linux-kernel - Re: [uml-devel] [PATCH 1/1] hostfs: fix UML crash

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Message-Id: <201010182203.42690.richard@nod.at>
Date:	Mon, 18 Oct 2010 22:03:42 +0200
From:	Richard Weinberger <richard@....at>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, toralf.foerster@....de,
	jdike@...toit.com, linux-kernel@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net,
	viro@...iv.linux.org.uk
Subject: Re: [uml-devel] [PATCH 1/1] hostfs: fix UML crash

Am Montag 18 Oktober 2010, 21:43:44 schrieb Geert Uytterhoeven:
> On Mon, Oct 18, 2010 at 21:31, Richard Weinberger <richard@....at> wrote:
> > Am Montag 18 Oktober 2010, 21:22:31 schrieb Geert Uytterhoeven:
> >> On Mon, Oct 18, 2010 at 20:40, Andrew Morton <akpm@...ux-foundation.org>
> > 
> > wrote:
> >> > On Mon, 18 Oct 2010 18:36:54 +0200 Richard Weinberger <richard@....at>
> > 
> > wrote:
> >> >> 365b1818 resized f_spare within struct statfs.
> >> >> hostfs accesses f_spare directly and needs an update.
> >> >> 
> >> >> Signed-off-by: Richard Weinberger <richard@....at>
> >> >> Reported-by: Toralf F__rster <toralf.foerster@....de>
> >> >> Tested-by: Toralf F__rster <toralf.foerster@....de>
> >> >> ---
> >> >>  fs/hostfs/hostfs_user.c |    2 +-
> >> >>  1 files changed, 1 insertions(+), 1 deletions(-)
> >> >> 
> >> >> diff --git a/fs/hostfs/hostfs_user.c b/fs/hostfs/hostfs_user.c
> >> >> index 6777aa0..ce2f168 100644
> >> >> --- a/fs/hostfs/hostfs_user.c
> >> >> +++ b/fs/hostfs/hostfs_user.c
> >> >> @@ -388,6 +388,6 @@ int do_statfs(char *root, long *bsize_out, long
> >> >> long *blocks_out, spare_out[1] = buf.f_spare[1];
> >> >>       spare_out[2] = buf.f_spare[2];
> >> >>       spare_out[3] = buf.f_spare[3];
> >> >> -     spare_out[4] = buf.f_spare[4];
> >> >> +
> >> >>       return 0;
> >> >>  }
> >> > 
> >> > Thanks.
> >> > 
> >> > Is there any reason for hostfs to be playing with the f_spare field at
> >> > all?
> >> 
> >> It just copies it from struct statfs64 on the host to struct kstatfs
> >> on the guest.
> >> Probably a memcpy() is more future-safe, if it's combined with a
> >> BUILD_BUG_ON(sizeof(statfs64.f_spare) != sizeof(kstatfs.f_spare)).
> >> 
> >> Still, currently it doesn't copy the recently added f_flags field.
> >> To protect against future changes like that, an explicit
> >> BUILD_BUG_ON(sizeof(kstatfs.f_spare) != 4*sizeof(long)) may be even
> >> better...

IMHO using kstatfs within ubd_user.c is not very nice because is a internal 
kernel struct.

> > Anyway, why do we need to copy f_spare from the host to the guest?
> > I'm quite sure it can be omitted.
> 
> I guess it wants to preserve fields that are added in the future,
> which may be useful
> if the host is more recent than the guest.

Good point!

Thanks,
//richard
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