Re: [PATCH 5/5] fanotify: Add pidfd info record support to the fanotify API

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, May 20, 2021 at 12:11:51PM +1000, Matthew Bobrowski wrote:
> Introduce a new flag FAN_REPORT_PIDFD for fanotify_init(2) which
> allows userspace applications to control whether a pidfd info record
> containing a pidfd is to be returned with each event.
> 
> If FAN_REPORT_PIDFD is enabled for a notification group, an additional
> struct fanotify_event_info_pidfd object will be supplied alongside the
> generic struct fanotify_event_metadata within a single event. This
> functionality is analogous to that of FAN_REPORT_FID in terms of how
> the event structure is supplied to the userspace application. Usage of
> FAN_REPORT_PIDFD with FAN_REPORT_FID/FAN_REPORT_DFID_NAME is
> permitted, and in this case a struct fanotify_event_info_pidfd object
> will follow any struct fanotify_event_info_fid object.
> 
> Usage of FAN_REPORT_TID is not permitted with FAN_REPORT_PIDFD as the
> pidfd API only supports the creation of pidfds for thread-group
> leaders. Attempting to do so will result with a -EINVAL being returned
> when calling fanotify_init(2).
> 
> If pidfd creation fails via pidfd_create(), the pidfd field within
> struct fanotify_event_info_pidfd is set to FAN_NOPIDFD.
> 
> Signed-off-by: Matthew Bobrowski <repnop@google.com>
> ---
>  fs/notify/fanotify/fanotify_user.c | 65 +++++++++++++++++++++++++++---
>  include/linux/fanotify.h           |  3 +-
>  include/uapi/linux/fanotify.h      | 12 ++++++
>  3 files changed, 74 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 1e15f3222eb2..bba61988f4a0 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -106,6 +106,8 @@ struct kmem_cache *fanotify_perm_event_cachep __read_mostly;
>  #define FANOTIFY_EVENT_ALIGN 4
>  #define FANOTIFY_FID_INFO_HDR_LEN \
>  	(sizeof(struct fanotify_event_info_fid) + sizeof(struct file_handle))
> +#define FANOTIFY_PIDFD_INFO_HDR_LEN \
> +	sizeof(struct fanotify_event_info_pidfd)
>  
>  static int fanotify_fid_info_len(int fh_len, int name_len)
>  {
> @@ -141,6 +143,9 @@ static int fanotify_event_info_len(unsigned int info_mode,
>  	if (fh_len)
>  		info_len += fanotify_fid_info_len(fh_len, dot_len);
>  
> +	if (info_mode & FAN_REPORT_PIDFD)
> +		info_len += FANOTIFY_PIDFD_INFO_HDR_LEN;
> +
>  	return info_len;
>  }
>  
> @@ -401,6 +406,29 @@ static int copy_fid_info_to_user(__kernel_fsid_t *fsid,
>  	return info_len;
>  }
>  
> +static int copy_pidfd_info_to_user(struct pid *pid,
> +				   char __user *buf,
> +				   size_t count)
> +{
> +	struct fanotify_event_info_pidfd info = { };
> +	size_t info_len = FANOTIFY_PIDFD_INFO_HDR_LEN;
> +
> +	if (WARN_ON_ONCE(info_len > count))
> +		return -EFAULT;
> +
> +	info.hdr.info_type = FAN_EVENT_INFO_TYPE_PIDFD;
> +	info.hdr.len = info_len;
> +
> +	info.pidfd = pidfd_create(pid, 0);
> +	if (info.pidfd < 0)
> +		info.pidfd = FAN_NOPIDFD;
> +
> +	if (copy_to_user(buf, &info, info_len))
> +		return -EFAULT;

