From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Petr Mladek <pmladek@suse.com>
Cc: David Laight <David.Laight@aculab.com>,
'Peter Zijlstra' <peterz@infradead.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Bill Wendling <morbo@google.com>,
"x86@kernel.org" <x86@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"mark.rutland@arm.com" <mark.rutland@arm.com>,
"dvyukov@google.com" <dvyukov@google.com>,
"seanjc@google.com" <seanjc@google.com>,
"pbonzini@redhat.com" <pbonzini@redhat.com>,
"mbenes@suse.cz" <mbenes@suse.cz>,
"llvm@lists.linux.dev" <llvm@lists.linux.dev>,
"linux-toolchains@vger.kernel.org"
<linux-toolchains@vger.kernel.org>,
live-patching@vger.kernel.org
Subject: Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage
Date: Wed, 24 Nov 2021 09:42:13 -0800 [thread overview]
Message-ID: <20211124174213.mspehbgomdqarxea@treble> (raw)
In-Reply-To: <YZvXhAYjHrnc3/rv@alley>
On Mon, Nov 22, 2021 at 06:46:44PM +0100, Petr Mladek wrote:
> On Thu 2021-11-11 17:50:03, Josh Poimboeuf wrote:
> > On Wed, Nov 10, 2021 at 12:20:47PM +0000, David Laight wrote:
> > > > > Wouldn't moving part of a function to .text.cold (or .text.unlikely)
> > > > > generate the same problems with the stack backtrace code as the
> > > > > .text.fixup section you are removing had??
> > > >
> > > > GCC can already split a function into func and func.cold today (or
> > > > worse: func, func.isra.N, func.cold, func.isra.N.cold etc..).
> > > >
> > > > I'm assuming reliable unwind and livepatch know how to deal with this.
> > >
> > > They'll have 'proper' function labels at the top - so backtrace
> > > stands a chance.
> > > Indeed you (probably) want it to output "func.irsa.n.cold" rather
> > > than just "func" to help show which copy it is in. >
> > > I guess that livepatch will need separate patches for each
> > > version of the function - which might be 'interesting' if
> > > all the copies actually need patching at the same time.
> > > You'd certainly want a warning if there seemed to be multiple
> > > copies of the function.
> >
> > Hm, I think there is actually a livepatch problem here.
> >
> > If the .cold (aka "child") function actually had a fentry hook then we'd
> > be fine. Then we could just patch both "parent" and "child" functions
> > at the same time. We already have the ability to patch multiple
> > functions having dependent interface changes.
> >
> > But there's no fentry hook in the child, so we can only patch the
> > parent.
> >
> > If the child schedules out, and then the parent gets patched, things can
> > go off-script if the child later jumps back to the unpatched version of
> > the parent, and then for example the old parent tries to call another
> > patched function with a since-changed ABI.
>
> This thread seems to be motivation for the patchset
> https://lore.kernel.org/all/20211119090327.12811-1-mbenes@suse.cz/
> I am trying to understand the problem here, first. And I am
> a bit lost.
>
> How exactly is child called in the above scenario, please?
> How could parent get livepatched when child is sleeping?
>
> I imagine it the following way:
>
> parent_func()
> fentry
>
> /* some parent code */
> jmp child
> /* child code */
> jmp back_to_parent
> /* more parent code */
> ret
Right.
> In the above example, parent_func() would be on stack and could not
> get livepatched even when the process is sleeping in the child code.
>
> The livepatching is done via ftrace. Only code with fentry could be
> livepatched. And code called via fentry must be visible on stack.
How would parent_func() be on the stack? If it jumps to the child then
it leaves no trace on the stack.
--
Josh
next prev parent reply other threads:[~2021-11-24 17:42 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20211105171821.654356149@infradead.org>
[not found] ` <20211108164711.mr2cqdcvedin2lvx@treble>
[not found] ` <YYlshkTmf5zdvf1Q@hirez.programming.kicks-ass.net>
[not found] ` <CAKwvOdkFZ4PSN0GGmKMmoCrcp7_VVNjau_b0sNRm3MuqVi8yow@mail.gmail.com>
[not found] ` <YYov8SVHk/ZpFsUn@hirez.programming.kicks-ass.net>
[not found] ` <CAKwvOdn8yrRopXyfd299=SwZS9TAPfPj4apYgdCnzPb20knhbg@mail.gmail.com>
[not found] ` <20211109210736.GV174703@worktop.programming.kicks-ass.net>
[not found] ` <f6dbe42651e84278b44e44ed7d0ed74f@AcuMS.aculab.com>
[not found] ` <YYuogZ+2Dnjyj1ge@hirez.programming.kicks-ass.net>
[not found] ` <2734a37ebed2432291345aaa8d9fd47e@AcuMS.aculab.com>
2021-11-12 1:50 ` [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage Josh Poimboeuf
2021-11-12 9:33 ` Peter Zijlstra
2021-11-13 5:35 ` Josh Poimboeuf
2021-11-15 12:36 ` Miroslav Benes
2021-11-15 13:01 ` Joe Lawrence
2021-11-15 23:40 ` Josh Poimboeuf
2021-11-16 7:25 ` Miroslav Benes
2021-11-15 12:59 ` Miroslav Benes
2021-11-16 21:27 ` Josh Poimboeuf
2021-11-18 7:15 ` Miroslav Benes
2021-11-22 17:46 ` Petr Mladek
2021-11-24 17:42 ` Josh Poimboeuf [this message]
2021-11-25 8:18 ` Petr Mladek
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=20211124174213.mspehbgomdqarxea@treble \
--to=jpoimboe@redhat.com \
--cc=David.Laight@aculab.com \
--cc=dvyukov@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-toolchains@vger.kernel.org \
--cc=live-patching@vger.kernel.org \
--cc=llvm@lists.linux.dev \
--cc=mark.rutland@arm.com \
--cc=mbenes@suse.cz \
--cc=morbo@google.com \
--cc=ndesaulniers@google.com \
--cc=pbonzini@redhat.com \
--cc=peterz@infradead.org \
--cc=pmladek@suse.com \
--cc=seanjc@google.com \
--cc=x86@kernel.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).