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[RFC 3/3] lib/vsprintf.c: eliminate potential race in string()

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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@rasmusvillemoes.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

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Re: [PATCH] string: Improve the generic strlcpy() implementation Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-06 00:30 +0200
  Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-06 10:00 +0200
  Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-06 10:10 +0200
    Re: [PATCH] string: Improve the generic strlcpy() implementation Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-07 00:10 +0200
      Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-07 09:20 +0200
        Re: [PATCH] string: Improve the generic strlcpy() implementation Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-07 11:10 +0200
          Re: [PATCH] string: Improve the generic strlcpy() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-07 11:30 +0200
            Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-08 10:50 +0200
              Re: [PATCH] string: Improve the generic strlcpy() implementation Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 10:20 +0200
                [RFC 0/3] eliminate potential race in string() (was: [PATCH] string: Improve the generic strlcpy() implementation) Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 11:20 +0200
                [RFC 1/3] lib/vsprintf.c: pull out padding code from dentry_name() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 11:20 +0200
                [RFC 3/3] lib/vsprintf.c: eliminate potential race in string() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 11:20 +0200
                [RFC 2/3] lib/vsprintf.c: move string() below widen_string() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 11:20 +0200
                Re: [RFC 0/3] eliminate potential race in string() (was: [PATCH]  string: Improve the generic strlcpy() implementation) Ingo Molnar <mingo@kernel.org> - 2015-10-10 09:50 +0200

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