linux-kernel - Re: [PATCH V2] clk: zynq: do not allow kmalloc failure

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Message-ID: <20181130082926.GA6517@osadl.at>
Date:   Fri, 30 Nov 2018 09:29:26 +0100
From:   Nicholas Mc Guire <der.herr@...r.at>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Nicholas Mc Guire <hofrat@...dl.org>,
        Michal Simek <michal.simek@...inx.com>,
        Rob Herring <robh@...nel.org>, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] clk: zynq: do not allow kmalloc failure

On Fri, Nov 30, 2018 at 12:09:30AM -0800, Stephen Boyd wrote:
> Quoting Nicholas Mc Guire (2018-11-29 23:54:53)
> > On Thu, Nov 29, 2018 at 03:45:23PM -0800, Stephen Boyd wrote:
> > > Quoting Nicholas Mc Guire (2018-11-21 04:28:30)
> > > > The kmalloc here is small (< 16 bytes) and occurs during initialization
> > > > during system startup here (can not be built as module) thus if this
> > > > kmalloc failed it is an indication of something more serious going on
> > > > and it is fine to hang the system here rather than cause some harder
> > > > to understand error by dereferencing NULL.
> > > > 
> > > > Explicitly checking would not make that much sense here as the only
> > > > possible reaction would be would BUG() here anyway.
> > > > 
> > > > Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
> > > > Fixes: 0ee52b157b8e ("clk: zynq: Add clock controller driver")
> > > > Acked-by: Michal Simek <michal.simek@...inx.com>
> > > > ---
> > > 
> > > Nak. We don't have any __GFP_NOFAIL in drivers/clk and I don't see a
> > > reason why we would want it here either. Just handle the failure, or
> > > don't care if this is so critical to system boot.
> > >
> > It was not motivated by the criticality but by the low probability
> > and cluttering the code for this case did not seem good to me.
> > Effectively handling it here means BUG() - so more or less
> > the same result that hanging it on __GFP_NOFAIL if allocation
> > was not possible would cause.
> > 
> > Not clear what the objection to __GFP_NOFAIL here is - my understanding
> > was that it is intended precisely for cases like this - but
> > I´ll send a V2 handling it with BUG_ON(!clk_name) if that is prefered.
> > 
> 
> Or just WARN() and return. Maybe something else can get far enough to be
> helpful.
> 
> I would also appreciate if this sort of problem could be caught earlier
> in code review and/or with some automated scripting. Debating BUG_ON()
> and allocation failures is not what I look forward to doing so please
> try to make this exact sort of patch never make it to the list in the
> first place.
>
well it was found by a experimental coccinelle script
I´m trying to write up semi-formal specifications for
APIs so that this can be tested automatically and cought early.

If you put in a WARN() here it would still allow for the
NULL pointer to be passt on potentially corupting memory in the following 
snprintf()->vsnprintf() which does not seem to check for !buf 


thx!
hofrat