lib/kasprintf.c: add sanity check to kvasprintf

kasprintf relies on being able to replay the formatting and getting the
same result (in particular, the same length).  This will almost always
work, but it is possible that the object pointed to by a %s or %p
argument changed under us (so we might get truncated output).  Add a
somewhat paranoid sanity check and let's see if it ever triggers.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Joe Perches <joe@perches.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Maurizio Lombardi <mlombard@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/kasprintf.c b/lib/kasprintf.c
index f194e6e593e19db22ad6b7e50aeee0e7c46eafa6..7f6c506a494226479af1152f13f99f1870f554d1 100644 (file)
--- a/lib/kasprintf.c
+++ b/lib/kasprintf.c
@@ -13,19 +13,21 @@
 /* Simplified asprintf. */
 char *kvasprintf(gfp_t gfp, const char *fmt, va_list ap)
 {
-       unsigned int len;
+       unsigned int first, second;
        char *p;
        va_list aq;
 
        va_copy(aq, ap);
-       len = vsnprintf(NULL, 0, fmt, aq);
+       first = vsnprintf(NULL, 0, fmt, aq);
        va_end(aq);
 
-       p = kmalloc_track_caller(len+1, gfp);
+       p = kmalloc_track_caller(first+1, gfp);
        if (!p)
                return NULL;
 
-       vsnprintf(p, len+1, fmt, ap);
+       second = vsnprintf(p, first+1, fmt, ap);
+       WARN(first != second, "different return values (%u and %u) from vsnprintf(\"%s\", ...)",
+            first, second, fmt);
 
        return p;
 }