs/preload/net.c

@@ -14,6 +14,7 @@
 #include <linux/netlink.h>
 #include <netinet/in.h>
 #include <stdio.h>
+#include <limits.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/socket.h>
@@ -194,10 +195,44 @@ static int maybe_answer_lsof(const char *path, char *const argv[]) {
   _exit(0);
 }
 
+/* The x86-64 Steam client's web helper is the arm64 client's (bannerlator-x64-arm-webhelper, named
+ * by BL_X64_ARM_WEBHELPER, which bannerlator-steam-x64 sets for the client): steamrt64's
+ * steamwebhelper.sh ends in `exec ./steamwebhelper`, and that exec becomes the launcher with the same
+ * arguments. Steam's own files stay as shipped - the client checksums them as it starts and
+ * reinstalled itself over an edited launcher every time. The arm64 helper (steamrtarm64) never
+ * matches. Returns the launcher's argv to exec instead, or NULL. */
+static char **web_helper_redirect(const char *path, char *const argv[], const char **launcher) {
+  const char *want = getenv("BL_X64_ARM_WEBHELPER");
+  if (!want || !*want || !path) return NULL;
+  const char *base = strrchr(path, '/');
+  if (strcmp(base ? base + 1 : path, "steamwebhelper") != 0) return NULL;
+  if (!strstr(path, "/steamrt64/")) {
+    char cwd[PATH_MAX];
+    size_t n;
+    if (path[0] == '/' || !getcwd(cwd, sizeof(cwd))) return NULL;
+    n = strlen(cwd);
+    if (n < 10 || strcmp(cwd + n - 10, "/steamrt64") != 0) return NULL;
+  }
+  int argc = 0;
+  while (argv && argv[argc]) argc++;
+  char **out = calloc((size_t)argc + 2, sizeof(char *));
+  if (!out) return NULL;
+  out[0] = (char *)want;
+  for (int i = 1; i < argc; i++) out[i] = argv[i];
+  *launcher = want;
+  return out;
+}
+
 int execve(const char *path, char *const argv[], char *const envp[]) {
   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);
+  const char *launcher = NULL;
+  char **redirected = web_helper_redirect(path, argv, &launcher);
+  if (redirected) {
+    real(launcher, redirected, envp);
+    free(redirected); /* only on failure: the original exec runs */
+  }
   return real(path, argv, envp);
 }
 
@@ -229,6 +264,12 @@ int execv(const char *path, char *const argv[]) {
   static int (*real)(const char *, char *const[]);
   if (!real) real = (int (*)(const char *, char *const[]))dlsym(RTLD_NEXT, "execv");
   maybe_answer_lsof(path, argv);
+  const char *launcher = NULL;
+  char **redirected = web_helper_redirect(path, argv, &launcher);
+  if (redirected) {
+    real(launcher, redirected);
+    free(redirected);
+  }
   return real(path, argv);
 }