linux-kernel - Re: [PATCH v1 1/5] devres: Introduce devm_kmemdup

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Message-ID: <Z0Sl1gmYc3y11riD@google.com>
Date: Mon, 25 Nov 2024 08:29:10 -0800
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Raag Jadav <raag.jadav@...el.com>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	gregkh@...uxfoundation.org, linus.walleij@...aro.org,
	mika.westerberg@...ux.intel.com, broonie@...nel.org,
	pierre-louis.bossart@...ux.dev, linux-gpio@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-input@...r.kernel.org,
	linux-sound@...r.kernel.org
Subject: Re: [PATCH v1 1/5] devres: Introduce devm_kmemdup_array()

On Mon, Nov 25, 2024 at 05:08:13PM +0200, Raag Jadav wrote:
> On Mon, Nov 25, 2024 at 09:49:22AM +0200, Andy Shevchenko wrote:
> > On Sun, Nov 24, 2024 at 07:03:36AM +0000, Dmitry Torokhov wrote:
> > > On Sun, Nov 24, 2024 at 01:35:23AM +0530, Raag Jadav wrote:
> > > > Introduce '_array' variant of devm_kmemdup() for the users which lack
> > > > multiplication overflow check.
> > > 
> > > I am not sure that this new helper is needed. Unlike allocators for
> > > brand new objects, such as kmalloc_array(), devm_kmemdup() makes a copy
> > > of already existing object, which is supposed to be a valid object and
> > > therefore will have a reasonable size. So there should be no chance for
> > > hitting this overflow unless the caller is completely confused and calls
> > > devm_kmemdup() with random arguments (in which case all bets are off).
> > 
> > Don't we want to have a code more robust even if all what you say applies?
> > Also this makes the call consistent with zillions of others from the alloc
> > family of calls in the Linux kernel.

Having a clean API is fine, just do not bill it as something that is
"safer". As I mentioned, unlike other allocators this one is supposed to
operate with a valid source object and size passed to devm_kmemdup()
should not exceed the size of the source object. There is no chance of
overflowing.

> 
> Agree. Although shooting in the foot is never the expectation, it is
> atleast better than having to debug such unexpected cases.


Then maybe have a BUG() there instead of returning NULL? I know BUG()s
are frowned upon, but I think in this case overflow is really an
indicator of a hard error by the caller which is passing garbage
arguments to this function.

Hm, I see we have kmemdup_array() already. Ok. How about making your
devm_kmemdup_array() be similar to kmemdup_array()?

static inline void *devm_kmemdup_array(struct device *dev, const void *src,
				       size_t n, size_t size, gfp_t flags)
{
	return devm_kmemdup(dev, src, size_mul(size, n), flags);
}

This will trigger a warning on a too large order of allocation in
mm/page_alloc.c::__alloc_pages_noprof().

Thanks.

-- 
Dmitry