Re: [patch 2/3] kcmp: Add KCMP_EPOLL_TFD mode to compare epoll target files

[Andrei Vagin <avagin@virtuozzo.com>]

On Fri, Mar 10, 2017 at 11:16:57AM +0300, Cyrill Gorcunov wrote:
> With current epoll architecture target files are addressed
> with file_struct and file descriptor number, where the last
> is not unique. Moreover files can be transferred from another
> process via unix socket, added into queue and closed then
> so we won't find this descriptor in the task fdinfo list.
> 
> Thus to checkpoint and restore such processes CRIU needs to
> find out where exactly the target file is present to add it into
> epoll queue. For this sake one can use kcmp call where
> some particular target file from the queue is compared with
> arbitrary file passed as an argument.
> 
> Because epoll target files can have same file descriptor
> number but different file_struct a caller should explicitly
> specify the offset within.
> 
> To test if some particular file is matching entry inside
> epoll one have to
> 
>  - fill kcmp_epoll_slot structure with epoll file descriptor,
>    target file number and target file offset (in case if only
>    one target is present then it should be 0)
> 
>  - call kcmp as kcmp(pid1, pid2, KCMP_EPOLL_TFD, fd, &kcmp_epoll_slot)
>     - the kernel fetch file pointer matching file descriptor @fd of pid1
>     - lookups for file struct in epoll queue of pid2 and returns traditional
>       0,1,2 result for sorting purpose
> 
> v2:
>  - Use KCMP_FILES salt for files comparision (for convenience sake,
>    since the pointers are file structs so user can lookup over previously
>    collected files tree)
>  - Make kcmp_epoll_target as a separate helper instead of opencoding
>    it with #ifdef
> 
> v3:
>  - Use less if()s in kcmp_epoll_target for readability sake (by avagin@)
>  - Use u32 for kcmp_epoll_slot::toff instead of u64, which makes the less
>    memory pressue
>

Here is one question inline.

