netdev - Re: [PATCH iproute2 1/2] lib/libnetlink: re malloc buff if size is not enough

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Date:   Wed, 13 Sep 2017 17:26:58 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Michal Kubecek <mkubecek@...e.cz>
Cc:     Phil Sutter <phil@....cc>, netdev@...r.kernel.org,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH iproute2 1/2] lib/libnetlink: re malloc buff if size is
 not enough

Hi Michal,

Thanks a lot for all your explains. Phil has helped update the patch to support
return a newly allocated buffer. I will post it soon.

Thanks
Hangbin
On Tue, Sep 12, 2017 at 11:09:26AM +0200, Michal Kubecek wrote:
> > 
> > I checked again and arpd indeed isn't a problem. It doesn't seem to call
> > any of the two functions (directly or indirectly) and while it's linked
> > with "-lpthread", it's not really multithreaded.
> > 
> > But my concern was rather about other potential users of libnetlink
> > (i.e. those which are not part of iproute2). I must admit, though, that
> > I'm not sure if libnetlink code is reentrant as of now. (And people are
> > discouraged from using it in its own manual page.)
> > 
> > That being said, I still like Phil's idea for a different reason. While
> > investigating the issue with "ip link show dev eth ..." which led me to
> > commit 6599162b958e ("iplink: check for message truncation in
> > iplink_get()"), I quickly peeked at some other callers of rtnl_talk()
> > and I'm afraid there may be others which wouldn't handle truncated
> > message correctly. I assume the maxlen argument was always chosen to be
> > sufficient for any expected messages but as the example of iplink_get()
> > shows, messages returned by kernel my grow over time.
> > 
> > That's why I like the idea of __rtnl_talk() returning a pointer to newly
> > allocated buffer (of sufficient size) rather than copying the response
> > into a buffer provided by caller and potentially truncating it.
> 
> I'm sorry, I managed to forget that your patch 2 does already address
> this problem. But the fact that any caller must keep in mind that he
> must not call the same function again until the previous response is no
> longer needed still feels like a trap. It's something you need to keep
> in mind (where "you" in fact means any future contributor) and it's
> easy to forget. That's why I prefer the reentrant functions like
> strerror_r() or localtime_r() even in code which is not intended to be
> multithreaded. Getting an object which is "mine" to do with whatever
> I want until I no longer need it feels like a cleaner interface to me.
> 
> Michal Kubecek
>