Message-ID: <1335428498.6702.33.camel@perseus.themaw.net>
Date:	Thu, 26 Apr 2012 16:21:38 +0800
From:	Ian Kent <raven@...maw.net>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	Linux-kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Meyer <thomas@...3r.de>, stable@...nel.org,
	autofs@...r.kernel.org
Subject: Re: [PATCH] unbreak automount daemon after fixing autofs5 ABI

On Thu, 2012-04-26 at 10:26 +0400, Michael Tokarev wrote:
> Patch 499f286dc02cde6b668e2d757dfe100cb0c43445, comitted
> in Feb-2012, fixed a kernel<=>user interface which was
> broken for mixed 32/64 user/kernel environment since its
> introduction in 2006 by commit 5c0a32fc2cd0.  The problem
> with this fix is that it broke userspace which were adopted
> to the initial bug a year after its introduction, in
> autofs-5.0.1.  So patch 499f286dc02cde6b broke a working
> userspace.  This has been done because another user of
> this interface emerged, systemd, which had the same prob
> as old automount (<5.0.1) had.  So the end result was a
> breakage of an old, widely used -- by all current and
> previous distribution -- software (autofs5) in order to
> unbreak a new not yet widely adopted software (systemd).
> 
> The real fix for this issue, in my opinion, is to adopt
> the same fix in systemd as has been done in autofs5, and/or
> to have a new interface without a bug wich both userspace
> implementations will use.
> 
> Alternative is to find a way on the kernel side to see
> which variant of userspace we're dealing with, by seeing
> how many bytes for the structure in question the userspace
> requests in read operation.  But since autofs uses regular
> pipe code to read the data from kernel, and the said pipe
> is opened in userspace before connecting it with the autofs
> module, reads are not seen by autofs module directly, they're
> seen by the pipe code.
> 
> So for now, below is a minimal quick fix which should sort
> current mess.
> 
> Change 499f286dc02cde6b tries to fix the original interface
> bug for 32bit userspace running on 64bit kernel.  But the
> issue is that autofs5 already has a workaround for it, so
> the end result is that the two (kernel & user) disagree
> again.  "Fix" this by applying the in-kernel fix only if
> the process is not named "automount" -- which is how autofs5
> daemon is named.

No, you cannot be sure the autofs binary is named automount.
Not only that, this approach makes an unpleasant change even more
unpleasant.

I still think that my original proposed patches were the better approach
to handling this problem.

My recommendation was that the major autofs kernel protocol version be
bumped and a packed structure used from that version on. That allows
user space to request that protocol version for communication at mount
time. If systemd doesn't want to support earlier protocol versions then
it can complain, instructing the user to update to a later kernel.

I understand why that proposal was not well accepted but the fact
remains, I did make a mistake and I did chose to work around it in user
space rather than cope with the pain at the time. In hind site that may
have been the wrong thing to do but that's history. So I believe the
most sensible thing to do now is to take the approach that minimizes
breakage.

> 
> This change should be applied to all 3.x stable series too,
> since it breaks existing userspace for these kernels.
> 
> Signed-off-by: Michael Tokarev <mjt@....msk.ru>
> Cc: Ian Kent <raven@...maw.net>
> Cc: Thomas Meyer <thomas@...3r.de>
> Cc: stable@...nel.org
> Cc: autofs@...r.kernel.org
> ---
>  fs/autofs4/dev-ioctl.c |    3 ++-
>  fs/autofs4/inode.c     |    3 ++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c
> index 9dacb85..d5a4cb6 100644
> --- a/fs/autofs4/dev-ioctl.c
> +++ b/fs/autofs4/dev-ioctl.c
> @@ -385,7 +385,8 @@ static int autofs_dev_ioctl_setpipefd(struct file *fp,
>  		sbi->pipefd = pipefd;
>  		sbi->pipe = pipe;
>  		sbi->catatonic = 0;
> -		sbi->compat_daemon = is_compat_task();
> +		sbi->compat_daemon =
> +			is_compat_task() && strcmp(current->comm, "automount");

If this approach is used it might be better to use a length limited
comparison so that if a distribution needs to have multiple packages
side by side, perhaps v4 or 32 and 64 bit, they would most likely append
something on the end of the binary name.
 
>  	}
>  out:
>  	mutex_unlock(&sbi->wq_mutex);
> diff --git a/fs/autofs4/inode.c b/fs/autofs4/inode.c
> index d8dc002..b59f6a0 100644
> --- a/fs/autofs4/inode.c
> +++ b/fs/autofs4/inode.c
> @@ -225,7 +225,8 @@ int autofs4_fill_super(struct super_block *s, void *data, int silent)
>  	set_autofs_type_indirect(&sbi->type);
>  	sbi->min_proto = 0;
>  	sbi->max_proto = 0;
> -	sbi->compat_daemon = is_compat_task();
> +	sbi->compat_daemon =
> +		is_compat_task() && strcmp(current->comm, "automount");
>  	mutex_init(&sbi->wq_mutex);
>  	mutex_init(&sbi->pipe_mutex);
>  	spin_lock_init(&sbi->fs_lock);


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