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From: Chris Brannon <chris@the-brannons.com>
To: "Speakup is a screen review system for Linux." <speakup@linux-speakup.org>
Subject: Re: ot memory allocation question
References: <41356f9b-e7c3-4522-b960-142abea1cedd@default>
Date: Fri, 19 Jul 2013 05:37:52 -0700
In-Reply-To: <41356f9b-e7c3-4522-b960-142abea1cedd@default> (Don Raikes's
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Don Raikes <don.raikes@oracle.com> writes:
> My solution:
>
> Allocate a new larger buffer inside of the userspace and copy_to_user
> into the new buffer and then when I pass control to the "real"
> sys_write function point it to the new buffer.
> But the problem is how do I allocate this new buffer?

There's no easy way to do this.  You can't just pass your kernel
buffer to the system call you are intercepting, since the intercepted
call expects a user-space buffer.
Have a look at this link for some inspiration:
http://web.cs.wpi.edu/~cs4513/b05/proj1note2.txt

Also be careful about the return value of the real sys_write system
call.  If you're passing it a buffer larger than the n bytes passed in
from userspace, its return value can be greater than n.  Don't just use
it unmodified as the return value for your function.

-- Chris