mem-pool: don't assume uintmax_t is aligned enough for all types

mem_pool_alloc uses sizeof(uintmax_t) as a proxy for what should be
_Alignof(max_align_t) in C11. On most architectures this is sufficient
(though on m68k it is in fact overly strict, since the de-facto ABI,
which differs from the specified System V ABI, has the maximum alignment
of all types as 2 bytes), but on CHERI, and thus Arm's Morello
prototype, it is insufficient for any type that stores a pointer, which
must be aligned to 128 bits (on 64-bit architectures extended with
CHERI), whilst uintmax_t is a 64-bit integer.

Fix this by introducing our own approximation for max_align_t and a
means to compute _Alignof it without relying on C11. Currently this
union only contains uintmax_t and void *, but more types can be added as
needed.

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/mem-pool.c b/mem-pool.c
index ccdcad2e3d622fdd9491d3d1cbdc26f673bb41df..599d8e895f81216e0e0cb44696734cb2a157309b 100644 (file)
--- a/mem-pool.c
+++ b/mem-pool.c
@@ -7,6 +7,26 @@
 
 #define BLOCK_GROWTH_SIZE (1024 * 1024 - sizeof(struct mp_block))
 
+/*
+ * The inner union is an approximation for C11's max_align_t, and the
+ * struct + offsetof computes _Alignof. This can all just be replaced
+ * with _Alignof(max_align_t) if/when C11 is part of the baseline.
+ * Note that _Alignof(X) need not be the same as sizeof(X); it's only
+ * required to be a (possibly trivial) factor. They are the same for
+ * most architectures, but m68k for example has only 2-byte alignment
+ * for its 4-byte and 8-byte types, so using sizeof would waste space.
+ *
+ * Add more types to the union if the current set is insufficient.
+ */
+struct git_max_alignment {
+       char unalign;
+       union {
+               uintmax_t max_align_uintmax;
+               void *max_align_pointer;
+       } aligned;
+};
+#define GIT_MAX_ALIGNMENT offsetof(struct git_max_alignment, aligned)
+
 /*
  * Allocate a new mp_block and insert it after the block specified in
  * `insert_after`. If `insert_after` is NULL, then insert block at the
@@ -69,9 +89,9 @@ void *mem_pool_alloc(struct mem_pool *pool, size_t len)
        struct mp_block *p = NULL;
        void *r;
 
-       /* round up to a 'uintmax_t' alignment */
-       if (len & (sizeof(uintmax_t) - 1))
-               len += sizeof(uintmax_t) - (len & (sizeof(uintmax_t) - 1));
+       /* round up to a 'GIT_MAX_ALIGNMENT' alignment */
+       if (len & (GIT_MAX_ALIGNMENT - 1))
+               len += GIT_MAX_ALIGNMENT - (len & (GIT_MAX_ALIGNMENT - 1));
 
        if (pool->mp_block &&
            pool->mp_block->end - pool->mp_block->next_free >= len)