[PATCH v4 next 09/23] tools/nolibc: Implement strerror() in terms of strerror_r()

[david.laight.linux@gmail.com]

From: David Laight <david.laight.linux@gmail.com>

strerror() can be the only part of a program that has a .data section.
This requres 4k in the program file.

Add a simple implementation of strerror_r() (ignores buflen) and use
that in strerror() so that the "errno=" string is copied at run-time.
Use __builtin_memcpy() because that optimises away the input string
and just writes the required constants to the target buffer.

Ignoring buflen is unlikely to be a problem given that the output is
always short.

Code size change largely depends on whether the inlining decision for
strerror() changes.

Change the tests to use the normal EXPECT_VFPRINTF() when testing %m.
Skip the tests when !is_nolibc.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
---

v4:
- Leave the NOLIBC_IGNORE_ERRNO check in __nolibc_printf().
- Don't rename the errno parameter to strerror() in this patch.

 tools/include/nolibc/stdio.h                 | 18 +++++++++++++++---
 tools/testing/selftests/nolibc/nolibc-test.c | 20 ++------------------
 2 files changed, 17 insertions(+), 21 deletions(-)

diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index a4df72d9a2d3..03fcd0229f90 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -722,14 +722,26 @@ int setvbuf(FILE *stream __attribute__((unused)),
 	return 0;
 }
 
+static __attribute__((unused,))
+int strerror_r(int errnum, char *buf, size_t buflen __attribute__((unused)))
+{
+	__builtin_memcpy(buf, "errno=", 6);
+	return 6 + i64toa_r(errnum, buf + 6);
+}
+
 static __attribute__((unused))
 const char *strerror(int errno)
 {
-	static char buf[18] = "errno=";
+	static char buf[18];
+	char *b = buf;
+
+	/* Force gcc to use 'register offset' to access buf[]. */
+	_NOLIBC_OPTIMIZER_HIDE_VAR(b);
 
-	i64toa_r(errno, &buf[6]);
+	/* Use strerror_r() to avoid having the only .data in small programs. */
+	strerror_r(errno, b, sizeof(buf));
 
-	return buf;
+	return b;
 }
 
 #endif /* _NOLIBC_STDIO_H */
diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index 9ebebe4ff253..638f18fc5123 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -1786,23 +1786,6 @@ static int test_scanf(void)
 	return 0;
 }
 
-int test_strerror(void)
-{
-	char buf[100];
-	ssize_t ret;
-
-	memset(buf, 'A', sizeof(buf));
-
-	errno = EINVAL;
-	ret = snprintf(buf, sizeof(buf), "%m");
-	if (is_nolibc) {
-		if (ret < 6 || memcmp(buf, "errno=", 6))
-			return 1;
-	}
-
-	return 0;
-}
-
 static int test_printf_error(void)
 {
 	int fd, ret, saved_errno;
@@ -1852,8 +1835,9 @@ static int run_printf(int min, int max)
 		CASE_TEST(string_width); EXPECT_VFPRINTF(1, "         1", "%10s", "1"); break;
 		CASE_TEST(number_width); EXPECT_VFPRINTF(1, "         1", "%10d", 1); break;
 		CASE_TEST(width_trunc);  EXPECT_VFPRINTF(1, "                        1", "%25d", 1); break;
+		CASE_TEST(errno);        errno = 22; EXPECT_VFPRINTF(is_nolibc, "errno=22", "%m"); break;
+		CASE_TEST(errno-neg);    errno = -22; EXPECT_VFPRINTF(is_nolibc, "   errno=-22", "%12m"); break;
 		CASE_TEST(scanf);        EXPECT_ZR(1, test_scanf()); break;
-		CASE_TEST(strerror);     EXPECT_ZR(1, test_strerror()); break;
 		CASE_TEST(printf_error); EXPECT_ZR(1, test_printf_error()); break;
 		case __LINE__:
 			return ret; /* must be last */
-- 
2.39.5