BUILD: ebtree: improve architecture-specific alignment
Commit 2c315ee75e ("BUG/MEDIUM: ebtree: don't set attribute packed
without unaligned access support") addressed alignment issues in
ebtrees in a way that is not really optimal since it will leave holes
in eb32trees for example.
This fix is better in that it restores the packed attribute on ebnode
but enforces proper alignment on the carrying nodes where necessary.
This also has the benefit of closing holes wherever possible and to
align data to the minimally required size.
The only thing it cannot close is the 32-bit hole at the end of ebmbnode
due to the required 64-bit on certain archs but at least it guarantees
that the key correctly points to the end of the node and that there is
never a hole after it.
This is a better fix than the one above and should be backported to
branches where the one above will be backported.
diff --git a/ebtree/ebmbtree.h b/ebtree/ebmbtree.h
index 3200a12..8262ec6 100644
--- a/ebtree/ebmbtree.h
+++ b/ebtree/ebmbtree.h
@@ -36,11 +36,16 @@
* is, the end user is not meant to manipulate internals, so this is pointless.
* The 'node.bit' value here works differently from scalar types, as it contains
* the number of identical bits between the two branches.
+ * Note that we take a great care of making sure the key is located exactly at
+ * the end of the struct even if that involves holes before it, so that it
+ * always aliases any external key a user would append after. This is why the
+ * key uses the same alignment as the struct.
*/
struct ebmb_node {
struct eb_node node; /* the tree node, must be at the beginning */
+ ALWAYS_ALIGN(sizeof(void*));
unsigned char key[0]; /* the key, its size depends on the application */
-};
+} ALIGNED(sizeof(void*));
/*
* Exported functions and macros.