linux-kernel - Re: chmod fails though it pretends to succeed

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Message-ID: <4A1B2DD6.3050405@gmail.com>
Date:	Mon, 25 May 2009 17:46:30 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	Christoph Pleger <Christoph.Pleger@...tu-dortmund.de>
CC:	linux-kernel@...r.kernel.org
Subject: Re: chmod fails though it pretends to succeed

Christoph Pleger wrote:
> Hello,
> 
> I have a strange problem here with a C program that calls chmod.
> 
> The C program is a PAM module called pam-devperm. The purpose of this
> module is to set appropriate device ownerships and permissions when a
> user logs in. But the module does not work as expected on my Ubuntu
> 9.04 system. When a user logs in the first time after system boot, only
> the device ownerships are changed, the permissions are not. When the
> user logs out and in again, permissions are changed, too.
> 
> I added some lines of code to the the source of of pam-devperm so that
> it produces more debug output when changing the properties of /dev/dsp.
> The debug output showed me that chmod has been called to set the
> permissions of /dev/dsp to 0600 and that chmod returned 0, which means
> success. In spite of that, device permissions remain at 660.
> 
> Now I have two questions:
> 
> 1. What can be the reason for chmod to return a value of 0 though
> device permissions have not been changed?
> 
> 2. What can be the reason why this chmod fails on the first login of a
> user, but succeeds on further logins?

Are you sure that some other process is not changing the device 
permissions also?

> 
> Regards
>   Christoph  

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