argv_split(): teach it to handle mutable strings

argv_split() allocates argv[count_argc(str)] array and assumes that it
will find the same number of arguments later.  This is obviously wrong if
this string can be changed, say, by sysctl.

With this patch argv_split() kstrndup's the whole string and does not
split it, we simply replace the spaces with zeroes and keep the allocated
memory in argv[-1] for argv_free(arg).

We do not use argv[0] because:

	- str can be all-spaces or empty. In fact this case is fine,
	  we could kfree() it before return, but:

	- str can have a space at the start, and we can not rely on
	  kstrndup(skip_spaces(str)) because it can equally race if
	  this string is mutable.

Also, simplify count_argc() and kill the no longer used skip_arg().

Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Andi Kleen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/argv_split.c

@@ -8,23 +8,17 @@
 #include <linux/slab.h>
 #include <linux/export.h>
 
-static const char *skip_arg(const char *cp)
-{
- while (*cp && !isspace(*cp))
- cp++;
-
- return cp;
-}
-
 static int count_argc(const char *str)
 {
  int count = 0;
+ bool was_space;
 
- while (*str) {
- str = skip_spaces(str);
- if (*str) {
+ for (was_space = true; *str; str++) {
+ if (isspace(*str)) {
+ was_space = true;
+ } else if (was_space) {
+ was_space = false;
  count++;
- str = skip_arg(str);
  }
  }
 
@@ -39,10 +33,8 @@ static int count_argc(const char *str)
  */
 void argv_free(char **argv)
 {
- char **p;
- for (p = argv; *p; p++)
- kfree(*p);
-
+ argv--;
+ kfree(argv[0]);
  kfree(argv);
 }
 EXPORT_SYMBOL(argv_free);
@@ -59,43 +51,44 @@ EXPORT_SYMBOL(argv_free);
  * considered to be a single argument separator. The returned array
  * is always NULL-terminated. Returns NULL on memory allocation
  * failure.
+ *
+ * The source string at `str' may be undergoing concurrent alteration via
+ * userspace sysctl activity (at least). The argv_split() implementation
+ * attempts to handle this gracefully by taking a local copy to work on.
  */
 char **argv_split(gfp_t gfp, const char *str, int *argcp)
 {
- int argc = count_argc(str);
- char **argv = kzalloc(sizeof(*argv) * (argc+1), gfp);
- char **argvp;
-
- if (argv == NULL)
- goto out;
-
- if (argcp)
- *argcp = argc;
-
- argvp = argv;
-
- while (*str) {
- str = skip_spaces(str);
-
- if (*str) {
- const char *p = str;
- char *t;
-
- str = skip_arg(str);
+ char *argv_str;
+ bool was_space;
+ char **argv, **argv_ret;
+ int argc;
+
+ argv_str = kstrndup(str, KMALLOC_MAX_SIZE - 1, gfp);
+ if (!argv_str)
+ return NULL;
+
+ argc = count_argc(argv_str);
+ argv = kmalloc(sizeof(*argv) * (argc + 2), gfp);
+ if (!argv) {
+ kfree(argv_str);
+ return NULL;
+ }
 
- t = kstrndup(p, str-p, gfp);
- if (t == NULL)
- goto fail;
- *argvp++ = t;
+ *argv = argv_str;
+ argv_ret = ++argv;
+ for (was_space = true; *argv_str; argv_str++) {
+ if (isspace(*argv_str)) {
+ was_space = true;
+ *argv_str = 0;
+ } else if (was_space) {
+ was_space = false;
+ *argv++ = argv_str;
  }
  }
- *argvp = NULL;
-
- out:
- return argv;
+ *argv = NULL;
 
- fail:
- argv_free(argv);
- return NULL;
+ if (argcp)
+  *argcp = argc;
+ return argv_ret;
 }
 EXPORT_SYMBOL(argv_split);