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

[Taylor Blau <me@ttaylorr.com>]

(+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?

I suppose the way you wrote it here is done in order to prevent padding
on the end of the git_max_align union from artificially increasing the
value of GIT_MAX_ALIGNMENT.

In any case, I *think* what you wrote here is right. The typedef's are
uncommon to our codebase, though. I wonder how much of this is all
necessary.

Thanks,
Taylor