LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Daniel Axtens <dja@axtens.net>
To: kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	paulmck@kernel.org, rcu@vger.kernel.org
Subject: instrument_atomic_read()/_write() in noinstr functions?
Date: Wed, 06 Oct 2021 12:05:00 +1100	[thread overview]
Message-ID: <871r4z55fn.fsf@dja-thinkpad.axtens.net> (raw)

Hi,

commit b58e733fd774 ("rcu: Fixup noinstr warnings") adds some
instrument_atomic_read calls to rcu_nmi_enter - a function marked
noinstr. Similar calls are added to some other functions as well.

This is causing me some grief on powerpc64 while trying to enable
KASAN. powerpc64 book3s takes some NMIs in real mode, and in real mode
we can't access where I'm proposing to put the KASAN shadow - we can
only access it with translations on. So I end up taking a fault in the
kasan_check_read path via rcu_nmi_enter.

As far as I can tell `instrumentation_begin()` and
`instrumentation_end()` don't make it safe to call instrumentation, they
just tell the developer that instrumentation is safe. (And they are used
to check the balance of _begin()/_end() blocks.)

Is the underlying assumption that the KASAN shadow will always be safe
to access, even in functions marked noinstr? It seems to undercut what
an architecture can assume about a function marked noinstr...

Kind regards,
Daniel

P.S. On a more generic note instrumentation_begin()/_end() is now
scattered across the kernel and it makes me a bit nervous. It's making a
statement about something that is in part a property of how the arch
implements instrumentation. Are arches expected to implement things in
such a way as to make these blocks accurate? For example in
arch/powerpc/include/asm/interrupt.h::interrupt_nmi_enter_prepare we
currently sometimes call nmi_enter in real mode; should we instead only
call it when we have translations on?

             reply	other threads:[~2021-10-06  1:05 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-06  1:05 Daniel Axtens [this message]
2021-10-06  7:57 ` Peter Zijlstra
2021-10-06 12:08   ` Daniel Axtens

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=871r4z55fn.fsf@dja-thinkpad.axtens.net \
    --to=dja@axtens.net \
    --cc=kasan-dev@googlegroups.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=paulmck@kernel.org \
    --cc=peterz@infradead.org \
    --cc=rcu@vger.kernel.org \
    --subject='Re: instrument_atomic_read()/_write() in noinstr functions?' \
    /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

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).