@matheusbsbs / matheus-bagliano / commits / e38bcc66d8

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>

Jessica Clarke committed Jan 23, 2022 at 20:33 UTC e38bcc66d8af1a46203abe9b6ffc8aa124865803
1 file changed +23 -3
mem-pool.c
+23 -3
@@ -7,6 +7,26 @@
7
8 #define BLOCK_GROWTH_SIZE (1024 * 1024 - sizeof(struct mp_block))
9
10 +/*
11 + * The inner union is an approximation for C11's max_align_t, and the
12 + * struct + offsetof computes _Alignof. This can all just be replaced
13 + * with _Alignof(max_align_t) if/when C11 is part of the baseline.
14 + * Note that _Alignof(X) need not be the same as sizeof(X); it's only
15 + * required to be a (possibly trivial) factor. They are the same for
16 + * most architectures, but m68k for example has only 2-byte alignment
17 + * for its 4-byte and 8-byte types, so using sizeof would waste space.
18 + *
19 + * Add more types to the union if the current set is insufficient.
20 + */
21 +struct git_max_alignment {
22 + char unalign;
23 + union {
24 + uintmax_t max_align_uintmax;
25 + void *max_align_pointer;
26 + } aligned;
27 +};
28 +#define GIT_MAX_ALIGNMENT offsetof(struct git_max_alignment, aligned)
29 +
30 /*
31 * Allocate a new mp_block and insert it after the block specified in
32 * `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)
89 struct mp_block *p = NULL;
90 void *r;
91
72 - /* round up to a 'uintmax_t' alignment */
73 - if (len & (sizeof(uintmax_t) - 1))
74 - len += sizeof(uintmax_t) - (len & (sizeof(uintmax_t) - 1));
92 + /* round up to a 'GIT_MAX_ALIGNMENT' alignment */
93 + if (len & (GIT_MAX_ALIGNMENT - 1))
94 + len += GIT_MAX_ALIGNMENT - (len & (GIT_MAX_ALIGNMENT - 1));
95
96 if (pool->mp_block &&
97 pool->mp_block->end - pool->mp_block->next_free >= len)