From: Petr Mladek <pmladek@suse.com>
To: Josh Poimboeuf <jpoimboe@redhat.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: Thu, 25 Nov 2021 09:18:09 +0100 [thread overview]
Message-ID: <YZ9Gwclfit3pviA1@alley> (raw)
In-Reply-To: <20211124174213.mspehbgomdqarxea@treble>
On Wed 2021-11-24 09:42:13, Josh Poimboeuf wrote:
> 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.
Grr, sure. It was off-by-one error on my side. /o\
Thanks for explanation.
Best Regards,
Petr
next prev parent reply other threads:[~2021-11-25 8:20 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-05 17:10 [PATCH 00/22] x86: Remove anonymous out-of-line fixups Peter Zijlstra
2021-11-05 17:10 ` [PATCH 01/22] bitfield.h: Fix "type of reg too small for mask" test Peter Zijlstra
2021-11-05 17:10 ` [PATCH 02/22] x86,mmx_32: Remove .fixup usage Peter Zijlstra
2021-11-05 17:10 ` [PATCH 03/22] x86,copy_user_64: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 04/22] x86,copy_mc_64: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 05/22] x86,entry_64: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 06/22] x86,entry_32: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 07/22] x86,extable: Extend extable functionality Peter Zijlstra
2021-11-05 17:10 ` [PATCH 08/22] x86,msr: Remove .fixup usage Peter Zijlstra
2021-11-05 17:10 ` [PATCH 09/22] x86,futex: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 10/22] x86,uaccess: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 11/22] x86,xen: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 12/22] x86,fpu: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 13/22] x86,segment: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 14/22] x86,vmx: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 15/22] x86,checksum_32: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 16/22] x86,sgx: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 17/22] x86,kvm: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 18/22] x86,usercopy_32: Simplify __copy_user_intel_nocache() Peter Zijlstra
2021-11-05 17:10 ` [PATCH 19/22] x86,usercopy: Remove .fixup usage Peter Zijlstra
2021-11-05 17:10 ` [PATCH 20/22] x86,word-at-a-time: " Peter Zijlstra
2021-11-05 18:01 ` Josh Poimboeuf
2021-11-05 18:07 ` Peter Zijlstra
2021-11-08 16:47 ` Josh Poimboeuf
2021-11-08 18:29 ` Peter Zijlstra
2021-11-08 18:53 ` Nick Desaulniers
2021-11-09 8:23 ` Peter Zijlstra
2021-11-09 19:22 ` Nick Desaulniers
2021-11-09 20:59 ` Bill Wendling
2021-11-09 21:21 ` Peter Zijlstra
2021-11-09 21:25 ` Nick Desaulniers
2021-11-09 22:11 ` Peter Zijlstra
2021-11-09 22:15 ` Nick Desaulniers
2021-11-09 21:07 ` Peter Zijlstra
2021-11-10 10:18 ` Peter Zijlstra
2021-11-10 10:46 ` David Laight
2021-11-10 11:09 ` Peter Zijlstra
2021-11-10 12:20 ` David Laight
2021-11-12 1:50 ` 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
2021-11-25 8:18 ` Petr Mladek [this message]
2021-11-10 12:14 ` Segher Boessenkool
2021-11-05 17:10 ` [PATCH 21/22] x86: Remove .fixup section Peter Zijlstra
2021-11-05 17:10 ` [PATCH 22/22] objtool: Remove .fixup handling Peter Zijlstra
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=YZ9Gwclfit3pviA1@alley \
--to=pmladek@suse.com \
--cc=David.Laight@aculab.com \
--cc=dvyukov@google.com \
--cc=jpoimboe@redhat.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=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).