From 67538b64ea1b90a0d01e4f6ebd45713ec03c9222 Mon Sep 17 00:00:00 2001
From: Eric Seppanen <eds@reric.net>
Date: Fri, 16 Jul 2021 12:22:02 -0700
Subject: Allow explicit padding (#2060)

If a struct needs to be serialized in its native format (padding bytes
and all), for example writing it to a file or sending it on the network,
then explicit padding fields are necessary, as anything reading the
padding bytes of a struct may lead to Undefined Behavior.
---
 src/codegen/struct_layout.rs | 42 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 37 insertions(+), 5 deletions(-)

(limited to 'src/codegen/struct_layout.rs')

diff --git a/src/codegen/struct_layout.rs b/src/codegen/struct_layout.rs
index 2e4b9735..1c6b977b 100644
--- a/src/codegen/struct_layout.rs
+++ b/src/codegen/struct_layout.rs
@@ -217,8 +217,11 @@ impl<'a> StructLayoutTracker<'a> {
         let padding_layout = if self.is_packed || is_union {
             None
         } else {
+            let force_padding = self.ctx.options().force_explicit_padding;
+
             // Otherwise the padding is useless.
-            let need_padding = padding_bytes >= field_layout.align ||
+            let need_padding = force_padding ||
+                padding_bytes >= field_layout.align ||
                 field_layout.align > MAX_GUARANTEED_ALIGN;
 
             debug!(
@@ -236,11 +239,14 @@ impl<'a> StructLayoutTracker<'a> {
                 field_layout
             );
 
+            let padding_align = if force_padding {
+                1
+            } else {
+                cmp::min(field_layout.align, MAX_GUARANTEED_ALIGN)
+            };
+
             if need_padding && padding_bytes != 0 {
-                Some(Layout::new(
-                    padding_bytes,
-                    cmp::min(field_layout.align, MAX_GUARANTEED_ALIGN),
-                ))
+                Some(Layout::new(padding_bytes, padding_align))
             } else {
                 None
             }
@@ -262,6 +268,32 @@ impl<'a> StructLayoutTracker<'a> {
         padding_layout.map(|layout| self.padding_field(layout))
     }
 
+    pub fn add_tail_padding(
+        &mut self,
+        comp_name: &str,
+        comp_layout: Layout,
+    ) -> Option<proc_macro2::TokenStream> {
+        // Only emit an padding field at the end of a struct if the
+        // user configures explicit padding.
+        if !self.ctx.options().force_explicit_padding {
+            return None;
+        }
+
+        if self.latest_offset == comp_layout.size {
+            // This struct does not contain tail padding.
+            return None;
+        }
+
+        trace!(
+            "need a tail padding field for {}: offset {} -> size {}",
+            comp_name,
+            self.latest_offset,
+            comp_layout.size
+        );
+        let size = comp_layout.size - self.latest_offset;
+        Some(self.padding_field(Layout::new(size, 0)))
+    }
+
     pub fn pad_struct(
         &mut self,
         layout: Layout,
-- 
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