[PATCH 13/13] lib/kasprintf.c: add sanity check to kvasprintf

[Rasmus Villemoes]

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@xxxxxxxxxxxxxxxxxx>
---
 lib/kasprintf.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/lib/kasprintf.c b/lib/kasprintf.c
index 32f12150fc4f..a30bf3042020 100644
--- 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;
 }
-- 
2.6.1

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