Hm, well this kinda sucks. The caller can end up with a pidfd in their
fd table and when the copy_to_user() failed they won't know what fd it
is. I think this might be better served by sm like (see what I did in
kernel/fork.c): 

	pidfd = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
	if (pidfd < 0)
		/* error handling */
	
	pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid, O_RDWR | O_CLOEXEC);
	if (IS_ERR(pidfile)) {
		put_unused_fd(pidfd);
		pidfd = PTR_ERR(pidfile);
		/* error handling */
	}
	get_pid(pid);	/* held by pidfile now */

	info.hdr.info_type = FAN_EVENT_INFO_TYPE_PIDFD;
	info.hdr.len = info_len;
	info.pidfd = pidfd;

	if (copy_to_user(buf, &info, info_len)) {
		fput(pidfile);
		put_unused_fd(pidfd);
		return -EFAULT;
	}

	fd_install(pidfd, pidfile);
	return 0;

> +
> +	return info_len;
> +}
> +
>  static int copy_info_to_user(struct fanotify_event *event,
>  			     struct fanotify_info *info,
>  			     unsigned int info_mode,
> @@ -408,9 +436,12 @@ static int copy_info_to_user(struct fanotify_event *event,
>  {
>  	int ret, info_type = 0;
>  	unsigned int fid_mode = info_mode & FANOTIFY_FID_BITS;
> +	unsigned int pidfd_mode = info_mode & FAN_REPORT_PIDFD;
>  
>  	/*
>  	 * Event info records order is as follows: dir fid + name, child fid.
> +	 * If FAN_REPORT_PIDFD has been specified, then a pidfd info record
> +	 * will follow the fid info records.
>  	 */
>  	if (fanotify_event_dir_fh_len(event)) {
>  		info_type = info->name_len ? FAN_EVENT_INFO_TYPE_DFID_NAME :
> @@ -465,10 +496,18 @@ static int copy_info_to_user(struct fanotify_event *event,
>  		}
>  
>  		ret = copy_fid_info_to_user(fanotify_event_fsid(event),
> -					    fanotify_event_object_fh(event),
> -					    info_type, dot, dot_len,
> -					    buf, count);
> -	}
> +					    fanotify_event_object_fh(event),
> +					    info_type, dot, dot_len,
> +					    buf, count);
> +		if (ret < 0)
> +			return ret;
> +
> +		buf += ret;
> +		count -= ret;
> +	}
> +
> +	if (pidfd_mode)
> +		return copy_pidfd_info_to_user(event->pid, buf, count);
>  
>  	return ret;
>  }
> @@ -530,6 +569,15 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>  		fd_install(fd, f);
>  
>  	if (info_mode) {
> +		/*
> +		 * Complain if FAN_REPORT_PIDFD and FAN_REPORT_TID mutual
> +		 * exclusion is ever lifted. At the time of implementing
> +		 * FAN_REPORT_PIDFD, the pidfd API only supported the creation
> +		 * of pidfds on thread-group leaders.
> +		 */
> +		WARN_ON_ONCE((info_mode & FAN_REPORT_PIDFD) &&
> +			     FAN_GROUP_FLAG(group, FAN_REPORT_TID));
> +
>  		ret = copy_info_to_user(event, info, info_mode, buf, count);
>  		if (ret < 0)
>  			return ret;
> @@ -1079,6 +1127,13 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
>  #endif
>  		return -EINVAL;
>  
> +	/*
> +	 * A pidfd can only be returned for a thread-group leader; thus
> +	 * FAN_REPORT_TID and FAN_REPORT_PIDFD need to be mutually exclusive.
> +	 */
> +	if ((flags & FAN_REPORT_PIDFD) && (flags & FAN_REPORT_TID))
