linux-kernel - Re: [PATCH 2/2 V2] livepatch: handle kzalloc failure properly

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Message-ID: <2d712c92-e661-a32c-06cf-4de93ad93f77@redhat.com>
Date:   Thu, 13 Dec 2018 11:39:25 -0500
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Nicholas Mc Guire <der.herr@...r.at>
Cc:     Nicholas Mc Guire <hofrat@...dl.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...nel.org>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2 V2] livepatch: handle kzalloc failure properly

On 12/13/2018 10:39 AM, Nicholas Mc Guire wrote:
> On Thu, Dec 13, 2018 at 09:14:18AM -0500, Joe Lawrence wrote:
>> On 12/13/2018 09:05 AM, Nicholas Mc Guire wrote:
>>> kzalloc() return should be checked. On dummy_alloc() failing
>>> in kzalloc() NULL should be returned.
>>>
>>> Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
>>> ---
>>>
>>> Problem was located with an experimental coccinelle script
>>>
>>> V2: returning NULL is ok but not without cleanup - thanks to
>>>     Petr Mladek <pmladek@...e.com> for catching this.
>>>
>>> Patch was compile tested with: x86_64_defconfig + FTRACE=y
>>> FUNCTION_TRACER=y, EXPERT=y, LATENCYTOP=y, SAMPLES=y, SAMPLE_LIVEPATCH=y
>>> (with a number of unrelated sparse warnings on symbols not being static)
>>>
>>> Patch is against 4.20-rc6 (localversion-next is next-20181213)
>>>
>>>  samples/livepatch/livepatch-shadow-mod.c | 4 ++++
>>>  1 file changed, 4 insertions(+)
>>>
>>> diff --git a/samples/livepatch/livepatch-shadow-mod.c b/samples/livepatch/livepatch-shadow-mod.c
>>> index 4c54b25..4aa8a88 100644
>>> --- a/samples/livepatch/livepatch-shadow-mod.c
>>> +++ b/samples/livepatch/livepatch-shadow-mod.c
>>> @@ -118,6 +118,10 @@ noinline struct dummy *dummy_alloc(void)
>>>  
>>>  	/* Oops, forgot to save leak! */
>>>  	leak = kzalloc(sizeof(int), GFP_KERNEL);
>>> +	if (!leak) {
>>> +		kfree(d);
>>> +		return NULL;
>>> +	}
>>>  
>>>  	pr_info("%s: dummy @ %p, expires @ %lx\n",
>>>  		__func__, d, d->jiffies_expire);
>>>
>>
>> Hi Nicholas,
>>
>> Thanks for finding and fixing these up... can we either squash these two
>> patches into a single commit or give them unique subject lines?  Code
>> looks good (including Petr's suggested fix) otherwise.
>>
> yup - makes sense to pop it into a single patch - I assumed that this
> would not be acceptable - so I actually split it up :)
> I´ll send a V3 then.

I don't know if there is a hard rule, but I always thought that unique
subject lines were desired to avoid grep/search confusion.

As far as one or two commits, I'd prefer a single commit since these are
so small.  Personal preference, you could just say that you're fixing
samples/livepatch as a whole.

> 
> BTW: wanted to fix up the sparse warnings but I think thats not going
> to be that simple as the functions/structs sparse complains about
> are actually being shared:

Ok, these are welcome too, separate commit...

>   CHECK   samples/livepatch/livepatch-shadow-fix1.c
> samples/livepatch/livepatch-shadow-fix1.c:74:14: warning: symbol 'livepatch_fix1_dummy
> alloc' was not declared. Should it be static?
> samples/livepatch/livepatch-shadow-fix1.c:116:6: warning: symbol 'livepatch_fix1_dummy
> free' was not declared. Should it be static?
> 
>   CHECK   samples/livepatch/livepatch-shadow-mod.c
> samples/livepatch/livepatch-shadow-mod.c:99:1: warning: symbol 'dummy_list' was not declared. Should it be static?
> samples/livepatch/livepatch-shadow-mod.c:100:1: warning: symbol 'dummy_list_mutex' was not declared. Should it be static?
> samples/livepatch/livepatch-shadow-mod.c:107:23: warning: symbol 'dummy_alloc' was not declared. Should it be static?
> samples/livepatch/livepatch-shadow-mod.c:132:15: warning: symbol 'dummy_free' was not declared. Should it be static?
> samples/livepatch/livepatch-shadow-mod.c:140:15: warning: symbol 'dummy_check' was not declared. Should it be static?
> 
> so to clean that appropriate declarations should probably
> go into a .h file. Technically its maybe not important as this
> is not production code - it would though be nice if sample
> code is sparse/smatch/cocci clean.
> 
> would it be acceptable to clean this up with an additional
> livepatch-shadow-mod.h ?

I'm not a C language expert, but as I understand it: static functions
are only a namespacing game for the compiler.  So I think it is safe to
pass around and call function pointers to static functions between
compilation units.  At least I see this throughout the kernel, so that
is my assumption :)

-- Joe