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SubjectRe: [RFC 11/39] richacl: Compute maximum file masks from an acl
From
On Fri, Mar 27, 2015 at 05:50:09PM +0100, Andreas Gruenbacher wrote:
> Compute upper bound owner, group, and other file masks with as few
> permissions as possible without denying any permissions that the NFSv4
> acl in a richacl grants.
>
> This algorithm is used when a file inherits an acl at create time and
> when an acl is set via a mechanism that does not specify file modes
> (such as via nfsd). When user-space sets an acl, the file masks are
> passed in as part of the xattr.
>
> When setting a richacl, the file masks determine what the file
> permission bits will be set to; see richacl_masks_to_mode().
>
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> ---
> fs/richacl_base.c | 128 ++++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/richacl.h | 1 +
> 2 files changed, 129 insertions(+)
>
> diff --git a/fs/richacl_base.c b/fs/richacl_base.c
> index 9cd15ca..d9e557a 100644
> --- a/fs/richacl_base.c
> +++ b/fs/richacl_base.c
> @@ -173,3 +173,131 @@ richacl_want_to_mask(unsigned int want)
> return mask;
> }
> EXPORT_SYMBOL_GPL(richacl_want_to_mask);
> +
> +/**
> + * richacl_allowed_to_who - mask flags allowed to a specific who value
> + *
> + * Computes the mask values allowed to a specific who value, taking
> + * EVERYONE@ entries into account.
> + */
> +static unsigned int richacl_allowed_to_who(struct richacl *acl,
> + struct richace *who)
> +{
> + struct richace *ace;
> + unsigned int allowed = 0;
> +
> + richacl_for_each_entry_reverse(ace, acl) {
> + if (richace_is_inherit_only(ace))
> + continue;
> + if (richace_is_same_identifier(ace, who) ||
> + richace_is_everyone(ace)) {
> + if (richace_is_allow(ace))
> + allowed |= ace->e_mask;
> + else if (richace_is_deny(ace))
> + allowed &= ~ace->e_mask;
> + }
> + }
> + return allowed;
> +}
> +
> +/**
> + * richacl_group_class_allowed - maximum permissions the group class is allowed
> + *
> + * See richacl_compute_max_masks().
> + */
> +static unsigned int richacl_group_class_allowed(struct richacl *acl)
> +{
> + struct richace *ace;
> + unsigned int everyone_allowed = 0, group_class_allowed = 0;
> + int had_group_ace = 0;
> +
> + richacl_for_each_entry_reverse(ace, acl) {
> + if (richace_is_inherit_only(ace) ||
> + richace_is_owner(ace))
> + continue;
> +
> + if (richace_is_everyone(ace)) {
> + if (richace_is_allow(ace))
> + everyone_allowed |= ace->e_mask;
> + else if (richace_is_deny(ace))
> + everyone_allowed &= ~ace->e_mask;
> + } else {
> + group_class_allowed |=
> + richacl_allowed_to_who(acl, ace);
> +
> + if (richace_is_group(ace))
> + had_group_ace = 1;
> + }
> + }
> + if (!had_group_ace)
> + group_class_allowed |= everyone_allowed;
> + return group_class_allowed;
> +}
> +
> +/**
> + * richacl_compute_max_masks - compute upper bound masks
> + *
> + * Computes upper bound owner, group, and other masks so that none of
> + * the mask flags allowed by the acl are disabled (for any choice of the
> + * file owner or group membership).
> + */
> +void richacl_compute_max_masks(struct richacl *acl)
> +{
> + unsigned int gmask = ~0;
> + struct richace *ace;
> +
> + /*
> + * @gmask contains all permissions which the group class is ever
> + * allowed. We use it to avoid adding permissions to the group mask
> + * from everyone@ allow aces which the group class is always denied
> + * through other aces. For example, the following acl would otherwise
> + * result in a group mask or rw:
> + *
> + * group@:w::deny
> + * everyone@:rw::allow
> + *
> + * Avoid computing @gmask for acls which do not include any group class
> + * deny aces: in such acls, the group class is never denied any
> + * permissions from everyone@ allow aces.
> + */

Cripes, by the time I got to this point I'd already written one response
where I complained this was wrong, another where I explained why it was
right, and gone back and forth a couple more times.

Feel free to add a

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

but maybe we could use a couple more examples here. Or a paper.

--b.

> +
> +restart:
> + acl->a_owner_mask = 0;
> + acl->a_group_mask = 0;
> + acl->a_other_mask = 0;
> +
> + richacl_for_each_entry_reverse(ace, acl) {
> + if (richace_is_inherit_only(ace))
> + continue;
> +
> + if (richace_is_owner(ace)) {
> + if (richace_is_allow(ace))
> + acl->a_owner_mask |= ace->e_mask;
> + else if (richace_is_deny(ace))
> + acl->a_owner_mask &= ~ace->e_mask;
> + } else if (richace_is_everyone(ace)) {
> + if (richace_is_allow(ace)) {
> + acl->a_owner_mask |= ace->e_mask;
> + acl->a_group_mask |= ace->e_mask & gmask;
> + acl->a_other_mask |= ace->e_mask;
> + } else if (richace_is_deny(ace)) {
> + acl->a_owner_mask &= ~ace->e_mask;
> + acl->a_group_mask &= ~ace->e_mask;
> + acl->a_other_mask &= ~ace->e_mask;
> + }
> + } else {
> + if (richace_is_allow(ace)) {
> + acl->a_owner_mask |= ace->e_mask & gmask;
> + acl->a_group_mask |= ace->e_mask & gmask;
> + } else if (richace_is_deny(ace) && gmask == ~0) {
> + gmask = richacl_group_class_allowed(acl);
> + if (likely(gmask != ~0))
> + /* should always be true */
> + goto restart;
> + }
> + }
> + }
> +
> + acl->a_flags &= ~RICHACL_MASKED;
> +}
> +EXPORT_SYMBOL_GPL(richacl_compute_max_masks);
> diff --git a/include/linux/richacl.h b/include/linux/richacl.h
> index 2120aa6..25f45f7 100644
> --- a/include/linux/richacl.h
> +++ b/include/linux/richacl.h
> @@ -288,5 +288,6 @@ extern struct richacl *richacl_alloc(int, gfp_t);
> extern int richacl_masks_to_mode(const struct richacl *);
> 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 *);
>
> #endif /* __RICHACL_H */
> --
> 2.1.0
>
> --
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