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Subject[PATCH v4 2/4] vm: add a syscall to map a process memory into a pipe
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From: Andrei Vagin <avagin@virtuozzo.com>

It is a hybrid of process_vm_readv() and vmsplice().

vmsplice can map memory from a current address space into a pipe.
process_vm_readv can read memory of another process.

A new system call can map memory of another process into a pipe.

ssize_t process_vmsplice(pid_t pid, int fd, const struct iovec *iov,
unsigned long nr_segs, unsigned int flags)

All arguments are identical with vmsplice except pid which specifies a
target process.

Currently if we want to dump a process memory to a file or to a socket,
we can use process_vm_readv() + write(), but it works slow, because data
are copied into a temporary user-space buffer.

A second way is to use vmsplice() + splice(). It is more effective,
because data are not copied into a temporary buffer, but here is another
problem. vmsplice works with the currect address space, so it can be
used only if we inject our code into a target process.

The second way suffers from a few other issues:
* a process has to be stopped to run a parasite code
* a number of pipes is limited, so it may be impossible to dump all
memory in one iteration, and we have to stop process and inject our
code a few times.
* pages in pipes are unreclaimable, so it isn't good to hold a lot of
memory in pipes.

The introduced syscall allows to use a second way without injecting any
code into a target process.

My experiments shows that process_vmsplice() + splice() works two time
faster than process_vm_readv() + write().

It is particularly useful on a pre-dump stage. On this stage we enable a
memory tracker, and then we are dumping a process memory while a
process continues work. On the first iteration we are dumping all
memory, and then we are dumpung only modified memory from a previous
iteration. After a few pre-dump operations, a process is stopped and
dumped finally. The pre-dump operations allow to significantly decrease
a process downtime, when a process is migrated to another host.

v2: move this syscall under CONFIG_CROSS_MEMORY_ATTACH
give correct flags to get_user_pages_remote()

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
---
fs/splice.c | 205 ++++++++++++++++++++++++++++++++++++++
include/linux/compat.h | 3 +
include/linux/syscalls.h | 4 +
include/uapi/asm-generic/unistd.h | 5 +-
kernel/sys_ni.c | 2 +
5 files changed, 218 insertions(+), 1 deletion(-)

diff --git a/fs/splice.c b/fs/splice.c
index 7f1ffc5..72397d2 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -34,6 +34,7 @@
#include <linux/socket.h>
#include <linux/compat.h>
#include <linux/sched/signal.h>
+#include <linux/sched/mm.h>

#include "internal.h"

@@ -1373,6 +1374,210 @@ SYSCALL_DEFINE4(vmsplice, int, fd, const struct iovec __user *, iov,
return error;
}

