From 1e3e03a47b73966c68a4585b5ecbc7f4efb66b34 Mon Sep 17 00:00:00 2001
From: Evan Brass <brassevan@gmail.com>
Date: Tue, 8 Jul 2025 15:14:38 -0700
Subject: [PATCH 1/2] Add a borrowing cmsg api

This is mainly to support implementing custom control messages.
---
 src/sys/socket/mod.rs | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/src/sys/socket/mod.rs b/src/sys/socket/mod.rs
index 30c4e0a082..bb60316a17 100644
--- a/src/sys/socket/mod.rs
+++ b/src/sys/socket/mod.rs
@@ -725,19 +725,43 @@ impl Iterator for CmsgIterator<'_> {
     type Item = ControlMessageOwned;
 
     fn next(&mut self) -> Option<ControlMessageOwned> {
-        match self.cmsghdr {
+        match self.next_raw() {
             None => None,   // No more messages
-            Some(hdr) => {
+            Some((hdr, _)) => {
                 // Get the data.
                 // Safe if cmsghdr points to valid data returned by recvmsg(2)
-                let cm = unsafe { Some(ControlMessageOwned::decode_from(hdr))};
+                unsafe { Some(ControlMessageOwned::decode_from(hdr))}
+            }
+        }
+    }
+}
+
+impl CmsgIterator<'_> {
+    pub fn next_raw(&mut self) -> Option<(&cmsghdr, &[u8])> {
+        match self.cmsghdr {
+            None => None,   // No more messages
+            Some(header) => {
+                // Get the data.
+                let p = unsafe { CMSG_DATA(header) };
+
+                // Determine the length of the control message data
+                // The cast is not unnecessary on all platforms.
+                #[allow(clippy::unnecessary_cast)]
+                let len = header as *const _ as usize + header.cmsg_len as usize
+                    - p as usize;
+
+                // Read the value as a slice (This slice is the same one that ControlMessageOwned::Unknown copies)
+                let value = unsafe { core::slice::from_raw_parts(p, len) };
+
                 // Advance the internal pointer.  Safe if mhdr and cmsghdr point
                 // to valid data returned by recvmsg(2)
                 self.cmsghdr = unsafe {
-                    let p = CMSG_NXTHDR(self.mhdr as *const _, hdr as *const _);
+                    let p = CMSG_NXTHDR(self.mhdr as *const _, header as *const _);
                     p.as_ref()
                 };
-                cm
+
+                // Return the header and data
+                Some((header, value))
             }
         }
     }

From 10c04451984d6e6f934d19bda10e30ddbd454d00 Mon Sep 17 00:00:00 2001
From: Evan Brass <brassevan@gmail.com>
Date: Wed, 9 Jul 2025 12:14:22 -0700
Subject: [PATCH 2/2] use map

---
 src/sys/socket/mod.rs | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/src/sys/socket/mod.rs b/src/sys/socket/mod.rs
index bb60316a17..4af588d770 100644
--- a/src/sys/socket/mod.rs
+++ b/src/sys/socket/mod.rs
@@ -725,14 +725,8 @@ impl Iterator for CmsgIterator<'_> {
     type Item = ControlMessageOwned;
 
     fn next(&mut self) -> Option<ControlMessageOwned> {
-        match self.next_raw() {
-            None => None,   // No more messages
-            Some((hdr, _)) => {
-                // Get the data.
-                // Safe if cmsghdr points to valid data returned by recvmsg(2)
-                unsafe { Some(ControlMessageOwned::decode_from(hdr))}
-            }
-        }
+        // Safe if cmsghdr points to valid data returned by recvmsg(2)
+        self.next_raw().map(|(hdr, _)| unsafe { ControlMessageOwned::decode_from(hdr) })
     }
 }