I recently upgraded to clang-18, and after compiling musl with it, I noticed that all my getaddrinfo() calls are failing. I tracked this to be an issue in __res_msend_rc(), where the 'sin6' member of union 'sa' is initialized to garbage, rather than 0. Then later bind() fails with EADDRNOTAVAIL.
- According to strict interpretation of the C standard, initializing a union with '{0}', only initializes the first member of the union to 0 (in this case, sin4), and "default" initializes the rest. This interpretation is still up for debate. The proper way to initialize the entire union is '{ }' not '{ 0 }'.
- There is currently a bug in clang-18 that treats '{ }' to be the same as '{ 0 }'. The proposed fix is to just zero out the entire union for both "{ 0 }" and "{ }". However we cannot rely on "{ 0 }" to always zero out the entire union in the future.
musl should be fixed to use "{ }" for initialization. And to work around the current buggy release of clang-18, perhaps flip the order to make sin6 the first member of the struct? I've attached a patch that works for me. There may be other instances of the same bug in the musl code base.
--- a/src/network/res_msend.c
+++ b/src/network/res_msend.c
@@ -83,9 +83,9 @@ int __res_msend_rc(int nqueries, const unsigned char *const *queries,
int fd;
int timeout, attempts, retry_interval, servfail_retry;
union {
- struct sockaddr_in sin;
struct sockaddr_in6 sin6;
- } sa = {0}, ns[MAXNS] = {{0}};
+ struct sockaddr_in sin;
+ } sa = {}, ns[MAXNS] = {{}};
socklen_t sl = sizeof sa.sin;
int nns = 0;
int family = AF_INET;