shared: add helper function to add and check for overflow

Use _builtin_uaddll_overflow/_builtin_uaddl_overflow when available,
abstracting the type to use it with uint64_t.

Otherwise fallback to the implementation as added in 67466f2 ("Prevent
offset + size overflow.").

This also adds the tests for this new helper in the testsuite.

4 files changed, 39 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 30eea86..30eb37f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -49,6 +49,8 @@ AC_CHECK_FUNCS_ONCE([finit_module])
 
 CC_CHECK_FUNC_BUILTIN([__builtin_clz])
 CC_CHECK_FUNC_BUILTIN([__builtin_types_compatible_p])
+CC_CHECK_FUNC_BUILTIN([__builtin_uaddl_overflow], [ ], [ ])
+CC_CHECK_FUNC_BUILTIN([__builtin_uaddll_overflow], [ ], [ ])
 
 # dietlibc doesn't have st.st_mtim struct member
 AC_CHECK_MEMBERS([struct stat.st_mtim], [], [], [#include <sys/stat.h>])
diff --git a/m4/attributes.m4 b/m4/attributes.m4
index 7922746..985a3ed 100644
--- a/m4/attributes.m4
+++ b/m4/attributes.m4
@@ -262,6 +262,8 @@ AC_DEFUN([CC_CHECK_FUNC_BUILTIN], [
        m4_case([$1],
          [__builtin_clz], [$1(0)],
          [__builtin_types_compatible_p], [$1(int, int)],
+         [__builtin_uaddl_overflow], [$1(0UL, 0UL, (void*)0)],
+         [__builtin_uaddll_overflow], [$1(0ULL, 0ULL, (void*)0)],
          [__builtin_expect], [$1(0, 0)]
        )])],
        [cc_cv_have_$1=yes],
diff --git a/shared/util.h b/shared/util.h
index 4c59705..6f602d3 100644
--- a/shared/util.h
+++ b/shared/util.h
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <inttypes.h>
 #include <limits.h>
 #include <stdbool.h>
 #include <stdlib.h>
@@ -72,3 +73,18 @@ static inline void freep(void *p) {
         free(*(void**) p);
 }
 #define _cleanup_free_ _cleanup_(freep)
+
+static inline bool addu64_overflow(uint64_t a, uint64_t b, uint64_t *res)
+{
+#if (HAVE___BUILTIN_UADDL_OVERFLOW && HAVE___BUILTIN_UADDLL_OVERFLOW)
+#if __SIZEOF_LONG__ == 8
+	return __builtin_uaddl_overflow(a, b, res);
+#elif __SIZEOF_LONG_LONG__ == 8
+	return __builtin_uaddll_overflow(a, b, res);
+#else
+#error "sizeof(long long) != 8"
+#endif
+#endif
+	*res = a + b;
+	return ULLONG_MAX - a < b;
+}
diff --git a/testsuite/test-util.c b/testsuite/test-util.c
index b0c4ce6..5e25e58 100644
--- a/testsuite/test-util.c
+++ b/testsuite/test-util.c
@@ -206,5 +206,24 @@ DEFINE_TEST(test_write_str_safe,
 		},
 	});
 
+static int test_addu64_overflow(const struct test *t)
+{
+	uint64_t res;
+	bool overflow;
+
+	overflow = addu64_overflow(UINT64_MAX - 1, 1, &res);
+	assert_return(!overflow, EXIT_FAILURE);
+	assert_return(res == UINT64_MAX, EXIT_FAILURE);
+
+	overflow = addu64_overflow(UINT64_MAX, 1, &res);
+	assert_return(overflow, EXIT_FAILURE);
+
+	return EXIT_SUCCESS;
+}
+DEFINE_TEST(test_addu64_overflow,
+	.description = "check implementation of addu4_overflow()",
+	.need_spawn = false,
+	);
+
 
 TESTSUITE_MAIN();