Fix ifaddrs error handling.

An NLMSG_ERROR packet includes an errno value that we should use. Also report
failures to create a socket immediately, rather than falling through to the
send and reporting EBADF.

Bug: http://b/32145516
Bug: http://b/31038971
Test: bionic ifaddr tests on ryu (with broken kernel) and flounder
Change-Id: I84c480c5b75077eb90d40426a9d66d7bffbd3d51

libc/bionic/bionic_netlink.cpp

@@ -61,6 +61,7 @@ bool NetlinkConnection::SendRequest(int type) {
   // Did we open a netlink socket yet?
   if (fd_ == -1) {
     fd_ = socket(PF_NETLINK, SOCK_RAW | SOCK_CLOEXEC, NETLINK_ROUTE);
+    if (fd_ == -1) return false;
   }
 
   // Construct and send the message.
@@ -83,7 +84,11 @@ bool NetlinkConnection::ReadResponses(void callback(void*, nlmsghdr*), void* con
     nlmsghdr* hdr = reinterpret_cast<nlmsghdr*>(data_);
     for (; NLMSG_OK(hdr, static_cast<size_t>(bytes_read)); hdr = NLMSG_NEXT(hdr, bytes_read)) {
       if (hdr->nlmsg_type == NLMSG_DONE) return true;
-      if (hdr->nlmsg_type == NLMSG_ERROR) return false;
+      if (hdr->nlmsg_type == NLMSG_ERROR) {
+        nlmsgerr* err = reinterpret_cast<nlmsgerr*>(NLMSG_DATA(hdr));
+        errno = (hdr->nlmsg_len >= NLMSG_LENGTH(sizeof(nlmsgerr))) ? -err->error : EIO;
+        return false;
+      }
       callback(context, hdr);
     }
   }

tests/ifaddrs_test.cpp

@@ -19,6 +19,7 @@
 #include <ifaddrs.h>
 
 #include <dirent.h>
+#include <fcntl.h>
 #include <linux/if_packet.h>
 #include <net/ethernet.h>
 #include <net/if.h>
@@ -28,6 +29,7 @@
 
 #include <algorithm>
 #include <map>
+#include <thread>
 #include <vector>
 
 TEST(ifaddrs, freeifaddrs_null) {
@@ -267,3 +269,40 @@ TEST(ifaddrs, inet6_scope_ids) {
 
   freeifaddrs(addrs);
 }
+
+TEST(ifaddrs, kernel_bug_31038971) {
+  // Some kernels had a bug that would lead to an NLMSG_ERROR response,
+  // but bionic wasn't setting errno based on the value in the message.
+  // This is the test for the kernel bug, but on a device with a bad
+  // kernel this test was also useful for testing the bionic errno fix.
+  std::vector<std::thread*> threads;
+  for (size_t i = 0; i < 128; ++i) {
+    threads.push_back(new std::thread([]() {
+      ifaddrs* addrs = nullptr;
+      ASSERT_EQ(0, getifaddrs(&addrs)) << strerror(errno);
+      freeifaddrs(addrs);
+    }));
+  }
+  for (auto& t : threads) {
+    t->join();
+    delete t;
+  }
+}
+
+TEST(ifaddrs, errno_EMFILE) {
+  std::vector<int> fds;
+  while (true) {
+    int fd = open("/dev/null", O_RDONLY|O_CLOEXEC);
+    if (fd == -1) {
+      ASSERT_EQ(EMFILE, errno);
+      break;
+    }
+    fds.push_back(fd);
+  }
+
+  ifaddrs* addrs;
+  EXPECT_EQ(-1, getifaddrs(&addrs));
+  EXPECT_EQ(EMFILE, errno);
+
+  for (int fd : fds) close(fd);
+}