Re: [PATCH] Properly align memory allocations and temporary buffers

> 
> (+cc René as another possible reviewer)
> 
> On Wed, Jan 05, 2022 at 01:23:24PM +0000, Jessica Clarke wrote:
>> Currently git_qsort_s allocates a buffer on the stack that has no
>> alignment, and mem_pool_alloc assumes uintmax_t's size is adequate
>> alignment for any type.
>> 
>> On CHERI, and thus Arm's Morello prototype, pointers are implemented as
>> hardware capabilities which, as well as having a normal integer address,
>> have additional bounds, permissions and other metadata in a second word,
>> so on a 64-bit architecture they are 128-bit quantities, including their
>> alignment requirements. Despite being 128-bit, their integer component
>> is still only a 64-bit field, so uintmax_t remains 64-bit, and therefore
>> uintmax_t does not sufficiently align an allocation.
>> 
>> Moreover, these capabilities have an additional "129th" tag bit, which
>> tracks the validity of the capability and is cleared on any invalid
>> operation that doesn't trap (e.g. partially overwriting a capability
>> will invalidate it) which, combined with the architecture's strict
>> checks on capability manipulation instructions, ensures it is
>> architecturally impossible to construct a capability that gives more
>> rights than those you were given in the first place. To store these tag
>> bits, each capability sized and aligned word in memory gains a single
>> tag bit that is stored in unaddressable (to the processor) memory. This
>> means that it is impossible to store a capability at an unaligned
>> address: a normal load or store of a capability will always take an
>> alignment fault even if the (micro)architecture supports unaligned
>> loads/stores for other data types, and a memcpy will, if the destination
>> is not appropriately aligned, copy the byte representation but lose the
>> tag, meaning that if it is eventually copied back and loaded from an
>> aligned location any attempt to dereference it will trap with a tag
>> fault. Thus, even char buffers that are memcpy'ed to or from must be
>> properly aligned on CHERI architectures if they are to hold pointers.
>> 
>> Address both of these by introducing a new git_max_align type put in a
>> union with the on-stack buffer to force its alignment, as well as a new
>> GIT_MAX_ALIGNMENT macro whose value is the alignment of git_max_align
>> that gets used for mem_pool_alloc. As well as making the code work on
>> CHERI, the former change likely also improves performance on some
>> architectures by making memcpy faster (either because it can use larger
>> block sizes or because the microarchitecture has inefficient unaligned
>> accesses).
>> 
>> Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
>> ---
>> compat/qsort_s.c  | 11 +++++++----
>> git-compat-util.h | 11 +++++++++++
>> mem-pool.c        |  6 +++---
>> 3 files changed, 21 insertions(+), 7 deletions(-)
>> 
>> diff --git a/compat/qsort_s.c b/compat/qsort_s.c
>> index 52d1f0a73d..1ccdb87451 100644
>> --- a/compat/qsort_s.c
>> +++ b/compat/qsort_s.c
>> @@ -49,16 +49,19 @@ int git_qsort_s(void *b, size_t n, size_t s,
>> 		int (*cmp)(const void *, const void *, void *), void *ctx)
>> {
>> 	const size_t size = st_mult(n, s);
>> -	char buf[1024];
>> +	union {
>> +		char buf[1024];
>> +		git_max_align align;
>> +	} u;
> 
> I'm not sure I understand. Clearly this union aligns buf along the width
> of git_max_align. But what about the preimage makes buf unaligned?

>> diff --git a/git-compat-util.h b/git-compat-util.h
>> index 5fa54a7afe..28581a45c5 100644
>> --- a/git-compat-util.h
>> +++ b/git-compat-util.h
>> @@ -274,6 +274,17 @@ typedef unsigned long uintptr_t;
>> #define _ALL_SOURCE 1
>> #endif
>> 
>> +typedef union {
>> +	uintmax_t max_align_uintmax;
>> +	void *max_align_pointer;
>> +} git_max_align;
> 
> OK, the purpose of this union is to be as wide as the least common
> alignment between uintmax_t and void *, yes?

>> +
>> +typedef struct {
>> +	char unalign;
>> +	git_max_align aligned;
>> +} git_max_alignment;
>> +#define GIT_MAX_ALIGNMENT offsetof(git_max_alignment, aligned)
> 
> ...then the offset of the aligned field within the git_max_alignment
> struct is going to be that common alignment? Could you not `#define
> GIT_MAX_ALIGNMENT` to be `sizeof(git_max_align)` directly, or is there
> something I'm missing?