exec: introduce get_user_arg_ptr() helper

Introduce get_user_arg_ptr() helper, convert count() and copy_strings()
to use it.

No functional changes, preparation. This helper is trivial, it just
reads the pointer from argv/envp user-space array.

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>

1 files changed, 25 insertions, 11 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 5e62d26a4fec..b12e24fe1c5e 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -398,6 +398,17 @@ err:
         return err;
 }
 
+static const char __user *
+get_user_arg_ptr(const char __user * const __user *argv, int nr)
+{
+        const char __user *ptr;
+
+        if (get_user(ptr, argv + nr))
+                return ERR_PTR(-EFAULT);
+
+        return ptr;
+}
+
 /*
  * count() counts the number of strings in array ARGV.
  */
@@ -407,13 +418,14 @@ static int count(const char __user * const __user * argv, int max)
 
         if (argv != NULL) {
                 for (;;) {
-                        const char __user * p;
+                        const char __user *p = get_user_arg_ptr(argv, i);
 
-                        if (get_user(p, argv))
-                                return -EFAULT;
                         if (!p)
                                 break;
-                        argv++;
+
+                        if (IS_ERR(p))
+                                return -EFAULT;
+
                         if (i++ >= max)
                                 return -E2BIG;
 
@@ -443,16 +455,18 @@ static int copy_strings(int argc, const char __user *const __user *argv,
                 int len;
                 unsigned long pos;
 
-                if (get_user(str, argv+argc) ||
-                                !(len = strnlen_user(str, MAX_ARG_STRLEN))) {
-                        ret = -EFAULT;
+                ret = -EFAULT;
+                str = get_user_arg_ptr(argv, argc);
+                if (IS_ERR(str))
                         goto out;
-                }
 
-                if (!valid_arg_len(bprm, len)) {
-                        ret = -E2BIG;
+                len = strnlen_user(str, MAX_ARG_STRLEN);
+                if (!len)
+                        goto out;
+
+                ret = -E2BIG;
+                if (!valid_arg_len(bprm, len))
                         goto out;
-                }
 
                 /* We're going to work our way backwords. */
                 pos = bprm->p;