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SubjectRe: [v4,resend,2/2] kcmp: Add KCMP_EPOLL_TFD mode to compare epoll target files
On Mon, Apr 24, 2017 at 06:39:28PM +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
>
> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> Acked-by: Andrey Vagin <avagin@openvz.org>
> CC: Al Viro <viro@zeniv.linux.org.uk>
> CC: Andrew Morton <akpm@linuxfoundation.org>
> 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>
> ---
> fs/eventpoll.c | 42 +++++++++++++++++++++++++++++++++
> include/linux/eventpoll.h | 3 ++
> include/uapi/linux/kcmp.h | 10 ++++++++
> kernel/kcmp.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 112 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 +98,56 @@ 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);
> +
> + if (filp_epoll) {
> + filp_tgt = get_epoll_tfile_raw_ptr(filp_epoll, slot.tfd, slot.toff);
> + fput(filp_epoll);
> + } else
I think this "else" is unnecessary here.

> +
> + 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
> 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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