linux-kernel - Re: [PATCH v2] HID: leave dev_rdesc unmodified and use it for comparisons

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Date:	Wed, 19 Sep 2012 20:32:31 +0200
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Kevin Daughtridge <kevin@...u.com>
Cc:	Jiri Slaby <jslaby@...e.cz>, Jiri Kosina <jkosina@...e.cz>,
	linux-input@...r.kernel.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] HID: leave dev_rdesc unmodified and use it for
 comparisons

Hi Kevin,

Thanks for looking to this.

> Hmm. I hadn't noticed that the other drivers are returning a static
> structure. In that case, it seems that report_fixup itself is broken
> from a memory perspective, in that it returns pointers to
> inconsistent storage types depending on the driver.

The driver can either modify the existing buffer, of return a pointer
to a buffer managed by the driver. The former requires a kmemdup
before, the latter a kmemdup after.

> 1. Ugly workaround: make a temporary copy of the dev_rdesc, give it
> to report_fixup, make a copy of the return, store that copy in
> rdesc, free the temporary copy. Though ugly, this would at least
> involve the smallest diff.

Yes, it is correct and ugly, in no particular order.

> 2. Standardize the behavior of the drivers' report_fixup
> implementations. Given that some of them need to change the size of
> the descriptor, modifying the passed structure is not an option.
> Probably all of them should return a newly allocated structure,
> either a modified copy of the input or a copy of their static, that
> can then be stored directly in rdesc. Especially since report_fixup
> is only ever called right before a copy is going to be taken anyway.

Relying on the returned pointer to be properly alloc'd is not a good
idea, in particular since it changes semantics rather drastically.

Since the current function performs two different things, perhaps
there should be two different functions instead.

> (Adding constness to a parameter isn't considered a severe ABI
> break, is it?)

Inside the kernel there is no ABI. Go wild.

Thanks,
Henrik
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