linux-kernel - Re: [PATCH v4 00/10] klp-convert livepatch build tooling

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Message-ID: <20190614163655.GC15002@fm.suse.cz>
Date:   Fri, 14 Jun 2019 18:36:55 +0200
From:   Libor Pechacek <lpechacek@...e.cz>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Petr Mladek <pmladek@...e.com>, Miroslav Benes <mbenes@...e.cz>,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        linux-kbuild@...r.kernel.org
Subject: Re: [PATCH v4 00/10] klp-convert livepatch build tooling

On Fri 14-06-19 10:20:09, Joe Lawrence wrote:
> On 6/14/19 4:34 AM, Petr Mladek wrote:
[...]
> > Anyway, I am curious about one thing. I saw:
> > 
> > function __load_mod() {
> > 	local mod="$1"; shift
> > 
> > 	local msg="% modprobe $mod $*"
> > 	log "${msg%% }"
> > 	ret=$(modprobe "$mod" "$@" 2>&1)
> > 	if [[ "$ret" != "" ]]; then
> > 		die "$ret"
> > 	fi
> > 
> > 	# Wait for module in sysfs ...
> > 	loop_until '[[ -e "/sys/module/$mod" ]]' ||
> > 		die "failed to load module $mod"
> > }
> > 
> > Is the waiting for sysfs really necessary here?
> > 
> > Note that it is /sys/module and not /sys/kernel/livepatch/.
> 
> I can't remember if that was just paranoid-protective-bash coding or
> actually required.  Libor provided great feedback on the initial patch
> series that introduced the self-tests, perhaps he remembers.

I don't recall analyzing this spot in detail but looking at it now I don't see
anything wrong with it. While the check is likely superfluous, I'm not against
keeping it in place.

> > My understanding is that modprobe waits until the module succesfully
> > loaded. mod_sysfs_setup() is called before the module init callback.
> > Therefore the sysfs interface should be read before modprobe returns.
> > Do I miss something?
> >
> > If it works different way then there might be some races because
> > mod_sysfs_setup() is called before the module is alive.
> 
> All of this is called from a single bash script function, so in a call stack
> fashion, something like this would occur when loading a livepatch module:
> 
>   [ mod_sysfs_setup() ]
>   modprobe waits for:         .init complete, MODULE_STATE_LIVE
>   __load_mod() waits for:     /sys/module/$mod
>   load_lp_nowait() waits for: /sys/kernel/livepatch/$mod
>   load_lp() waits for:        /sys/kernel/livepatch/$mod/transition = 0
>   test-script.sh
> 
> So I would think that by calling modprobe, we ensure that the module code is
> ready to go.  The /sys/module/$mod check might be redundant as you say, but
> because modprobe completed, we should be safe, no?
> 
> The only "nowait" function we have is load_lp_nowait(), which would let us
> march onward before the livepatch transition may have completed.

And even that one is waiting for the live patch module name appear under
/sys/kernel/livepatch/. This is IMHO acceptable level of paranoia.

Libor
-- 
Libor Pechacek
SUSE Labs