> +		return -EINVAL;
> +
>  	if (event_f_flags & ~FANOTIFY_INIT_ALL_EVENT_F_BITS)
>  		return -EINVAL;
>  
> @@ -1477,7 +1532,7 @@ static int __init fanotify_user_setup(void)
>  	max_marks = clamp(max_marks, FANOTIFY_OLD_DEFAULT_MAX_MARKS,
>  				     FANOTIFY_DEFAULT_MAX_USER_MARKS);
>  
> -	BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 10);
> +	BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 11);
>  	BUILD_BUG_ON(HWEIGHT32(FANOTIFY_MARK_FLAGS) != 9);
>  
>  	fanotify_mark_cache = KMEM_CACHE(fsnotify_mark,
> diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
> index f76c7635efc8..bb2898240e5a 100644
> --- a/include/linux/fanotify.h
> +++ b/include/linux/fanotify.h
> @@ -27,7 +27,7 @@ extern struct ctl_table fanotify_table[]; /* for sysctl */
>  
>  #define FANOTIFY_FID_BITS	(FAN_REPORT_FID | FAN_REPORT_DFID_NAME)
>  
> -#define FANOTIFY_INFO_MODES	(FANOTIFY_FID_BITS)
> +#define FANOTIFY_INFO_MODES	(FANOTIFY_FID_BITS | FAN_REPORT_PIDFD)
>  
>  /*
>   * fanotify_init() flags that require CAP_SYS_ADMIN.
> @@ -37,6 +37,7 @@ extern struct ctl_table fanotify_table[]; /* for sysctl */
>   */
>  #define FANOTIFY_ADMIN_INIT_FLAGS	(FANOTIFY_PERM_CLASSES | \
>  					 FAN_REPORT_TID | \
> +					 FAN_REPORT_PIDFD | \
>  					 FAN_UNLIMITED_QUEUE | \
>  					 FAN_UNLIMITED_MARKS)
>  
> diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
> index fbf9c5c7dd59..36c3bddcf690 100644
> --- a/include/uapi/linux/fanotify.h
> +++ b/include/uapi/linux/fanotify.h
> @@ -55,6 +55,7 @@
>  #define FAN_REPORT_FID		0x00000200	/* Report unique file id */
>  #define FAN_REPORT_DIR_FID	0x00000400	/* Report unique directory id */
>  #define FAN_REPORT_NAME		0x00000800	/* Report events with name */
> +#define FAN_REPORT_PIDFD	0x00001000	/* Report pidfd for event->pid */
>  
>  /* Convenience macro - FAN_REPORT_NAME requires FAN_REPORT_DIR_FID */
>  #define FAN_REPORT_DFID_NAME	(FAN_REPORT_DIR_FID | FAN_REPORT_NAME)
> @@ -123,6 +124,7 @@ struct fanotify_event_metadata {
>  #define FAN_EVENT_INFO_TYPE_FID		1
>  #define FAN_EVENT_INFO_TYPE_DFID_NAME	2
>  #define FAN_EVENT_INFO_TYPE_DFID	3
> +#define FAN_EVENT_INFO_TYPE_PIDFD	4
>  
>  /* Variable length info record following event metadata */
>  struct fanotify_event_info_header {
> @@ -148,6 +150,15 @@ struct fanotify_event_info_fid {
>  	unsigned char handle[0];
>  };
>  
> +/*
> + * This structure is used for info records of type FAN_EVENT_INFO_TYPE_PIDFD.
> + * It holds a pidfd for the pid responsible for generating an event.
> + */
> +struct fanotify_event_info_pidfd {
> +	struct fanotify_event_info_header hdr;
> +	__s32 pidfd;
> +};
> +
>  struct fanotify_response {
>  	__s32 fd;
>  	__u32 response;
> @@ -160,6 +171,7 @@ struct fanotify_response {
>  
>  /* No fd set in event */
>  #define FAN_NOFD	-1
> +#define FAN_NOPIDFD	FAN_NOFD
>  
>  /* Helper functions to deal with fanotify_event_metadata buffers */
>  #define FAN_EVENT_METADATA_LEN (sizeof(struct fanotify_event_metadata))
> -- 
> 2.31.1.751.gd2f1c929bd-goog
> 
> /M