Re: [uml-devel] [PATCH] fix writing into /dev/kmem

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

Alle 03:02, giovedì 4 marzo 2004, Jeff Dike ha scritto:
> blaisorblade_spam@yahoo.it said:
> > Well, on *i386* that is 0xc0000001; since klogd is built with i386
> > kernel  headers, it will use those.
>
> This was the stupidity that I was refusing to believe in.  This means that
> if you take a normal klogd and move it to a 2G/2G system, where the kernel
> starts at 0x80000000, it won't work.

Well, that can be stupid (and is), but needing to recompile iptables when 
upgrading kernel from 2.4.21-Mandrake (i.e. 2.4.21-rc#) to 2.4.21 is even 
more stupid, right? However that was real.

> > Anyway, here comes that copy_{to,from}_user are buggy. Since fixing
> > those  would have a good performance hit (to test, but that will
> > probably be  sensitive
>
> In skas mode, I wasn't expecting any legitimate unfixable kernel mode
> access faults, so I wasn't setting up the fault_catcher.  So, what I did
> was add a wrapper around the guts of copy_user which does a setjmp, and
> returns whether it got longjmp-ed back to.

Yes, fault_catcher (i.e. memory exceptions) works for kernel addresses at very 
little cost. Not for userspace addresses, sadly.

> Reading /dev/kmem now produces -EFAULT.

Well, if you only changed copy_user, there should be some working addresses to 
read there, right? Even if klogd would need recompilation to handle it right.

In the case /dev/kmem does not work at all we could live with it, but it is a 
bit unclean.

> Writing to it still causes a hang
> because write_kmem doesn't check the return value of do_write_mem.  With
> that fixed, everything produces -EFAULT.

I'm sending the return check fix to LKML+ Andrew Morton, together with some 
cleanup for the unused params of do_write_mem.
-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729



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