linux-kernel - Re: [PATCH] livepatch: Enforce reliable stack trace as config dependency

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Date:   Tue, 12 Feb 2019 10:46:08 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Miroslav Benes <mbenes@...e.cz>, Jiri Kosina <jikos@...nel.org>
Subject: Re: [PATCH] livepatch: Enforce reliable stack trace as config
 dependency

On Mon 2019-02-11 08:08:13, Josh Poimboeuf wrote:
> Anyway, I'm not sure about this approach.  This patch makes the s390
> livepatch code no longer compilable, turning it into completely dead
> code.  So if something changes in the s390 code which causes it to stop
> compiling, nobody will notice.

Good point. Well, it is only small win when a code is buildable
but it could not get really used. Also the amount of arch-specific
code is really minimal.

> I think I'd rather go in the opposite direction: allow the patches to be
> loaded.  Then they can be forced, if needed.  That enables both compile
> and runtime testing.  That way we don't make any backward progress,
> until such arches get reliable stacktraces.

Do you mean to convert the error into warning?

For example, the change below. Note that I did not mention
the possibility to force the transition by intention. It is risky
and people should not get used to it.

Heh, I think that this was the main reason why it was the error.
We did not want to get people used to forcing livepatches.


diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index d1af69e9f0e3..8d9bce251516 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -1035,11 +1035,10 @@ int klp_enable_patch(struct klp_patch *patch)
 		return -ENODEV;
 
 	if (!klp_have_reliable_stack()) {
-		pr_err("This architecture doesn't have support for the livepatch consistency model.\n");
-		return -EOPNOTSUPP;
+		pr_warn("This architecture doesn't have support for the livepatch consistency model.\n");
+		pr_warn("Only one livepatch can be installed.\n");
 	}
 
-
 	mutex_lock(&klp_mutex);
 
 	ret = klp_init_patch_early(patch);


Best Regards,
Petr