[PATCH v5 next 03/17] 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() 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.

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.

Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: David Laight <david.laight.linux@gmail.com>
---

v5:
- strerror_r() needs to return 0 on success.
- Return ERANGE if the buffer is too short to contain the longest
  possible output.

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                 | 22 +++++++++++++++++---
 tools/testing/selftests/nolibc/nolibc-test.c | 20 ++----------------
 2 files changed, 21 insertions(+), 21 deletions(-)

diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index a4df72d9a2d3..af2d5be47aa3 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -722,14 +722,30 @@ int setvbuf(FILE *stream __attribute__((unused)),
 	return 0;
 }
 
+static __attribute__((unused,))
+int strerror_r(int errnum, char *buf, size_t buflen)
+{
+	if (buflen < 18)
+		return ERANGE;
+
+	__builtin_memcpy(buf, "errno=", 6);
+	i64toa_r(errnum, buf + 6);
+	return 0;
+}
+
 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