ot memory allocation question

[Don Raikes]

Chris,

Could I do something like:

Mm_segment_t cur_fs = get_fs();
Set_fs(get_ds()); // which puts me in userspace
Char * newbuf[required_size];
Set_fs(cur_fs);
If (copy_to_user(newbuf,srcbuf,required_size);
Ret = sys_write_orig(fd, newbuf,required_size);



-----Original Message-----
From: Chris Brannon [mailto:chris at the-brannons.com] 
Sent: Friday, July 19, 2013 5:38 AM
To: Speakup is a screen review system for Linux.
Subject: Re: ot memory allocation question

Don Raikes <don.raikes at 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
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