lmkd --reinit hack

commit eb8e5359f6c0a5deccb1a17a6fa130c042479519
Author: Juhyung Park <qkrwngud825@gmail.com>
Date:   Sun Sep 19 02:56:25 2021 +0900

    exec: add a hack to disable lmkd reloading props

    /vendor/etc/perf/perfconfigstore.xml enables ro.lmk.enable_userspace_lmk,
    and when the lmkd properties are re-read, lmkd tries to honor it and ignores
    the presence of in-kernel lmk and exits repeatedly, which causes init to
    trigger a reboot.

    Signed-off-by: Juhyung Park <qkrwngud825@gmail.com>

diff --git a/fs/exec.c b/fs/exec.c
index d713797fab9aa..be1e483b093a3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1724,6 +1724,27 @@ static int exec_binprm(struct linux_binprm *bprm)
    return ret;
 }

+static noinline bool is_lmkd_reinit(struct user_arg_ptr *argv)
+{
+   const char __user *str;
+   char buf[10];
+   int len;
+
+   str = get_user_arg_ptr(*argv, 1);
+   if (IS_ERR(str))
+       return false;
+
+   // strnlen_user() counts NULL terminator
+   len = strnlen_user(str, MAX_ARG_STRLEN);
+   if (len != 9)
+       return false;
+
+   if (copy_from_user(buf, str, len))
+       return false;
+
+   return !strcmp(buf, "--reinit");
+}
+
 /*
  * sys_execve() executes a new program.
  */
@@ -1828,6 +1849,15 @@ static int __do_execve_file(int fd, struct filename *filename,
    if (retval < 0)
        goto out;

+   // Super nasty hack to disable lmkd reloading props
+   if (unlikely(strcmp(bprm.filename, "/system/bin/lmkd") == 0)) {
+       if (is_lmkd_reinit(&argv)) {
+           pr_info("sys_execve(): prevented /system/bin/lmkd --reinit\n");
+           retval = -ENOENT;
+           goto out;
+       }
+   }
+
    retval = exec_binprm(&bprm);
    if (retval < 0)
        goto out;