[PATCH v3] mem-pool: Don't assume uintmax_t is aligned enough for all types
- From
Jessica Clarke <jrtc27@jrtc27.com>
- Date
- Jan 23, 2022, 20:33 UTC
- Message-ID
- <20220123203347.74869-1-jrtc27@jrtc27.com>
- In-Reply-To
- <20220123152458.72540-1-jrtc27@jrtc27.com>
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> ---
Changed in v3: * Removed first "Currently" from commit message body * Added explanation for _Alignof vs sizeof in comment
mem-pool.c | 26 +++++++++++++++++++++++--- 1 file changed, 23 insertions(+), 3 deletions(-)
diff --git a/mem-pool.c b/mem-pool.c index ccdcad2e3d..599d8e895f 100644 --- 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)
-- 2.33.1