From 028441d22fee121d129126c55938690be38bd3d9 Mon Sep 17 00:00:00 2001
From: Samuel Williams <samuel.williams@oriontransfer.co.nz>
Date: Mon, 12 Jul 2021 16:28:43 +1200
Subject: [PATCH] Avoid calling `fstat` on things we already know are valid
 sockets.

---
 ext/socket/basicsocket.c | 30 ++++++++++++++++++++++++++++--
 ext/socket/init.c        | 18 ------------------
 2 files changed, 28 insertions(+), 20 deletions(-)

diff --git a/ext/socket/basicsocket.c b/ext/socket/basicsocket.c
index 05172100c8..8477a37a6d 100644
--- a/ext/socket/basicsocket.c
+++ b/ext/socket/basicsocket.c
@@ -10,6 +10,28 @@
 
 #include "rubysocket.h"
 
+#ifdef _WIN32
+#define is_socket(fd) rb_w32_is_socket(fd)
+#else
+static int
+is_socket(int fd)
+{
+    struct stat sbuf;
+
+    if (fstat(fd, &sbuf) < 0)
+        rb_sys_fail("fstat(2)");
+    return S_ISSOCK(sbuf.st_mode);
+}
+#endif
+
+static void
+rsock_validate_descriptor(int descriptor)
+{
+    if (!is_socket(descriptor) || rb_reserved_fd_p(descriptor)) {
+        rb_syserr_fail(EBADF, "not a socket file descriptor");
+    }
+}
+
 /*
  * call-seq:
  *   BasicSocket.for_fd(fd) => basicsocket
@@ -22,10 +44,14 @@
  *
  */
 static VALUE
-bsock_s_for_fd(VALUE klass, VALUE fd)
+bsock_s_for_fd(VALUE klass, VALUE _descriptor)
 {
     rb_io_t *fptr;
-    VALUE sock = rsock_init_sock(rb_obj_alloc(klass), NUM2INT(fd));
+
+    int descriptor = RB_NUM2INT(_descriptor);
+    rsock_validate_descriptor(descriptor);
+
+    VALUE sock = rsock_init_sock(rb_obj_alloc(klass), descriptor);
 
     GetOpenFile(sock, fptr);
 
diff --git a/ext/socket/init.c b/ext/socket/init.c
index 5859c33e29..359696e626 100644
--- a/ext/socket/init.c
+++ b/ext/socket/init.c
@@ -54,20 +54,6 @@ rsock_raise_socket_error(const char *reason, int error)
 #endif
 }
 
-#ifdef _WIN32
-#define is_socket(fd) rb_w32_is_socket(fd)
-#else
-static int
-is_socket(int fd)
-{
-    struct stat sbuf;
-
-    if (fstat(fd, &sbuf) < 0)
-        rb_sys_fail("fstat(2)");
-    return S_ISSOCK(sbuf.st_mode);
-}
-#endif
-
 #if defined __APPLE__
 # define do_write_retry(code) do {ret = code;} while (ret == -1 && errno == EPROTOTYPE)
 #else
@@ -79,10 +65,6 @@ rsock_init_sock(VALUE sock, int fd)
 {
     rb_io_t *fp;
 
-    if (!is_socket(fd) || rb_reserved_fd_p(fd)) {
-	rb_syserr_fail(EBADF, "not a socket file descriptor");
-    }
-
     rb_update_max_fd(fd);
     MakeOpenFile(sock, fp);
     fp->fd = fd;