libdrgn: work around "undefined reference to '__muloti4'" when using Clang

Older versions of Clang generate a call to __muloti4() for
__builtin_mul_overflow() with mixed signed and unsigned types. However,
Clang doesn't link to compiler-rt by default. Work around it by making
all of our calls to __builtin_mul_overflow() use unsigned types only.

1: https://bugs.llvm.org/show_bug.cgi?id=16404

Author: osandov

_drgn/type.c

@@ -27,17 +27,17 @@ static DrgnType *DrgnType_new(enum drgn_qualifiers qualifiers, size_t nmemb,
 			      size_t size)
 {
 	DrgnType *type_obj;
-	Py_ssize_t nitems;
+	size_t bytes;
 
-	if (__builtin_mul_overflow(nmemb, size, &nitems) ||
-	    __builtin_add_overflow(nitems, sizeof(struct drgn_type), &nitems) ||
-	    __builtin_add_overflow(nitems, DrgnType_type.tp_itemsize - 1,
-				   &nitems)) {
+	if (__builtin_mul_overflow(nmemb, size, &bytes) ||
+	    __builtin_add_overflow(bytes, sizeof(struct drgn_type), &bytes) ||
+	    __builtin_add_overflow(bytes, sizeof(void *) - 1, &bytes) ||
+	    bytes / sizeof(void *) > PY_SSIZE_T_MAX - sizeof(DrgnType)) {
 		PyErr_NoMemory();
 		return NULL;
 	}
-	nitems /= DrgnType_type.tp_itemsize;
-	type_obj = (DrgnType *)DrgnType_type.tp_alloc(&DrgnType_type, nitems);
+	type_obj = (DrgnType *)DrgnType_type.tp_alloc(&DrgnType_type,
+						      bytes / sizeof(void *));
 	if (!type_obj)
 		return NULL;
 	type_obj->qualifiers = qualifiers;

libdrgn/program.c

@@ -542,12 +542,12 @@ static struct drgn_error *parse_nt_file(const char *desc, size_t descsz,
 	if (is_64_bit) {
 		if (!read_u64(&p, end, bswap, &count) ||
 		    !read_u64(&p, end, bswap, &page_size) ||
-		    __builtin_mul_overflow(count, 24, &paths_offset))
+		    __builtin_mul_overflow(count, 24U, &paths_offset))
 			goto invalid;
 	} else {
 		if (!read_u32_into_u64(&p, end, bswap, &count) ||
 		    !read_u32_into_u64(&p, end, bswap, &page_size) ||
-		    __builtin_mul_overflow(count, 12, &paths_offset))
+		    __builtin_mul_overflow(count, 12U, &paths_offset))
 			goto invalid;
 	}
 

libdrgn/type.c

@@ -648,7 +648,7 @@ struct drgn_error *drgn_type_bit_size(struct drgn_type *type, uint64_t *ret)
 	err = drgn_type_sizeof(type, ret);
 	if (err)
 		return err;
-	if (__builtin_mul_overflow(*ret, 8, ret)) {
+	if (__builtin_mul_overflow(*ret, 8U, ret)) {
 		return drgn_error_create(DRGN_ERROR_OVERFLOW,
 					 "type bit size is too large");
 	}