From: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
To: Kees Cook <keescook@chromium.org>
Cc: Christine Caulfield <ccaulfie@redhat.com>,
David Teigland <teigland@redhat.com>,
cluster-devel@redhat.com, linux-kernel@vger.kernel.org,
linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2][next] dlm: Replace one-element array with flexible-array member
Date: Sun, 9 Oct 2022 15:05:17 +1300 [thread overview]
Message-ID: <Y0IsXXYnS4DnWkMW@mail.google.com> (raw)
In-Reply-To: <378C6BDE-0A68-4938-86CD-495BD5F35BE6@chromium.org>
On Sat, Oct 08, 2022 at 05:18:35PM -0700, Kees Cook wrote:
> >$ diff <(objdump -M intel -j .text -D dlm.old) <(objdump -M intel -j
> >.text -D dlm.new)
>
> I'd suggest different options here, this is harder to map back to the source line.
> See https://outflux.net/blog/archives/2022/06/24/finding-binary-differences/
> for lots of details. :)
>
Just read the blog entry, it's really interesting. I will be using it
from now onwards :)
> >
> >13778c13778
> >< c693: 49 8d bc 24 c0 08 00 lea rdi,[r12+0x8c0]
> >---
> >> c693: 49 8d bc 24 c1 08 00 lea rdi,[r12+0x8c1]
>
> This implies something unexpected changed.
>
I will add more details about this line at the other point you made
below to avoid repeating myself. But to cut a long story, short.. this
[reg + displacement + 1] difference is caused because I deliberately add
the NUL-terminator space to the kzalloc() call.
> This has trailing padding, so the struct size didn't actually change.
>
> >- ls = kzalloc(sizeof(struct dlm_ls) + namelen, GFP_NOFS);
> >+ ls = kzalloc(sizeof(struct dlm_ls) + namelen + 1, GFP_NOFS);
>
> This is allocating 1 more byte than before, since the struct size didn't change. But this has always allocated too much space, due to the struct padding. For a "no binary changes" patch, the above "+ 1" needs to be left off.
That's true. I agree that leaving "+ 1" would work and produce a
no-binary-changes patch due to the existing padding that the structure
has. OTOH, I thought that relying on that space could bite us in the
future if anyone tweaks the struct again...so my reaction was to ensure
that the NUL-terminator space was always guaranteed to be there.
Hence, the change on c693 (objdump above).
What do you think? Should we keep or leave the above
"+ 1" after the rationale above?
>
> I would expect the correct allocation size to be:
> offsetof(typeof(*ls), ls_name) + namelen
Fair point, I will make this change.
>
> Question, though: is ls_name _expected_ to be %NUL terminated
Yes, it is. I tracked down ls_name's utilisations and it is passed down to
a bunch of routines that expects it to be NUL-terminated such as
snprintf and vsnprintf.
>, and was the prior 3 bytes of extra allocation accidentally required?
>
I am assuming that you are refering to ls_namelen in the struct dlm_ls
(please correct me if this isn't what you meant).
ls_namelen member is only used within the routine which kzalloc
the space for the struct (fs/dlm/lockspace.c:new_lockspace).
There are no external references to this member outside of that method in the
kernel. One could say that ls_namelen can be removed without side effects but
I wouldn't suggest doing it in this patch, that's why I didn't touch it :)
Paulo A.
next prev parent reply other threads:[~2022-10-09 2:05 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-08 23:17 [PATCH v2][next] dlm: Replace one-element array with flexible-array member Paulo Miguel Almeida
2022-10-09 0:18 ` Kees Cook
2022-10-09 2:05 ` Paulo Miguel Almeida [this message]
2022-10-09 4:03 ` Kees Cook
2022-10-10 21:00 ` David Teigland
2022-10-10 22:35 ` Kees Cook
2022-10-11 15:20 ` David Teigland
2022-10-11 18:44 ` Paulo Miguel Almeida
2022-10-11 20:04 ` [PATCH v4] [next] dlm: replace one-element array with fixed size array Paulo Miguel Almeida
2022-10-11 20:06 ` Kees Cook
2022-10-11 20:11 ` Paulo Miguel Almeida
2022-10-11 20:23 ` [PATCH v5] " Paulo Miguel Almeida
2022-10-11 20:26 ` Gustavo A. R. Silva
2022-10-11 22:18 ` Kees Cook
2022-11-04 5:00 ` Paulo Miguel Almeida
2022-11-04 17:50 ` [Cluster-devel] " Alexander Aring
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