linux-kernel - [PATCH v3 10/14] lib/test_printf.c: test precision quirks

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Date:	Thu,  3 Dec 2015 21:51:09 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH v3 10/14] lib/test_printf.c: test precision quirks

The kernel's printf doesn't follow the standards in a few corner cases
(which are probably mostly irrelevant). Add tests that document the
current behaviour.

Cc: Kees Cook <keescook@...omium.org>
Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 lib/test_printf.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/lib/test_printf.c b/lib/test_printf.c
index 1ce1a1dd8faf..3393d667c6b8 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -166,14 +166,23 @@ test_string(void)
 	test("", "%s%.0s", "", "123");
 	test("ABCD|abc|123", "%s|%.3s|%.*s", "ABCD", "abcdef", 3, "123456");
 	test("1  |  2|3  |  4|5  ", "%-3s|%3s|%-*s|%*s|%*s", "1", "2", 3, "3", 3, "4", -3, "5");
+	test("1234      ", "%-10.4s", "123456");
+	test("      1234", "%10.4s", "123456");
 	/*
-	 * POSIX and C99 say that a missing precision should be
-	 * treated as a precision of 0. However, the kernel's printf
-	 * implementation treats this case as if the . wasn't
-	 * present. Let's add a test case documenting the current
-	 * behaviour; should anyone ever feel the need to follow the
-	 * standards more closely, this can be revisited.
+	 * POSIX and C99 say that a negative precision (which is only
+	 * possible to pass via a * argument) should be treated as if
+	 * the precision wasn't present, and that if the precision is
+	 * omitted (as in %.s), the precision should be taken to be
+	 * 0. However, the kernel's printf behave exactly opposite,
+	 * treating a negative precision as 0 and treating an omitted
+	 * precision specifier as if no precision was given.
+	 *
+	 * These test cases document the current behaviour; should
+	 * anyone ever feel the need to follow the standards more
+	 * closely, this can be revisited.
 	 */
+	test("    ", "%4.*s", -5, "123456");
+	test("123456", "%.s", "123456");
 	test("a||", "%.s|%.0s|%.*s", "a", "b", 0, "c");
 	test("a  |   |   ", "%-3.s|%-3.0s|%-3.*s", "a", "b", 0, "c");
 }
-- 
2.6.1

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