[libc-commits] [libc] [libc] Add struct cmsghdr and associated macros (PR #193756)

[Jeff Bailey via libc-commits]

================
@@ -73,6 +80,122 @@ TEST_F(LlvmLibcSendMsgRecvMsgTest, SucceedsWithSocketPair) {
   ASSERT_THAT(LIBC_NAMESPACE::close(sockpair[1]), Succeeds(0));
 }
 
+TEST_F(LlvmLibcSendMsgRecvMsgTest, CmsgDetails) {
+  ASSERT_EQ(CMSG_ALIGN(0), static_cast<size_t>(0));
+  ASSERT_EQ(CMSG_ALIGN(1), sizeof(size_t));
+
+  // Some implementations align struct cmsghdr in various size computations, but
+  // this is a noop. This verifies that.
+  ASSERT_EQ(CMSG_ALIGN(sizeof(struct cmsghdr)), sizeof(struct cmsghdr));
+
+  char buf[0x100] = {};
+
+  struct msghdr msg;
+  msg.msg_control = buf;
+
+  // We shouldn't be able to get the first header if there's not enough space
+  // for it.
+  msg.msg_controllen = 0;
+  ASSERT_EQ(CMSG_FIRSTHDR(&msg), nullptr);
+  msg.msg_controllen = sizeof(struct cmsghdr) - 1;
+  ASSERT_EQ(CMSG_FIRSTHDR(&msg), nullptr);
+  msg.msg_controllen = sizeof(struct cmsghdr);
+  ASSERT_EQ(CMSG_FIRSTHDR(&msg), reinterpret_cast<struct cmsghdr *>(buf));
+  msg.msg_controllen = sizeof(buf);
+  struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+  ASSERT_EQ(cmsg, reinterpret_cast<struct cmsghdr *>(buf));
+
+  // We shouldn't be able to get the next header if this one is too big.
+  cmsg->cmsg_len = 0x1000;
+  ASSERT_EQ(CMSG_NXTHDR(&msg, cmsg), nullptr);
+  cmsg->cmsg_len = sizeof(buf) - sizeof(struct cmsghdr) + 1;
+  ASSERT_EQ(CMSG_NXTHDR(&msg, cmsg), nullptr);
+
+  cmsg->cmsg_len = sizeof(buf) - sizeof(struct cmsghdr);
+  struct cmsghdr *cmsg2 = CMSG_NXTHDR(&msg, cmsg);
+  ASSERT_LT(buf, reinterpret_cast<char *>(cmsg2));
+  ASSERT_LT(reinterpret_cast<char *>(cmsg2), buf + sizeof(buf));
+
+  // POSIX allows explicitly does not specify whether CMSG_NXTHDR returns the
----------------
kaladron wrote:

```suggestion
  // POSIX explicitly does not specify whether CMSG_NXTHDR returns the
```

https://github.com/llvm/llvm-project/pull/193756