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Date: Fri, 9 Oct 2015 11:14:33 +0200 From: Rasmus Villemoes <linux@...musvillemoes.dk> To: Ingo Molnar <mingo@...nel.org> Cc: Chris Metcalf <cmetcalf@...hip.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Rasmus Villemoes <linux@...musvillemoes.dk>, linux-kernel@...r.kernel.org Subject: [RFC 3/3] lib/vsprintf.c: eliminate potential race in string() 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. Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk> --- lib/vsprintf.c | 28 +++++++++------------------- 1 file changed, 9 insertions(+), 19 deletions(-) diff --git a/lib/vsprintf.c b/lib/vsprintf.c index acead77594b5..d518849b6b75 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 -- 2.1.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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