[PATCH 3/4] selftests/nolibc: add abs() range test

[Ammar Faizi <ammarfaizi2@openresty.com>]

The existing abs/abs_noop cases only cover abs(-10) and abs(10), and
labs() and llabs() have no coverage at all.  Nothing exercises the type
minimum, where negating the argument overflows.

Add a case walking all three functions over the interesting points of
their argument type: the minimum, the minimum plus one, an ordinary
negative value, zero and the maximum. At the minimum the absolute
value is not representable and the argument is returned unchanged.

Both the arguments and the results have to be hidden from the
optimizer. The compiler knows these functions never return a negative
value, so it folds the comparisons at build time otherwise, and that
would equally hide the overflow being tested for.

Note that the overflow itself is only reported through the:

    -fsanitize=undefined -fsanitize-trap=all

flags the suite already builds with; without them the wrapped result
is the same as the correct one. With the flags, and with the preceding
fix reverted, the run dies with SIGILL on this test on both i386 and
x86-64.

Cc: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Signed-off-by: Ammar Faizi <ammarfaizi2@openresty.com>
---
 tools/testing/selftests/nolibc/nolibc-test.c | 47 ++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index 996e8d13508e..4dc2ffea63bd 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -1728,6 +1728,52 @@ int test_alloca(void)
 	return *x - 0x1234;
 }
 
+/* abs(), labs() and llabs() over the whole range of their argument type */
+int test_abs_range(void)
+{
+	int i, ri;
+	long l, rl;
+	long long ll, rll;
+
+	/*
+	 * Both the inputs and the results have to stay opaque: the compiler
+	 * knows abs() and friends never return a negative value and would
+	 * otherwise fold the comparisons below at build time, which would also
+	 * hide the undefined behavior that is being tested for.
+	 */
+	i = INT_MIN; l = LONG_MIN; ll = LLONG_MIN;
+	__asm__ ("" : "+r" (i), "+r" (l), "+r" (ll));
+	ri = abs(i); rl = labs(l); rll = llabs(ll);
+	__asm__ ("" : "+r" (ri), "+r" (rl), "+r" (rll));
+	/* the absolute value is not representable, the input is returned */
+	if (ri != INT_MIN || rl != LONG_MIN || rll != LLONG_MIN)
+		return 1;
+
+	i = INT_MIN + 1; l = LONG_MIN + 1; ll = LLONG_MIN + 1;
+	__asm__ ("" : "+r" (i), "+r" (l), "+r" (ll));
+	ri = abs(i); rl = labs(l); rll = llabs(ll);
+	__asm__ ("" : "+r" (ri), "+r" (rl), "+r" (rll));
+	if (ri != INT_MAX || rl != LONG_MAX || rll != LLONG_MAX)
+		return 2;
+
+	i = -42; l = -42; ll = -42;
+	__asm__ ("" : "+r" (i), "+r" (l), "+r" (ll));
+	if (abs(i) != 42 || labs(l) != 42 || llabs(ll) != 42)
+		return 3;
+
+	i = 0; l = 0; ll = 0;
+	__asm__ ("" : "+r" (i), "+r" (l), "+r" (ll));
+	if (abs(i) != 0 || labs(l) != 0 || llabs(ll) != 0)
+		return 4;
+
+	i = INT_MAX; l = LONG_MAX; ll = LLONG_MAX;
+	__asm__ ("" : "+r" (i), "+r" (l), "+r" (ll));
+	if (abs(i) != INT_MAX || labs(l) != LONG_MAX || llabs(ll) != LLONG_MAX)
+		return 5;
+
+	return 0;
+}
+
 int test_difftime(void)
 {
 	if (difftime(200., 100.) != 100.)
@@ -1943,6 +1989,7 @@ int run_stdlib(int min, int max)
 		CASE_TEST(toupper_noop);            EXPECT_EQ(1, toupper('A'), 'A'); break;
 		CASE_TEST(abs);                     EXPECT_EQ(1, abs(-10), 10); break;
 		CASE_TEST(abs_noop);                EXPECT_EQ(1, abs(10), 10); break;
+		CASE_TEST(abs_range);               EXPECT_ZR(1, test_abs_range()); break;
 		CASE_TEST(alloca);                  EXPECT_ZR(1, test_alloca()); break;
 		CASE_TEST(difftime);                EXPECT_ZR(1, test_difftime()); break;
 		CASE_TEST(memchr_foobar6_o);        EXPECT_STREQ(1, memchr("foobar", 'o', 6), "oobar"); break;
-- 
Ammar Faizi