Re: [PATCH v2 2/3] fanotify: define struct members to hold response decision context

On 2022-05-02 20:16, Paul Moore wrote: > On Thu, Apr 28, 2022 at 8:45 PM Richard Guy Briggs <rgb(a)redhat.com&gt; wrote: > > This patch adds 2 structure members to the response returned from user > > space on a permission event. The first field is 16 bits for the context > > type. The context type will describe what the meaning is of the second > > field. The default is none. The patch defines one additional context > > type which means that the second field is a 32-bit rule number. This > > will allow for the creation of other context types in the future if > > other users of the API identify different needs. The second field size > > is defined by the context type and can be used to pass along the data > > described by the context. > > > > To support this, there is a macro for user space to check that the data > > being sent is valid. Of course, without this check, anything that > > overflows the bit field will trigger an EINVAL based on the use of > > FAN_INVALID_RESPONSE_MASK in process_access_response(). > > > > Suggested-by: Steve Grubb <sgrubb(a)redhat.com&gt; > > Link: https://lore.kernel.org/r/2745105.e9J7NaK4W3@x2 > > Suggested-by: Jan Kara <jack(a)suse.cz&gt; > > Link: https://lore.kernel.org/r/20201001101219.GE17860@quack2.suse.cz > > Signed-off-by: Richard Guy Briggs <rgb(a)redhat.com&gt; > > Link: https://lore.kernel.org/r/17660b3f2817e5c0a19d1e9e5d40b53ff4561845.165117... > > --- > > fs/notify/fanotify/fanotify.c | 1 - > > fs/notify/fanotify/fanotify.h | 4 +- > > fs/notify/fanotify/fanotify_user.c | 59 ++++++++++++++++++++---------- > > include/linux/fanotify.h | 3 ++ > > include/uapi/linux/fanotify.h | 27 +++++++++++++- > > 5 files changed, 72 insertions(+), 22 deletions(-)

> > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c > > index 694516470660..f1ff4cf683fb 100644 > > --- a/fs/notify/fanotify/fanotify_user.c > > +++ b/fs/notify/fanotify/fanotify_user.c > > @@ -289,13 +289,19 @@ static int create_fd(struct fsnotify_group *group, struct path *path, > > */ > > static void finish_permission_event(struct fsnotify_group *group, > > struct fanotify_perm_event *event, > > - __u32 response) > > + struct fanotify_response *response) > > __releases(&group->notification_lock) > > { > > bool destroy = false; > > > > assert_spin_locked(&group->notification_lock); > > - event->response = response; > > + event->response = response->response; > > + event->extra_info_type = response->extra_info_type; > > + switch (event->extra_info_type) { > > + case FAN_RESPONSE_INFO_AUDIT_RULE: > > + memcpy(event->extra_info_buf, response->extra_info_buf, > > + sizeof(struct fanotify_response_audit_rule)); > > Since the fanotify_perm_event:extra_info_buf and > fanotify_response:extra_info_buf are the same type/length, and they > will be the same regardless of the extra_info_type field, why not > simply get rid of the above switch statement and do something like > this: > > memcpy(event->extra_info_buf, response->extra_info_buf, > sizeof(response->extra_info_buf)); I've been wrestling with the possibility of doing a split between what is presented to userspace and what's used in the kernel for struct fanotify_response, while attempting to future-proof it.

> > @@ -827,26 +845,25 @@ static ssize_t fanotify_read(struct file *file, char __user *buf, > > > > static ssize_t fanotify_write(struct file *file, const char __user *buf, size_t count, loff_t *pos) > > { > > - struct fanotify_response response = { .fd = -1, .response = -1 }; > > + struct fanotify_response response; > > struct fsnotify_group *group; > > int ret; > > + size_t size = min(count, sizeof(struct fanotify_response)); > > > > if (!IS_ENABLED(CONFIG_FANOTIFY_ACCESS_PERMISSIONS)) > > return -EINVAL; > > > > group = file->private_data; > > > > - if (count < sizeof(response)) > > + if (count < offsetof(struct fanotify_response, extra_info_buf)) > > return -EINVAL; > > Is this why you decided to shrink the fanotify_response:response field > from 32-bits to 16-bits? I hope not. I would suggest both keeping > the existing response field as 32-bits and explicitly checking for > writes that are either the existing/compat length as well as the > newer, longer length. No. I shrank it at Jan's suggestion. I think I agree with you that the response field should be kept at u32 as it is defined in userspace and purge the doubt about what would happen with a new userspace with an old kernel.

> > diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h > > index e8ac38cc2fd6..efb5a3a6f814 100644 > > --- a/include/uapi/linux/fanotify.h > > +++ b/include/uapi/linux/fanotify.h > > @@ -179,9 +179,34 @@ struct fanotify_event_info_error { > > __u32 error_count; > > }; > > > > +/* > > + * User space may need to record additional information about its decision. > > + * The extra information type records what kind of information is included. > > + * The default is none. We also define an extra informaion buffer whose > > + * size is determined by the extra information type. > > + * > > + * If the context type is Rule, then the context following is the rule number > > + * that triggered the user space decision. > > + */ > > + > > +#define FAN_RESPONSE_INFO_AUDIT_NONE 0 > > +#define FAN_RESPONSE_INFO_AUDIT_RULE 1 > > + > > +struct fanotify_response_audit_rule { > > + __u32 rule; > > +}; > > + > > +#define FANOTIFY_RESPONSE_EXTRA_LEN_MAX \ > > + (sizeof(union { \ > > + struct fanotify_response_audit_rule r; \ > > + /* add other extra info structures here */ \ > > + })) > > + > > struct fanotify_response { > > __s32 fd; > > - __u32 response; > > + __u16 response; > > + __u16 extra_info_type; > > + char extra_info_buf[FANOTIFY_RESPONSE_EXTRA_LEN_MAX]; > > }; > > Since both the kernel and userspace are going to need to agree on the > content and formatting of the fanotify_response:extra_info_buf field, > why is it hidden behind a char array? You might as well get rid of > that abstraction and put the union directly in the fanotify_response > struct. It is possible you could also get rid of the > fanotify_response_audit_rule struct this way too and just access the > rule scalar directly. This does make sense and my only concern would be a variable-length type. There isn't any reason to hide it. If userspace chooses to use the old interface and omit the type field then it defaults to NONE.