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From: Junio C Hamano <gitster@pobox.com>
To: Taylor Blau <me@ttaylorr.com>
Cc: "Jessica Clarke" <jrtc27@jrtc27.com>,
	"René Scharfe" <l.s.r@web.de>,
	git@vger.kernel.org
Subject: Re: [PATCH] Properly align memory allocations and temporary buffers
Date: Thu, 06 Jan 2022 14:27:25 -0800	[thread overview]
Message-ID: <xmqqh7agbiuq.fsf@gitster.g> (raw)
In-Reply-To: <YddjJ+uIQM34Gedo@nand.local> (Taylor Blau's message of "Thu, 6 Jan 2022 16:46:15 -0500")

Taylor Blau <me@ttaylorr.com> writes:

> (+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.

Alignment aside, if uintmax_t is 64-bit but your pointer needs
128-bit to store, saving a pointer value in uintmax_t variable would
not work correctly, I presume, as casting the 64-bit integral type
back into pointer would not be sufficient to recover the lost
information that used to be in the second word.

So, does the architecture have 128-bit uintptr_t and that is a safe
type both from the point of view of alignment and from the point of
view of not losing information?  

If that type is larger than uintmax_t, something smells wrong,
though.  max is not max anymore at that point.

IIRC, uintptr_t is optional in C99, so a simpler solution to use the
larger type between uintptr_t and uintmax_t as a replacement for how
we use uintmax_t would not quite work out of the box X-<.

  parent reply	other threads:[~2022-01-06 22:27 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
2022-01-06 22:27   ` Junio C Hamano [this message]
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

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