From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753354AbbJTUdP (ORCPT ); Tue, 20 Oct 2015 16:33:15 -0400 Received: from mail-lf0-f46.google.com ([209.85.215.46]:33693 "EHLO mail-lf0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752860AbbJTUae (ORCPT ); Tue, 20 Oct 2015 16:30:34 -0400 From: Rasmus Villemoes To: Andrew Morton Cc: Rasmus Villemoes , Ingo Molnar , linux-kernel@vger.kernel.org Subject: [PATCH 03/13] lib/vsprintf.c: eliminate potential race in string() Date: Tue, 20 Oct 2015 22:30:03 +0200 Message-Id: <1445373013-20207-4-git-send-email-linux@rasmusvillemoes.dk> X-Mailer: git-send-email 2.6.1 In-Reply-To: <1445373013-20207-1-git-send-email-linux@rasmusvillemoes.dk> References: <1445373013-20207-1-git-send-email-linux@rasmusvillemoes.dk> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If the string corresponding to a %s specifier can change under us, we might end up copying a \0 byte to the output buffer. There might be callers who expect the output buffer to contain a genuine C string whose length is exactly the snprintf return value (assuming truncation hasn't happened or has been checked for). We can avoid this by only passing over the source string once, stopping the first time we meet a nul byte (or when we reach the given precision), and then letting widen_string() handle left/right space padding. Cc: Ingo Molnar Signed-off-by: Rasmus Villemoes --- lib/vsprintf.c | 28 +++++++++------------------- 1 file changed, 9 insertions(+), 19 deletions(-) diff --git a/lib/vsprintf.c b/lib/vsprintf.c index a0145b5d114e..ba00eab1e703 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -557,32 +557,22 @@ char *widen_string(char *buf, int n, char *end, struct printf_spec spec) static noinline_for_stack char *string(char *buf, char *end, const char *s, struct printf_spec spec) { - int len, i; + int len = 0; + size_t lim = spec.precision; if ((unsigned long)s < PAGE_SIZE) s = "(null)"; - len = strnlen(s, spec.precision); - - if (!(spec.flags & LEFT)) { - while (len < spec.field_width--) { - if (buf < end) - *buf = ' '; - ++buf; - } - } - for (i = 0; i < len; ++i) { - if (buf < end) - *buf = *s; - ++buf; ++s; - } - while (len < spec.field_width--) { + while (lim--) { + char c = *s++; + if (!c) + break; if (buf < end) - *buf = ' '; + *buf = c; ++buf; + ++len; } - - return buf; + return widen_string(buf, len, end, spec); } static noinline_for_stack -- 2.6.1