Live-Patching Archive on lore.kernel.org
 help / color / Atom feed
From: Miroslav Benes <mbenes@suse.cz>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: live-patching@vger.kernel.org, linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH] Revert "kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled"
Date: Tue, 21 Jul 2020 13:17:00 +0200 (CEST)
Message-ID: <alpine.LSU.2.21.2007211316410.31851@pobox.suse.cz> (raw)
In-Reply-To: <696262e997359666afa053fe7d1a9fb2bb373964.1595010490.git.jpoimboe@redhat.com>

On Fri, 17 Jul 2020, Josh Poimboeuf wrote:

> Use of the new -flive-patching flag was introduced with the following
> commit:
> 
>   43bd3a95c98e ("kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled")
> 
> This flag has several drawbacks:
> 
> - It disables some optimizations, so it can have a negative effect on
>   performance.
> 
> - According to the GCC documentation it's not compatible with LTO, which
>   will become a compatibility issue as LTO support gets upstreamed in
>   the kernel.
> 
> - It was intended to be used for source-based patch generation tooling,
>   as opposed to binary-based patch generation tooling (e.g.,
>   kpatch-build).  It probably should have at least been behind a
>   separate config option so as not to negatively affect other livepatch
>   users.
> 
> - Clang doesn't have the flag, so as far as I can tell, this method of
>   generating patches is incompatible with Clang, which like LTO is
>   becoming more mainstream.
> 
> - It breaks GCC's implicit noreturn detection for local functions.  This
>   is the cause of several "unreachable instruction" objtool warnings.
> 
> - The broken noreturn detection is an obvious GCC regression, but we
>   haven't yet gotten GCC developers to acknowledge that, which doesn't
>   inspire confidence in their willingness to keep the feature working as
>   optimizations are added or changed going forward.
> 
> - While there *is* a distro which relies on this flag for their distro
>   livepatch module builds, there's not a publicly documented way to
>   create safe livepatch modules with it.  Its use seems to be based on
>   tribal knowledge.  It serves no benefit to those who don't know how to
>   use it.
> 
>   (In fact, I believe the current livepatch documentation and samples
>   are misleading and dangerous, and should be corrected.  Or at least
>   amended with a disclaimer.  But I don't feel qualified to make such
>   changes.)
> 
> Also, we have an idea for using objtool to detect function changes,
> which could potentially obsolete the need for this flag anyway.
> 
> At this point the flag has no benefits for upstream which would
> counteract the above drawbacks.  Revert it until it becomes more ready.
> 
> This reverts commit 43bd3a95c98e1a86b8b55d97f745c224ecff02b9.
> 
> Fixes: 43bd3a95c98e ("kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled")
> Reported-by: Randy Dunlap <rdunlap@infradead.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>

Acked-by: Miroslav Benes <mbenes@suse.cz>

M

  parent reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-17 18:29 Josh Poimboeuf
2020-07-20  3:35 ` Joe Lawrence
2020-07-20  8:50   ` Kamalesh Babulal
2020-07-20 11:47     ` Joe Lawrence
2020-07-21 11:27   ` Miroslav Benes
2020-07-21 11:17 ` Miroslav Benes [this message]
2020-08-06  9:24   ` 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=alpine.LSU.2.21.2007211316410.31851@pobox.suse.cz \
    --to=mbenes@suse.cz \
    --cc=jpoimboe@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=live-patching@vger.kernel.org \
    --cc=peterz@infradead.org \
    --cc=rdunlap@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

Live-Patching Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/live-patching/0 live-patching/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 live-patching live-patching/ https://lore.kernel.org/live-patching \
		live-patching@vger.kernel.org
	public-inbox-index live-patching

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.live-patching


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git