git@vger.kernel.org mailing list mirror (one of many)
 help / color / mirror / code / Atom feed
From: Jessica Clarke <jrtc27@jrtc27.com>
To: Taylor Blau <me@ttaylorr.com>
Cc: "René Scharfe" <l.s.r@web.de>, git@vger.kernel.org
Subject: Re: [PATCH] Properly align memory allocations and temporary buffers
Date: Thu, 6 Jan 2022 21:56:18 +0000	[thread overview]
Message-ID: <9F022B78-5EC7-41D8-8AA6-99470019BB95@jrtc27.com> (raw)
In-Reply-To: <YddjJ+uIQM34Gedo@nand.local>

On 6 Jan 2022, at 21:46, Taylor Blau <me@ttaylorr.com> wrote:
> 
> (+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?

It’s a char array, so it can have any alignment. Its address could be
0x10007. And it doesn’t align to the width of git_max_align, it aligns
to the alignment of git_max_align. Those don’t need to be the same, the
alignment just needs to be a factor of the size.

(Technically if git_max_align has a size > 1024 then it’d also make the
union bigger, but that’s clearly absurd for any real C implementation)

>> 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?

No, the purpose is for the union’s *alignment* to be the least common
alignment between uintmax_t and void *. The size doesn’t matter for
anything.

>> +
>> +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?

You could, but that would over-align in cases where the alignment of
git_max_align is smaller than its size. For example, uint32_t and
uint64_t only require 2-byte alignment on m68k. Using offsetof ensures
we actually query the thing we care about, the alignment, and not the
size, which is guaranteed to be a multiple of the alignment, but not
necessarily equal to it.

> 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.

So long as all those types have a size that is a power of two there
shouldn’t actually be any padding in the union, though it might be
legal for a hostile compiler to introduce it anyway for fun.

> 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.

If you’re willing to risk overaligning and wasting space then you can
just use sizeof the union. If you want it to be precise then I don’t
think you can cut any of it out (otherwise I would have done...).

Jess


  reply	other threads:[~2022-01-06 21:56 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-05 13:23 [PATCH] Properly align memory allocations and temporary buffers Jessica Clarke
2022-01-06 21:46 ` Taylor Blau
2022-01-06 21:56   ` Jessica Clarke [this message]
2022-01-06 22:27   ` Junio C Hamano
2022-01-06 22:56     ` Jessica Clarke
2022-01-07  0:10       ` Junio C Hamano
2022-01-07  0:22         ` Jessica Clarke
2022-01-07  0:31         ` brian m. carlson
2022-01-07  0:39           ` Jessica Clarke
2022-01-07  1:43             ` brian m. carlson
2022-01-07  2:08               ` Jessica Clarke
2022-01-07  2:11                 ` Jessica Clarke
2022-01-07 19:30               ` Junio C Hamano
2022-01-07 19:33                 ` Jessica Clarke
2022-01-07 20:56                 ` René Scharfe
2022-01-07 21:30                   ` Junio C Hamano
2022-01-07 23:30                     ` René Scharfe
2022-01-08  0:18                       ` Elijah Newren
2022-01-06 23:22 ` brian m. carlson
2022-01-06 23:31   ` Jessica Clarke
2022-01-07 14:57 ` Philip Oakley
2022-01-07 16:08 ` René Scharfe
2022-01-07 16:21   ` Jessica Clarke
2022-01-12 13:58 ` Jessica Clarke
2022-01-12 15:47   ` René Scharfe
2022-01-12 15:49     ` Jessica Clarke
2022-01-23 15:24 ` [PATCH v2] mem-pool: Don't assume uintmax_t is aligned enough for all types Jessica Clarke
2022-01-23 20:17   ` Junio C Hamano
2022-01-23 20:23     ` Jessica Clarke
2022-01-23 20:28       ` Junio C Hamano
2022-01-23 20:33   ` [PATCH v3] " Jessica Clarke
2022-01-24 17:11     ` Junio C Hamano

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: http://vger.kernel.org/majordomo-info.html

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=9F022B78-5EC7-41D8-8AA6-99470019BB95@jrtc27.com \
    --to=jrtc27@jrtc27.com \
    --cc=git@vger.kernel.org \
    --cc=l.s.r@web.de \
    --cc=me@ttaylorr.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this public inbox

	https://80x24.org/mirrors/git.git

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).