exec: introduce struct user_arg_ptr

No functional changes, preparation.

Introduce struct user_arg_ptr, change do_execve() paths to use it
instead of "char __user * const __user *argv".

This makes the argv/envp arguments opaque, we are ready to handle the
compat case which needs argv pointing to compat_uptr_t.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Tested-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

fs/exec.c

@@ -398,12 +398,15 @@ err:
 	return err;
 }
 
-static const char __user *
-get_user_arg_ptr(const char __user * const __user *argv, int nr)
+struct user_arg_ptr {
+	const char __user *const __user *native;
+};
+
+static const char __user *get_user_arg_ptr(struct user_arg_ptr argv, int nr)
 {
 	const char __user *ptr;
 
-	if (get_user(ptr, argv + nr))
+	if (get_user(ptr, argv.native + nr))
 		return ERR_PTR(-EFAULT);
 
 	return ptr;
@@ -412,11 +415,11 @@ get_user_arg_ptr(const char __user * const __user *argv, int nr)
 /*
  * count() counts the number of strings in array ARGV.
  */
-static int count(const char __user * const __user * argv, int max)
+static int count(struct user_arg_ptr argv, int max)
 {
 	int i = 0;
 
-	if (argv != NULL) {
+	if (argv.native != NULL) {
 		for (;;) {
 			const char __user *p = get_user_arg_ptr(argv, i);
 
@@ -442,7 +445,7 @@ static int count(const char __user * const __user * argv, int max)
  * processes's memory to the new process's stack.  The call to get_user_pages()
  * ensures the destination page is created and not swapped out.
  */
-static int copy_strings(int argc, const char __user *const __user *argv,
+static int copy_strings(int argc, struct user_arg_ptr argv,
 			struct linux_binprm *bprm)
 {
 	struct page *kmapped_page = NULL;
@@ -533,14 +536,19 @@ out:
 /*
  * Like copy_strings, but get argv and its values from kernel memory.
  */
-int copy_strings_kernel(int argc, const char *const *argv,
+int copy_strings_kernel(int argc, const char *const *__argv,
 			struct linux_binprm *bprm)
 {
 	int r;
 	mm_segment_t oldfs = get_fs();
+	struct user_arg_ptr argv = {
+		.native = (const char __user *const  __user *)__argv,
+	};
+
 	set_fs(KERNEL_DS);
-	r = copy_strings(argc, (const char __user *const  __user *)argv, bprm);
+	r = copy_strings(argc, argv, bprm);
 	set_fs(oldfs);
+
 	return r;
 }
 EXPORT_SYMBOL(copy_strings_kernel);
@@ -1393,10 +1401,10 @@ EXPORT_SYMBOL(search_binary_handler);
 /*
  * sys_execve() executes a new program.
  */
-int do_execve(const char * filename,
-	const char __user *const __user *argv,
-	const char __user *const __user *envp,
-	struct pt_regs * regs)
+static int do_execve_common(const char *filename,
+				struct user_arg_ptr argv,
+				struct user_arg_ptr envp,
+				struct pt_regs *regs)
 {
 	struct linux_binprm *bprm;
 	struct file *file;
@@ -1503,6 +1511,16 @@ out_ret:
 	return retval;
 }
 
+int do_execve(const char *filename,
+	const char __user *const __user *__argv,
+	const char __user *const __user *__envp,
+	struct pt_regs *regs)
+{
+	struct user_arg_ptr argv = { .native = __argv };
+	struct user_arg_ptr envp = { .native = __envp };
+	return do_execve_common(filename, argv, envp, regs);
+}
+
 void set_binfmt(struct linux_binfmt *new)
 {
 	struct mm_struct *mm = current->mm;