Acked-by: Andrei Vagin <avagin@virtuozzo.com>

> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> CC: Al Viro <viro@zeniv.linux.org.uk>
> CC: Andrew Morton <akpm@linuxfoundation.org>
> CC: Andrey Vagin <avagin@openvz.org>
> CC: Pavel Emelyanov <xemul@virtuozzo.com>
> CC: Michael Kerrisk <mtk.manpages@gmail.com>
> CC: Kir Kolyshkin <kir@openvz.org>
> CC: Jason Baron <jbaron@akamai.com>
> CC: Andy Lutomirski <luto@amacapital.net>
> ---
>  fs/eventpoll.c            |   42 +++++++++++++++++++++++++++++++++++
>  include/linux/eventpoll.h |    3 ++
>  include/uapi/linux/kcmp.h |   10 ++++++++
>  kernel/kcmp.c             |   55 ++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 110 insertions(+)
> 
> Index: linux-ml.git/fs/eventpoll.c
> ===================================================================
> --- linux-ml.git.orig/fs/eventpoll.c
> +++ linux-ml.git/fs/eventpoll.c
> @@ -1000,6 +1000,48 @@ static struct epitem *ep_find(struct eve
>  	return epir;
>  }
>  
> +static struct epitem *ep_find_tfd(struct eventpoll *ep, int tfd, unsigned long toff)
> +{
> +	struct rb_node *rbp;
> +	struct epitem *epi;
> +
> +	for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
> +		epi = rb_entry(rbp, struct epitem, rbn);
> +		if (epi->ffd.fd == tfd) {
> +			if (toff == 0)
> +				return epi;
> +			else
> +				toff--;
> +		}
> +		cond_resched();
> +	}
> +
> +	return NULL;
> +}
> +
> +struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd,
> +				     unsigned long toff)
> +{
> +	struct file *file_raw;
> +	struct eventpoll *ep;
> +	struct epitem *epi;
> +
> +	if (!is_file_epoll(file))
> +		return ERR_PTR(-EINVAL);
> +
> +	ep = file->private_data;
> +
> +	mutex_lock(&ep->mtx);
> +	epi = ep_find_tfd(ep, tfd, toff);
> +	if (epi)
> +		file_raw = epi->ffd.file;
> +	else
> +		file_raw = ERR_PTR(-ENOENT);
> +	mutex_unlock(&ep->mtx);
> +
> +	return file_raw;
> +}
> +
>  /*
>   * This is the callback that is passed to the wait queue wakeup
>   * mechanism. It is called by the stored file descriptors when they
> Index: linux-ml.git/include/linux/eventpoll.h
> ===================================================================
> --- linux-ml.git.orig/include/linux/eventpoll.h
> +++ linux-ml.git/include/linux/eventpoll.h
> @@ -14,6 +14,7 @@
>  #define _LINUX_EVENTPOLL_H
>  
>  #include <uapi/linux/eventpoll.h>
> +#include <uapi/linux/kcmp.h>
>  
>  
>  /* Forward declarations to avoid compiler errors */
> @@ -22,6 +23,8 @@ struct file;
>  
>  #ifdef CONFIG_EPOLL
>  
> +struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd, unsigned long toff);
> +
>  /* Used to initialize the epoll bits inside the "struct file" */
>  static inline void eventpoll_init_file(struct file *file)
>  {
> Index: linux-ml.git/include/uapi/linux/kcmp.h
> ===================================================================
> --- linux-ml.git.orig/include/uapi/linux/kcmp.h
> +++ linux-ml.git/include/uapi/linux/kcmp.h
> @@ -1,6 +1,8 @@
>  #ifndef _UAPI_LINUX_KCMP_H
>  #define _UAPI_LINUX_KCMP_H
>  
> +#include <linux/types.h>
> +
>  /* Comparison type */
>  enum kcmp_type {
>  	KCMP_FILE,
> @@ -10,8 +12,16 @@ enum kcmp_type {
>  	KCMP_SIGHAND,
>  	KCMP_IO,
>  	KCMP_SYSVSEM,
> +	KCMP_EPOLL_TFD,
>  
>  	KCMP_TYPES,
>  };
>  
> +/* Slot for KCMP_EPOLL_TFD */
> +struct kcmp_epoll_slot {
> +	__u32 efd;		/* epoll file descriptor */
> +	__u32 tfd;		/* target file number */
> +	__u32 toff;		/* target offset within same numbered sequence */
> +};
> +
>  #endif /* _UAPI_LINUX_KCMP_H */
> Index: linux-ml.git/kernel/kcmp.c
> ===================================================================
> --- linux-ml.git.orig/kernel/kcmp.c
> +++ linux-ml.git/kernel/kcmp.c
> @@ -11,6 +11,10 @@
>  #include <linux/bug.h>
>  #include <linux/err.h>
>  #include <linux/kcmp.h>
> +#include <linux/capability.h>
> +#include <linux/list.h>
> +#include <linux/eventpoll.h>
> +#include <linux/file.h>
>  
>  #include <asm/unistd.h>
>  
> @@ -94,6 +98,54 @@ static int kcmp_lock(struct mutex *m1, s
>  	return err;
>  }
>  
> +#ifdef CONFIG_EPOLL
> +static int kcmp_epoll_target(struct task_struct *task1,
> +			     struct task_struct *task2,
> +			     unsigned long idx1,
> +			     struct kcmp_epoll_slot __user *uslot)
> +{
> +	struct file *filp, *filp_epoll, *filp_tgt;
> +	struct kcmp_epoll_slot slot;
> +	struct files_struct *files;
> +
> +	if (copy_from_user(&slot, uslot, sizeof(slot)))
> +		return -EFAULT;
> +
> +	filp = get_file_raw_ptr(task1, idx1);
> +	if (!filp)
> +		return -EBADF;
> +
> +	files = get_files_struct(task2);
> +	if (!files)
> +		return -EBADF;
> +
> +	spin_lock(&files->file_lock);
> +	filp_epoll = fcheck_files(files, slot.efd);
> +	if (filp_epoll)
> +		get_file(filp_epoll);
> +	else
> +		filp_tgt = ERR_PTR(-EBADF);
> +	spin_unlock(&files->file_lock);
> +	put_files_struct(files);

Why can we not use fget here ^^^^ ?
> +
> +	if (filp_epoll) {
> +		filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
> +		fput(filp_epoll);
> +	}
> +
> +	return IS_ERR(filp_tgt) ? PTR_ERR(filp_tgt) :
> +		kcmp_ptr(filp, filp_tgt, KCMP_FILES);
> +}
> +#else
> +static int kcmp_epoll_target(struct task_struct *task1,
> +			     struct task_struct *task2,
> +			     unsigned long idx1,
> +			     struct kcmp_epoll_slot __user *uslot)
> +{
> +	return -EOPNOTSUPP;
> +}
> +#endif
> +
>  SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
>  		unsigned long, idx1, unsigned long, idx2)
>  {
> @@ -165,6 +217,9 @@ SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t
>  		ret = -EOPNOTSUPP;
>  #endif
>  		break;
> +	case KCMP_EPOLL_TFD:
> +		ret = kcmp_epoll_target(task1, task2, idx1, (void *)idx2);
> +		break;
>  	default:
>  		ret = -EINVAL;
>  		break;
>