linux-kernel - Re: [PATCH v2] livepatch: core: Return EOPNOTSUPP instead of ENOSYS

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Message-ID: <20190208093445.zvxkgaummsait5z6@pathway.suse.cz>
Date:   Fri, 8 Feb 2019 10:34:45 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Alice Ferrazzi <alicef@...cef.me>, jeyu@...nel.org,
        jikos@...nel.org, mbenes@...e.cz, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Alice Ferrazzi <alice.ferrazzi@...aclelinux.com>
Subject: Re: [PATCH v2] livepatch: core: Return EOPNOTSUPP instead of ENOSYS

On Fri 2019-02-08 11:50:05, Kamalesh Babulal wrote:
> On Wed, Feb 06, 2019 at 11:28:32AM +0100, Petr Mladek wrote:
> > On Tue 2019-02-05 09:59:33, Josh Poimboeuf wrote:
> > > On Tue, Feb 05, 2019 at 03:33:28AM +0900, Alice Ferrazzi wrote:
> > > > From: Alice Ferrazzi <alice.ferrazzi@...aclelinux.com>
> > > > 
> > > > As a result of an unsupported operation is better to use EOPNOTSUPP
> > > > as error code.
> > > > ENOSYS is only used for 'invalid syscall nr' and nothing else.
> > > > 
> > > > Signed-off-by: Alice Ferrazzi <alice.ferrazzi@...aclelinux.com>
> > > 
> > > Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > 
> > I have applied the patch into for-5.1/atomic-replace branch.
> 
> Sorry to jump into the discussion so late. Thinking a little more about
> the check itself, previously with immediate flag an architecture can do
> livepatching with limitations and without the reliable stack trace
> implemented yet.
> 
> After removal of the immediate flag by
> commit d0807da78e11 ("livepatch: Remove immediate feature"), every
> architecture enabling livepatching is required to have implemented
> reliable stack trace.  Is it a better idea to make
> HAVE_RELIABLE_STACKTRACE a config dependency, which will disable
> livepatching support for architectures without reliable stack trace
> function during kernel build?
> 
> The idea is to remove klp_have_reliable_stack() by moving
> CONFIG_HAVE_RELIABLE_STACKTRACE as a config dependency to Kconfig
> file

Looks like a nice cleanup.

> and adding the other CONFIG_STACKTRACE as a config dependency is not
> required, as it's selected via CONFIG_DYNAMIC_FTRACE_WITH_REGS
> dependency chain. With the patch on architecture without
> HAVE_RELIABLE_STACKTRACE, the user should see:

Hmm, I see the following in kernel/trace/Kconfig:

config TRACING
[...]
	select STACKTRACE if STACKTRACE_SUPPORT

It seems that the depency is not guaranted. Or do I miss anything?

Anyway, it is pretty indirect. I would prefer to add dependency
on STACKTRACE explicitly into config LIVEPATCH.

Best Regards,
Petr