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From 70d2687d85c314963cf280759b23fd4573ff0d82 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Wed, 19 Oct 2016 20:17:16 -0400
Subject: [PATCH 16/18] fix integer overflow in float printf needed-precision
computation
if the requested precision is close to INT_MAX, adding
LDBL_MANT_DIG/3+8 overflows. in practice the resulting undefined
behavior manifests as a large negative result, which is then used to
compute the new end pointer (z) with a wildly out-of-bounds value
(more overflow, more undefined behavior). the end result is at least
incorrect output and character count (return value); worse things do
not seem to happen, but detailed analysis has not been done.
this patch fixes the overflow by performing the intermediate
computation as unsigned; after division by 9, the final result
necessarily fits in int.
---
src/stdio/vfprintf.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/stdio/vfprintf.c b/src/stdio/vfprintf.c
index e439a07..cd17ad7 100644
--- a/src/stdio/vfprintf.c
+++ b/src/stdio/vfprintf.c
@@ -312,7 +312,7 @@ static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t)
}
while (e2<0) {
uint32_t carry=0, *b;
- int sh=MIN(9,-e2), need=1+(p+LDBL_MANT_DIG/3+8)/9;
+ int sh=MIN(9,-e2), need=1+(p+LDBL_MANT_DIG/3U+8)/9;
for (d=a; d<z; d++) {
uint32_t rm = *d & (1<<sh)-1;
*d = (*d>>sh) + carry;
--
2.10.1
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