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

> 
> On 2022-01-05 at 13:23:24, 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.
> 
> I think this is going to be a problem in a lot of places, not just in
> Git.  I'm pretty sure that copying data this way is specifically allowed
> by C and POSIX, and thus this approach is going to break a whole lot of
> things.

> For example, casting a void * to uintptr_t and back should produce two
> pointers that compare equal.  The C standard says that two pointers
> compare equal if they're both null, both point to the same object, or
> one points one past the end of an array and the other happens to point
> to the beginning of another object.  If the pointers aren't null and the
> original one points to valid data, then the resulting pointer (after the
> two casts) would point to the same object (since that's the only valid
> option that's left), and therefore could be used to access it, but that
> wouldn't necessarily work in this case.

> The CHERI paper I'm reading also specifically says it is not changing
> uintmax_t, which is a direct violation of the C standard.  If uintptr_t
> must be larger than 64 bits, then so must uintmax_t, even if that
> happens to be inconvenient (because it changes the ABI from the normal
> system ABI).  It sounds like, in fact, that you can't actually provide
> uintptr_t with the current architecture, because it can't be provided in
> a standard-compliant way.