RE: how do you call userspace syscalls (e.g. sys_rename) from inside kernel

[Aboo Valappil]

Hi,

In the past I looked in to open, read and write a file from a kernel
module.  But problem I faced using the kernel function was that it
checks for the permissions of the file and path against the "current"
process. For eg: open_namei() function ... My requirement was to open
the file regardless of the permissions on the file and also not by
modifying task_struct of the current process to change the permissions
first ! I also wanted not associate the file with the current/any
processes. 

Any ideas on this ?

Then I thought of using a work around and avoid opening files in kernel
mode.

Thanks

Aboo


-----Original Message-----
From: linux-kernel-owner@xxxxxxxxxxxxxxx
[mailto:linux-kernel-owner@xxxxxxxxxxxxxxx] On Behalf Of Luke Kenneth
Casson Leighton
Sent: Saturday, October 09, 2004 1:35 AM
To: Fabiano Ramos
Cc: linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: how do you call userspace syscalls (e.g. sys_rename) from
inside kernel

On Fri, Oct 08, 2004 at 10:07:04AM -0300, Fabiano Ramos wrote:
> On Fri, 2004-10-08 at 14:04 +0100, Luke Kenneth Casson Leighton wrote:
> > could someone kindly advise me on the location of some example code
in
> > the kernel which calls one of the userspace system calls from inside
the
> > kernel?
> > 
> > alternatively if this has never been considered before, please could
> > someone advise me as to how it might be achieved?
> > 
> 
> you cannot do that. For every sys_xx there is a do_xx, that can
> be called from inside the kernel.
 
 so, there's a do_rename (yes i found that and ISTRC that when
 i used it i can't exactly remember what the problem was:
 either i got an error code -14 or i got "warning symbol
 do_rename not found" when my module was linked together,
 even though it says EXPORT_SYMBOL(do_rename) in fs/namei.c,
 so i was forced to cut/paste sys_rename)

 and there's a do_open no there isn't, there's filp_open.

 and a do_pread64 no there isn't i had to cut/paste sys_pread64
 which was okay because it's pretty basic, just call vfs_read.

 and a do_mkdir no there isn't so i had to cut/paste that.


 basically what i am doing is writing a file system "proxy"
 module which re-calls back into the filesystem with a prefix
 onto the front of the pathname.

> > [p.s. i found asm/unistd.h, i found the macros syscall012345
> > etc., i believe i don't quite understand what these are for, and
> > may be on the wrong track.]
> 
> These are are available for you to make syscalls from user mode
> without library support (usually that brand new syscall you added).
> They are basically wrappers that expand into C code. _syscallx, 
> where x is the number of arguments the syscall needs.
 
 so, it's for use the other way round.  okay, thanks for keeping me off
 a broken line of enquiry.

 [oh, and i'll be abandoning this line of enquiry _entirely_ if i find
 that supermount-ng can do the same job - namely manage to keep
 userspace programs happy when users rip out media]
 
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