linux-kernel - Re: syscall in a module

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Message-ID: <20071220091605.GC7921@flint.arm.linux.org.uk>
Date:	Thu, 20 Dec 2007 09:16:05 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Pierre Savary <pierre.savary@...link.fr>
Cc:	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: syscall in a module

On Thu, Dec 20, 2007 at 10:03:53AM +0100, Pierre Savary wrote:
> I need some help about syscalls and modules. In fact I want to adapt the
> g_file_storage module to my own application.
> I need to use "sys_umount" and "sys_mount" but when I compile I obtain this
> message :
> [...]
>   Kernel: arch/arm/boot/zImage is ready
>   Building modules, stage 2.
>   MODPOST 4 modules
> ERROR: "sys_umount" [drivers/usb/gadget/g_file_storage.ko] undefined!
> make[1]: *** [__modpost] Error 1
> make: *** [modules] Error 2
> [...]
> 
> So, how can I use "sys_umount" and "sys_mount" in this module?

You don't.  Syscalls are NOT for modules because they modify the
filesystem, which depends on the user context.  For example, processes
running chroot have a different view of the filesystem from those which
aren't - and if you happen to be executing with the context of a
chrooted user process when you mount, and a non-chrooted process when
you umount.

I suggest you describe what you're trying to do in greater detail, and/or
ask on the main kernel list - but note that if you ask the same question
without explaining why you need it _and_ asking for alternative ideas,
you'll get the same kind of response there.
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