linux_inet_diag: get rid of alloca usage

alloca makes stack usage unpredictable and life difficult for
static analysis tools and compilers.  The 46 bytes of
INET6_ADDRSTRLEN is fine to keep on stack, but page size can be
several MB large in some architectures (but typically 4K on
common architectures).

Thus we handle page size-ed allocations via `rb_str_tmp_new'.
`rb_str_tmp_new' has been in public Ruby headers since the 1.9
days and used by the core `zlib', `digest', and `zlib'
extensions, so it should be safe to use (and `rb_str_resize' is
used in many more C extensions).

1 files changed, 11 insertions, 8 deletions

diff --git a/ext/raindrops/linux_inet_diag.c b/ext/raindrops/linux_inet_diag.c
index e1ae62a..2d4f503 100644
--- a/ext/raindrops/linux_inet_diag.c
+++ b/ext/raindrops/linux_inet_diag.c
@@ -212,24 +212,25 @@ static void bug_warn_nogvl(const char *fmt, ...)
 static struct listen_stats *stats_for(st_table *table, struct inet_diag_msg *r)
 {
         char *host, *key, *port, *old_key;
-        size_t alloca_len;
         struct listen_stats *stats;
         socklen_t hostlen;
         socklen_t portlen = (socklen_t)sizeof("65535");
         int n;
         const void *src = r->id.idiag_src;
+        char buf[INET6_ADDRSTRLEN];
+        size_t buf_len;
 
         switch (r->idiag_family) {
         case AF_INET: {
                 hostlen = INET_ADDRSTRLEN;
-                alloca_len = hostlen + portlen;
-                host = key = alloca(alloca_len);
+                buf_len = hostlen + portlen;
+                host = key = buf;
                 break;
                 }
         case AF_INET6: {
                 hostlen = INET6_ADDRSTRLEN;
-                alloca_len = 1 + hostlen + 1 + portlen;
-                key = alloca(alloca_len);
+                buf_len = 1 + hostlen + 1 + portlen;
+                key = buf;
                 host = key + 1;
                 break;
                 }
@@ -269,7 +270,7 @@ static struct listen_stats *stats_for(st_table *table, struct inet_diag_msg *r)
         old_key = key;
 
         if (r->idiag_state == TCP_ESTABLISHED) {
-                n = snprintf(key, alloca_len, "%s:%u",
+                n = snprintf(key, buf_len, "%s:%u",
                                  addr_any(r->idiag_family),
                                  ntohs(r->id.idiag_sport));
                 if (n <= 0) {
@@ -615,7 +616,7 @@ static VALUE tcp_listener_stats(int argc, VALUE *argv, VALUE self)
 {
         VALUE rv = rb_hash_new();
         struct nogvl_args args;
-        VALUE addrs, sock;
+        VALUE addrs, sock, buf;
 
         rb_scan_args(argc, argv, "02", &addrs, &sock);
 
@@ -624,8 +625,9 @@ static VALUE tcp_listener_stats(int argc, VALUE *argv, VALUE self)
          * buffer for recvmsg() later, we already checked for
          * OPLEN <= page_size at initialization
          */
+        buf = rb_str_buf_new(page_size);
         args.iov[2].iov_len = OPLEN;
-        args.iov[2].iov_base = alloca(page_size);
+        args.iov[2].iov_base = RSTRING_PTR(buf);
         args.table = NULL;
         sock = NIL_P(sock) ? rb_funcall(cIDSock, id_new, 0)
                         : rb_io_get_io(sock);
@@ -672,6 +674,7 @@ static VALUE tcp_listener_stats(int argc, VALUE *argv, VALUE self)
                 rb_hash_foreach(rv, drop_placeholders, Qfalse);
 
         /* let GC deal with corner cases */
+        rb_str_resize(buf, 0);
         if (argc < 2) rb_io_close(sock);
         return rv;
 }