linux-kernel - Re: [RFC v7 21/41] richacl: Move everyone@ aces down the acl

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Date:	Fri, 18 Sep 2015 15:35:24 -0400
From:	bfields@...ldses.org (J. Bruce Fields)
To:	Andreas Gruenbacher <agruenba@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org, linux-api@...r.kernel.org,
	linux-cifs@...r.kernel.org, linux-security-module@...r.kernel.org,
	Andreas Gruenbacher <agruen@...nel.org>
Subject: Re: [RFC v7 21/41] richacl: Move everyone@ aces down the acl

On Sat, Sep 05, 2015 at 12:27:16PM +0200, Andreas Gruenbacher wrote:
> The POSIX standard puts processes which are not the owner or a member in
> the owning group or which match any ace other then everyone@ on the
> other file class.  We only know if a process is in the other class after
> processing the entire acl.
> 
> Move all everyone@ aces in the acl down in the acl so that at most a
> single everyone@ allow ace remains at the end.  Permissions which are
> not explicitly allowed are implicitly denied, so an everyone@ deny ace
> is unneeded.
> 
> The everyone@ aces can be moved down the acl without changing the
> permissions that the acl grants.  This transformation simplifies the
> following algorithms, and eventually allows us to turn the final
> everyone@ allow ace into an entry for the other class.
> 
> Signed-off-by: Andreas Gruenbacher <agruen@...nel.org>
> ---
>  fs/richacl_compat.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
> 
> diff --git a/fs/richacl_compat.c b/fs/richacl_compat.c
> index 341e429..4f0acf5 100644
> --- a/fs/richacl_compat.c
> +++ b/fs/richacl_compat.c
> @@ -153,3 +153,68 @@ richace_change_mask(struct richacl_alloc *alloc, struct richace **ace,
>  	}
>  	return 0;
>  }
> +
> +/**
> + * richacl_move_everyone_aces_down  -  move everyone@ aces to the end of the acl
> + * @alloc:	acl and number of allocated entries
> + *
> + * Move all everyone aces to the end of the acl so that only a single everyone@
> + * allow ace remains at the end, and update the mask fields of all aces on the
> + * way.  The last ace of the resulting acl will be an everyone@ allow ace only
> + * if @acl grants any permissions to @everyone.  No @everyone deny aces will
> + * remain.
> + *
> + * This transformation does not alter the permissions that the acl grants.
> + * Having at most one everyone@ allow ace at the end of the acl helps us in the
> + * following algorithms.
> + */
> +static int
> +richacl_move_everyone_aces_down(struct richacl_alloc *alloc)
> +{
> +	struct richace *ace;
> +	unsigned int allowed = 0, denied = 0;
> +
> +	richacl_for_each_entry(ace, alloc->acl) {
> +		if (richace_is_inherit_only(ace))
> +			continue;
> +		if (richace_is_everyone(ace)) {
> +			if (richace_is_allow(ace))
> +				allowed |= (ace->e_mask & ~denied);
> +			else if (richace_is_deny(ace))
> +				denied |= (ace->e_mask & ~allowed);
> +			else
> +				continue;
> +			if (richace_change_mask(alloc, &ace, 0))
> +				return -1;
> +		} else {
> +			if (richace_is_allow(ace)) {
> +				if (richace_change_mask(alloc, &ace, allowed |
> +						(ace->e_mask & ~denied)))
> +					return -1;
> +			} else if (richace_is_deny(ace)) {
> +				if (richace_change_mask(alloc, &ace, denied |
> +						(ace->e_mask & ~allowed)))
> +					return -1;
> +			}
> +		}
> +	}
> +	if (allowed & ~RICHACE_POSIX_ALWAYS_ALLOWED) {
> +		struct richace *last_ace = ace - 1;
> +
> +		if (alloc->acl->a_entries &&
> +		    richace_is_everyone(last_ace) &&
> +		    richace_is_allow(last_ace) &&
> +		    richace_is_inherit_only(last_ace) &&
> +		    last_ace->e_mask == allowed)
> +			last_ace->e_flags &= ~RICHACE_INHERIT_ONLY_ACE;

That's a funny special case!  Is it even worth it, or could we just live
with an extra uninheritable EVERYONE ace in this case?

Anyway, again I like the way you've set this all up with the little
acl-editing helpers, it makes this easier to follow than it otherwise
would be....

	Reviewed-by: J. Bruce Fields <bfields@...hat.com>

--b.

> +		else {
> +			if (richacl_insert_entry(alloc, &ace))
> +				return -1;
> +			ace->e_type = RICHACE_ACCESS_ALLOWED_ACE_TYPE;
> +			ace->e_flags = RICHACE_SPECIAL_WHO;
> +			ace->e_mask = allowed;
> +			ace->e_id.special = RICHACE_EVERYONE_SPECIAL_ID;
> +		}
> +	}
> +	return 0;
> +}
> -- 
> 2.4.3
> 
> --
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