linux-kernel - Re: [PATCH v4] livepatch: introduce shadow variable API

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Date:   Mon, 21 Aug 2017 13:24:32 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH v4] livepatch: introduce shadow variable API

On Fri 2017-08-18 16:25:42, Joe Lawrence wrote:
> On 08/17/2017 10:05 AM, Petr Mladek wrote:
> > On Mon 2017-08-14 16:02:43, Joe Lawrence wrote:
> >> [ ... snip ... ]
> >> +	/* Allocate a new shadow variable for use inside the lock below */
> >> +	new_shadow = kzalloc(size + sizeof(*new_shadow), gfp_flags);
> > 
> > We should print an error message when the memory cannot be allocated.
> > Otherwise we will return NULL without explanation. It will be
> > especially helpful when a caller forgets to check for NULL.
> 
> Interesting, I hadn't seen this checkpatch complaint before:
> 
> WARNING: Possible unnecessary 'out of memory' message
> #416: FILE: kernel/livepatch/shadow.c:143:
> +       if (!new_shadow) {
> +               pr_err("failed to allocate shadow variable <0x%p, %lu>\n",
> 
> Discussion thread:
> https://lkml.org/lkml/2014/6/10/382

Interesting, I was not aware of this.

> Think the stack trace that the memory subsystem would emit is good
> enough, or would you like to see <obj, id> for debugging purposes?

I agree that the backtrace should be enough to locate the problematic call
quickly. Feel free to omit it.

Now, I just need to update my patterns when looking for problematic
code.

Best Regards,
Petr