Commit 0791e364 authored by Cyrill Gorcunov's avatar Cyrill Gorcunov Committed by Linus Torvalds
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kcmp: add KCMP_EPOLL_TFD mode to compare epoll target files 

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

Link: http://lkml.kernel.org/r/20170424154423.511592110@gmail.com


Signed-off-by: default avatarCyrill Gorcunov <gorcunov@openvz.org>
Acked-by: default avatarAndrey Vagin <avagin@openvz.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 77493f04

fs/eventpoll.c

+42 −0
Original line number Diff line number Diff line
@@ -1077,6 +1077,48 @@ static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd) 
	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

include/linux/eventpoll.h

+3 −0
Original line number Diff line number Diff line
@@ -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)

{

include/uapi/linux/kcmp.h

+10 −0
Original line number Diff line number Diff line 
#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 */

kernel/kcmp.c

+57 −0
Original line number Diff line number Diff line
@@ -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 +98,56 @@ static int kcmp_lock(struct mutex *m1, struct mutex *m2) 
	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);



	if (filp_epoll) {

		filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);

		fput(filp_epoll);

	} else



	if (IS_ERR(filp_tgt))

		return PTR_ERR(filp_tgt);



	return kcmp_ptr(filp, filp_tgt, KCMP_FILE);

}

#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 +219,9 @@ SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type, 
		ret = -EOPNOTSUPP;

#endif

		break;

	case KCMP_EPOLL_TFD:

		ret = kcmp_epoll_target(task1, task2, idx1, (void *)idx2);

		break;

	default:

		ret = -EINVAL;

		break;

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