wasip2: Fix poll's behavior on invalid fds (WebAssembly#680)

Skip negative fds, but fail on invalid fds.

Author: alexcrichton

libc-bottom-half/sources/poll-wasip2.c

@@ -60,10 +60,14 @@ int poll_wasip2(struct pollfd *fds, size_t nfds, int timeout)
 
         for (size_t i = 0; i < nfds; ++i) {
                 struct pollfd *pollfd = fds + i;
+                if (pollfd->fd < 0)
+                  continue;
                 state.pollfd = pollfd;
                 descriptor_table_entry_t *entry = descriptor_table_get_ref(pollfd->fd);
-                if (!entry)
-                    continue;
+                if (!entry) {
+                  errno = EBADF;
+                  return -1;
+                }
 
                 // If this descriptor has a custom registration function then
                 // use that exclusively.

test/src/poll.c

@@ -5,6 +5,7 @@
 #include <fcntl.h>
 #include <poll.h>
 #include "test.h"
+#include <errno.h>
 
 #define TEST(c, ...) ((c) ? 1 : (t_error(#c" failed: " __VA_ARGS__),0))
 
@@ -32,5 +33,15 @@ int main(void)
         if (fd > 2)
             TEST(unlink(tmp) != -1);
 
+        // skip negative fds
+        poll_fd.fd = -300;
+        r = poll(&poll_fd, 1, 0);
+        TEST(r==0, "poll returned %d, expected 0\n", r);
+
+        // fail on invalid fds
+        poll_fd.fd = 300;
+        r = poll(&poll_fd, 1, 0);
+        int err = errno;
+        TEST(r==-1 && err==EBADF, "poll returned %d, expected -1\n", r);
 	return t_status;
 }