printf-hook-builtin: Support GNU %m specifier

src/libstrongswan/tests/test_printf.c

@@ -15,6 +15,8 @@
 
 #include "test_suite.h"
 
+#include <errno.h>
+
 static void verify(char *expected, char *format, ...)
 {
 	FILE *mem;
@@ -45,6 +47,13 @@ START_TEST(test_printf_strings)
 }
 END_TEST
 
+START_TEST(test_printf_err)
+{
+	errno = EINVAL;
+	verify((char*)strerror(errno), "%m");
+}
+END_TEST
+
 START_TEST(test_printf_unsigned)
 {
 	verify("1 23 456", "%u %lu %llu", 1, (u_long)23, (u_int64_t)456);
@@ -93,6 +102,10 @@ Suite *printf_suite_create()
 	tcase_add_test(tc, test_printf_strings);
 	suite_add_tcase(s, tc);
 
+	tc = tcase_create("err");
+	tcase_add_test(tc, test_printf_err);
+	suite_add_tcase(s, tc);
+
 	tc = tcase_create("unsiged");
 	tcase_add_test(tc, test_printf_unsigned);
 	suite_add_tcase(s, tc);

src/libstrongswan/utils/printf_hook/printf_hook_builtin.c

@@ -46,6 +46,7 @@
 #include <stdio.h>
 #include <stdarg.h>
 #include <string.h>
+#include <errno.h>
 
 #define PRINTF_BUF_LEN 8192
 #define ARGS_MAX 3
@@ -682,6 +683,13 @@ int builtin_vsnprintf(char *buffer, size_t n, const char *format, va_list ap)
 								slen = strlen(sarg);
 								goto is_string;
 							}
+							case 'm':
+							{
+								/* glibc error string */
+								sarg = strerror(errno);
+								slen = strlen(sarg);
+								goto is_string;
+							}
 							is_string:
 							{
 								char sch;