Date: Thu, 8 Apr 2010 08:14:00 -0700 (PDT) From: Nate Eldredge <nate@thatsmathematics.com> To: Patrick Mahan <mahan@mahan.org> Cc: freebsd-hackers@freebsd.org Subject: Re: Modifying ELF files Message-ID: <Pine.GSO.4.64.1004080806540.5432@zeno.ucsd.edu> In-Reply-To: <4BBDE58A.9050502@mahan.org> References: <4BBDE58A.9050502@mahan.org>
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On Thu, 8 Apr 2010, Patrick Mahan wrote:
>
> In my job, we are producing applications and KLM's for our product
> that require them to be signed so that our installer will recognize
> and validate our images.
>
> The signature is stored in each app as
>
> unsigned char signature[40] __attribute__((section(".compsign")));
>
> What I need to do is open the file for writing, locate the ".compsign"
> section and stuff in the signature, write it out and close the file.
> (simple ELF manipulation)
>
> An 'ls -l' shows the following:
>
> % ls compklm.ko
> -rw-r--r-- 1 pmahan pmahan 125296 Apr 6 22:50
> /home/pmahan/temp/compklm.ko
>
> When I try to run my program
> ./signfile --signature=A203239897C8EB360D1EB2C84E8E77B16E5B7C9A compklm.ko
> open: Text file busy
>
> Googling and looking at the kernel sources, it seems that it detects
> this file contains 'shared text', that is, it is an executable file
> and does not allow me to open it for writing.
My understanding was that ETXTBSY occurs when you attempt to open for
writing a file which is actually being executed, i.e. is mapped into some
process. I'm not aware that open(2) actually looks at the file itself to
see if it is an executable; that would be very surprising to me.
What does "fstat -m compklm.ko" say?
What happens if you "cp compklm.ko foo.ko" and try to sign foo.ko? You
should then be able to do "mv foo.ko compklm.ko"; if compklm.ko is
in fact mapped into some process, it will continue to use the original
version, which will be kept around (invisibly) until all mappings go away.
This is what compilers, install(8), etc, normally do.
Does your signfile program do anything with the target file before
open(..., O_RDWR)?
--
Nate Eldredge
nate@thatsmathematics.com
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