From: Arvind Sankar <nivedita@alum.mit.edu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>,
Matthew Wilcox <willy@infradead.org>,
"Gustavo A. R. Silva" <gustavoars@kernel.org>,
Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
Christoph Lameter <cl@linux.com>, Linux-MM <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Kees Cook <keescook@chromium.org>
Subject: Re: [GIT PULL] percpu fix for v5.9-rc6
Date: Fri, 18 Sep 2020 22:53:36 -0400 [thread overview]
Message-ID: <20200919025336.GA3008405@rani.riverdale.lan> (raw)
In-Reply-To: <CAHk-=wg+3Poqh_HQ93nPMWOXLQHZhvYNuwScoQ-WaYWuriLYAg@mail.gmail.com>
On Fri, Sep 18, 2020 at 06:28:30PM -0700, Linus Torvalds wrote:
> On Fri, Sep 18, 2020 at 3:40 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > Ouch, offsetof() and sizeof() will give different results in the
> > presence of alignment padding.
>
> Indeed. But from an allocation standpoint, the offsetof()+size is I
> think the correct size. The padding at the end makes very little sense
> for something like "struct_size()".
I just meant that my suggestion doesn't actually work to assert that you
passed in the flexible array member to struct_size(), even outside of
any future warnings on sizeof().
And that it's another source of subtle bugs, although you'll err towards
over-allocating memory rather than under-allocating by using sizeof().
Is it ever necessary to allocate _at least_ sizeof() even if
offsetof()+size is smaller?
>
> Padding at the end is required for sizeof() for a very simple reason:
> arrays. The "sizeof()" needs to be aligned to the alignment of the
> entry, because if it isn't, then the standard C array traversal
> doesn't work.
>
> But you cannot sanely have arrays of these structures of variable size
> entries either - even if standard C cheerfully allows you to declare
> them (again: it will not behave like a variable sized array, it will
> behave like a zero-sized one).
I think you can't do this in standard C. It's a GCC extension.
A structure containing a flexible array member, or a union
containing such a structure (possibly recursively), may not be a
member of a structure or an element of an array. (However, these
uses are permitted by GCC as extensions.)
next prev parent reply other threads:[~2020-09-19 2:53 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-17 20:45 [GIT PULL] percpu fix for v5.9-rc6 Dennis Zhou
2020-09-18 1:05 ` Linus Torvalds
2020-09-18 16:23 ` Gustavo A. R. Silva
2020-09-18 17:23 ` Linus Torvalds
2020-09-18 19:34 ` Gustavo A. R. Silva
2020-09-18 19:37 ` Linus Torvalds
2020-09-18 20:02 ` Matthew Wilcox
2020-09-18 20:14 ` Linus Torvalds
2020-09-18 20:29 ` Arvind Sankar
2020-09-18 20:40 ` Linus Torvalds
2020-09-18 21:00 ` Arvind Sankar
2020-09-18 21:18 ` Linus Torvalds
2020-09-18 22:39 ` Arvind Sankar
2020-09-19 1:28 ` Linus Torvalds
2020-09-19 2:53 ` Arvind Sankar [this message]
2020-09-19 3:02 ` Matthew Wilcox
2020-09-19 3:04 ` Linus Torvalds
2020-09-19 2:45 ` Matthew Wilcox
2020-09-19 3:37 ` Arvind Sankar
2020-09-19 15:15 ` David Laight
2020-09-18 20:03 ` Gustavo A. R. Silva
2020-09-18 1:10 ` pr-tracker-bot
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
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200919025336.GA3008405@rani.riverdale.lan \
--to=nivedita@alum.mit.edu \
--cc=cl@linux.com \
--cc=dennis@kernel.org \
--cc=gustavoars@kernel.org \
--cc=keescook@chromium.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=tj@kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=willy@infradead.org \
/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.
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 NNTP newsgroup(s).