+#ifdef CONFIG_CROSS_MEMORY_ATTACH
+/*
+ * Map pages from a specified task into a pipe
+ */
+static int remote_single_vec_to_pipe(struct task_struct *task,
+ struct mm_struct *mm,
+ const struct iovec *rvec,
+ struct pipe_inode_info *pipe,
+ unsigned int flags,
+ size_t *total)
+{
+ struct pipe_buffer buf = {
+ .ops = &user_page_pipe_buf_ops,
+ .flags = flags
+ };
+ unsigned long addr = (unsigned long) rvec->iov_base;
+ unsigned long pa = addr & PAGE_MASK;
+ unsigned long start_offset = addr - pa;
+ unsigned long nr_pages;
+ ssize_t len = rvec->iov_len;
+ struct page *process_pages[16];
+ bool failed = false;
+ int ret = 0;
+
+ nr_pages = (addr + len - 1) / PAGE_SIZE - addr / PAGE_SIZE + 1;
+ while (nr_pages) {
+ long pages = min(nr_pages, 16UL);
+ int locked = 1;
+ ssize_t copied;
+
+ /*
+ * Get the pages we're interested in. We must
+ * access remotely because task/mm might not
+ * current/current->mm
+ */
+ down_read(&mm->mmap_sem);
+ pages = get_user_pages_remote(task, mm, pa, pages, 0,
+ process_pages, NULL, &locked);
+ if (locked)
+ up_read(&mm->mmap_sem);
+ if (pages <= 0) {
+ failed = true;
+ ret = -EFAULT;
+ break;
+ }
+
+ copied = pages * PAGE_SIZE - start_offset;
+ if (copied > len)
+ copied = len;
+ len -= copied;
+
+ ret = pages_to_pipe(process_pages, pipe, &buf, total, copied,
+ start_offset);
+ if (unlikely(ret < 0))
+ break;
+
+ start_offset = 0;
+ nr_pages -= pages;
+ pa += pages * PAGE_SIZE;
+ }
+ return ret < 0 ? ret : 0;
+}
+
+static ssize_t remote_iovec_to_pipe(struct task_struct *task,
+ struct mm_struct *mm,
+ const struct iovec *rvec,
+ unsigned long riovcnt,
+ struct pipe_inode_info *pipe,
+ unsigned int flags)
+{
+ size_t total = 0;
+ int ret = 0, i;
+
+ for (i = 0; i < riovcnt; i++) {
+ /* Work out address and page range required */
+ if (rvec[i].iov_len == 0)
+ continue;
+
+ ret = remote_single_vec_to_pipe(
+ task, mm, &rvec[i], pipe, flags, &total);
+ if (ret < 0)
+ break;
+ }
+ return total ? total : ret;
+}
+
+static long process_vmsplice_to_pipe(struct task_struct *task,
+ struct mm_struct *mm, struct file *file,
+ const struct iovec __user *uiov,
+ unsigned long nr_segs, unsigned int flags)
+{
+ struct pipe_inode_info *pipe;
+ struct iovec iovstack[UIO_FASTIOV];
+ struct iovec *iov = iovstack;
+ unsigned int buf_flag = 0;
+ long ret;
+
+ if (flags & SPLICE_F_GIFT)
+ buf_flag = PIPE_BUF_FLAG_GIFT;
+
+ pipe = get_pipe_info(file);
+ if (!pipe)
+ return -EBADF;
+
+ ret = rw_copy_check_uvector(CHECK_IOVEC_ONLY, uiov, nr_segs,
+ UIO_FASTIOV, iovstack, &iov);
+ if (ret < 0)
+ return ret;
+
+ pipe_lock(pipe);
+ ret = wait_for_space(pipe, flags);
+ if (!ret)
+ ret = remote_iovec_to_pipe(task, mm, iov,
+ nr_segs, pipe, buf_flag);
+ pipe_unlock(pipe);
+ if (ret > 0)
+ wakeup_pipe_readers(pipe);
+
+ if (iov != iovstack)
+ kfree(iov);
+ return ret;
+}
+
+/* process_vmsplice splices a process address range into a pipe. */
+SYSCALL_DEFINE5(process_vmsplice, int, pid, int, fd,
+ const struct iovec __user *, iov,
+ unsigned long, nr_segs, unsigned int, flags)
+{
+ struct task_struct *task;
+ struct mm_struct *mm;
+ struct fd f;
+ long ret;
+
+ if (unlikely(flags & ~SPLICE_F_ALL))
+ return -EINVAL;
+ if (unlikely(nr_segs > UIO_MAXIOV))
+ return -EINVAL;
+ else if (unlikely(!nr_segs))
+ return 0;
+
+ f = fdget(fd);
+ if (!f.file)
+ return -EBADF;
+
+ /* Get process information */
+ task = find_get_task_by_vpid(pid);
+ if (!task) {
+ ret = -ESRCH;
+ goto out_fput;
+ }
+
+ mm = mm_access(task, PTRACE_MODE_ATTACH_REALCREDS);
+ if (!mm || IS_ERR(mm)) {
+ ret = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH;
+ /*
+ * Explicitly map EACCES to EPERM as EPERM is a more a
+ * appropriate error code for process_vw_readv/writev
+ */
+ if (ret == -EACCES)
+ ret = -EPERM;
+ goto put_task_struct;
+ }
+
+ ret = -EBADF;
+ if (f.file->f_mode & FMODE_WRITE)
+ ret = process_vmsplice_to_pipe(task, mm, f.file,
+ iov, nr_segs, flags);
+ mmput(mm);
+
+put_task_struct:
+ put_task_struct(task);
+
+out_fput:
+ fdput(f);
+
+ return ret;
+}
+
+#ifdef CONFIG_COMPAT
+COMPAT_SYSCALL_DEFINE5(process_vmsplice, pid_t, pid, int, fd,
+ const struct compat_iovec __user *, iov32,
+ unsigned int, nr_segs, unsigned int, flags)
+{
+ struct iovec __user *iov;
+ unsigned int i;
+
+ if (nr_segs > UIO_MAXIOV)
+ return -EINVAL;
+
+ iov = compat_alloc_user_space(nr_segs * sizeof(struct iovec));
+ for (i = 0; i < nr_segs; i++) {
+ struct compat_iovec v;
+
+ if (get_user(v.iov_base, &iov32[i].iov_base) ||
+ get_user(v.iov_len, &iov32[i].iov_len) ||
+ put_user(compat_ptr(v.iov_base), &iov[i].iov_base) ||
+ put_user(v.iov_len, &iov[i].iov_len))
+ return -EFAULT;
+ }
+ return sys_process_vmsplice(pid, fd, iov, nr_segs, flags);
+}
+#endif
+#endif /* CONFIG_CROSS_MEMORY_ATTACH */
+
#ifdef CONFIG_COMPAT
COMPAT_SYSCALL_DEFINE4(vmsplice, int, fd, const struct compat_iovec __user *, iov32,
unsigned int, nr_segs, unsigned int, flags)
diff --git a/include/linux/compat.h b/include/linux/compat.h
index 0fc3640..11b3753 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -550,6 +550,9 @@ asmlinkage long compat_sys_getdents(unsigned int fd,
unsigned int count);
asmlinkage long compat_sys_vmsplice(int fd, const struct compat_iovec __user *,
unsigned int nr_segs, unsigned int flags);
+asmlinkage long compat_sys_process_vmsplice(pid_t pid, int fd,
+ const struct compat_iovec __user *,
+ unsigned int nr_segs, unsigned int flags);
asmlinkage long compat_sys_open(const char __user *filename, int flags,
umode_t mode);
asmlinkage long compat_sys_openat(int dfd, const char __user *filename,
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index a78186d..4ba9333 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -941,4 +941,8 @@ asmlinkage long sys_pkey_free(int pkey);
asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
unsigned mask, struct statx __user *buffer);

