Re: [RFC v7 13/41] richacl: Check if an acl is equivalent to a file mode

[J. Bruce Fields]

On Sat, Sep 05, 2015 at 12:27:08PM +0200, Andreas Gruenbacher wrote:
> ACLs are considered equivalent to file modes if they only consist of
> owner@, group@, and everyone@ entries, the owner@ permissions do not
> depend on whether the owner is a member in the owning group, and no
> inheritance flags are set.  This test is used to avoid storing richacls
> if the acl can be computed from the file permission bits.

We're assuming here that it's OK for us to silently rearrange an ACL as
long as the result is still equivalent (in the sense that the permission
algorithm would always produce the same result).

I guess that's OK by me, but it might violate user expectations in some
simple common cases, so may be worth mentioning in documentation
someplace if we don't already.

--b.

> 
> Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
> ---
>  fs/richacl_base.c       | 104 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/richacl.h |   1 +
>  2 files changed, 105 insertions(+)
> 
> diff --git a/fs/richacl_base.c b/fs/richacl_base.c
> index 3163152..106e988 100644
> --- a/fs/richacl_base.c
> +++ b/fs/richacl_base.c
> @@ -379,3 +379,107 @@ richacl_chmod(struct richacl *acl, mode_t mode)
>  	return clone;
>  }
>  EXPORT_SYMBOL_GPL(richacl_chmod);
> +
> +/**
> + * richacl_equiv_mode  -  compute the mode equivalent of @acl
> + *
> + * An acl is considered equivalent to a file mode if it only consists of
> + * owner@, group@, and everyone@ entries and the owner@ permissions do not
> + * depend on whether the owner is a member in the owning group.
> + */
> +int
> +richacl_equiv_mode(const struct richacl *acl, mode_t *mode_p)
> +{
> +	mode_t mode = *mode_p;
> +
> +	/*
> +	 * The RICHACE_DELETE_CHILD flag is meaningless for non-directories, so
> +	 * we ignore it.
> +	 */
> +	unsigned int x = S_ISDIR(mode) ? 0 : RICHACE_DELETE_CHILD;
> +	struct {
> +		unsigned int allowed;
> +		unsigned int defined;  /* allowed or denied */
> +	} owner = {
> +		.defined = RICHACE_POSIX_ALWAYS_ALLOWED |
> +			   RICHACE_POSIX_OWNER_ALLOWED  | x,
> +	}, group = {
> +		.defined = RICHACE_POSIX_ALWAYS_ALLOWED | x,
> +	}, everyone = {
> +		.defined = RICHACE_POSIX_ALWAYS_ALLOWED | x,
> +	};
> +	const struct richace *ace;
> +
> +	if (acl->a_flags & ~(RICHACL_WRITE_THROUGH | RICHACL_MASKED))
> +		return -1;
> +
> +	richacl_for_each_entry(ace, acl) {
> +		if (ace->e_flags & ~RICHACE_SPECIAL_WHO)
> +			return -1;
> +
> +		if (richace_is_owner(ace) || richace_is_everyone(ace)) {
> +			x = ace->e_mask & ~owner.defined;
> +			if (richace_is_allow(ace)) {
> +				unsigned int group_denied =
> +					group.defined & ~group.allowed;
> +
> +				if (x & group_denied)
> +					return -1;
> +				owner.allowed |= x;
> +			} else /* if (richace_is_deny(ace)) */ {
> +				if (x & group.allowed)
> +					return -1;
> +			}
> +			owner.defined |= x;
> +
> +			if (richace_is_everyone(ace)) {
> +				x = ace->e_mask;
> +				if (richace_is_allow(ace)) {
> +					group.allowed |=
> +						x & ~group.defined;
> +					everyone.allowed |=
> +						x & ~everyone.defined;
> +				}
> +				group.defined |= x;
> +				everyone.defined |= x;
> +			}
> +		} else if (richace_is_group(ace)) {
> +			x = ace->e_mask & ~group.defined;
> +			if (richace_is_allow(ace))
> +				group.allowed |= x;
> +			group.defined |= x;
> +		} else
> +			return -1;
> +	}
> +
> +	if (group.allowed & ~owner.defined)
> +		return -1;
> +
> +	if (acl->a_flags & RICHACL_MASKED) {
> +		if (acl->a_flags & RICHACL_WRITE_THROUGH) {
> +			owner.allowed = acl->a_owner_mask;
> +			everyone.allowed = acl->a_other_mask;
> +		} else {
> +			owner.allowed &= acl->a_owner_mask;
> +			everyone.allowed &= acl->a_other_mask;
> +		}
> +		group.allowed &= acl->a_group_mask;
> +	}
> +
> +	mode = (mode & ~S_IRWXUGO) |
> +	       (richacl_mask_to_mode(owner.allowed) << 6) |
> +	       (richacl_mask_to_mode(group.allowed) << 3) |
> +		richacl_mask_to_mode(everyone.allowed);
> +
> +	/* Mask flags we can ignore */
> +	x = S_ISDIR(mode) ? 0 : RICHACE_DELETE_CHILD;
> +
> +	if (((richacl_mode_to_mask(mode >> 6) ^ owner.allowed)    & ~x) ||
> +	    ((richacl_mode_to_mask(mode >> 3) ^ group.allowed)    & ~x) ||
> +	    ((richacl_mode_to_mask(mode)      ^ everyone.allowed) & ~x))
> +		return -1;
> +
> +	*mode_p = mode;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(richacl_equiv_mode);
> diff --git a/include/linux/richacl.h b/include/linux/richacl.h
> index d4a576c..6535ce5 100644
> --- a/include/linux/richacl.h
> +++ b/include/linux/richacl.h
> @@ -304,6 +304,7 @@ extern unsigned int richacl_mode_to_mask(mode_t);
>  extern unsigned int richacl_want_to_mask(unsigned int);
>  extern void richacl_compute_max_masks(struct richacl *);
>  extern struct richacl *richacl_chmod(struct richacl *, mode_t);
> +extern int richacl_equiv_mode(const struct richacl *, mode_t *);
>  
>  /* richacl_inode.c */
>  extern int richacl_permission(struct inode *, const struct richacl *, int);
> -- 
> 2.4.3
> 
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