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.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Joe Perches <joe@perches.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Maurizio Lombardi <mlombard@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files 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");
 }