+asmlinkage long sys_process_vmsplice(pid_t pid,
+ int fd, const struct iovec __user *iov,
+ unsigned long nr_segs, unsigned int flags);
+
#endif
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 8b87de0..37f1832 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -732,9 +732,12 @@ __SYSCALL(__NR_pkey_alloc, sys_pkey_alloc)
__SYSCALL(__NR_pkey_free, sys_pkey_free)
#define __NR_statx 291
__SYSCALL(__NR_statx, sys_statx)
+#define __NR_process_vmsplice 292
+__SC_COMP(__NR_process_vmsplice, sys_process_vmsplice,
+ compat_sys_process_vmsplice)

#undef __NR_syscalls
-#define __NR_syscalls 292
+#define __NR_syscalls 293

/*
* All syscalls below here should go away really,
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index b518976..a939fbb 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -158,8 +158,10 @@ cond_syscall(sys_sysfs);
cond_syscall(sys_syslog);
cond_syscall(sys_process_vm_readv);
cond_syscall(sys_process_vm_writev);
+cond_syscall(sys_process_vmsplice);
cond_syscall(compat_sys_process_vm_readv);
cond_syscall(compat_sys_process_vm_writev);
+cond_syscall(compat_sys_process_vmsplice);
cond_syscall(sys_uselib);
cond_syscall(sys_fadvise64);
cond_syscall(sys_fadvise64_64);
--
2.7.4
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