Ubuntu Manpage: process_vm_readv, process_vm_writev - transfer data between process address spaces

NAME

       process_vm_readv, process_vm_writev - transfer data between process address spaces

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/uio.h>

       ssize_t process_vm_readv(pid_t pid,
                              const struct iovec *local_iov,
                              unsigned long liovcnt,
                              const struct iovec *remote_iov,
                              unsigned long riovcnt,
                              unsigned long flags);
       ssize_t process_vm_writev(pid_t pid,
                              const struct iovec *local_iov,
                              unsigned long liovcnt,
                              const struct iovec *remote_iov,
                              unsigned long riovcnt,
                              unsigned long flags);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       process_vm_readv(), process_vm_writev():
           _GNU_SOURCE

DESCRIPTION

These system calls transfer data between the address space of the calling process ("the
local process") and the process identified by pid ("the remote process"). The data moves
directly between the address spaces of the two processes, without passing through kernel
space.

The process_vm_readv() system call transfers data from the remote process to the local
process. The data to be transferred is identified by remote_iov and riovcnt: remote_iov
is a pointer to an array describing address ranges in the process pid, and riovcnt
specifies the number of elements in remote_iov. The data is transferred to the locations
specified by local_iov and liovcnt: local_iov is a pointer to an array describing address
ranges in the calling process, and liovcnt specifies the number of elements in local_iov.

The process_vm_writev() system call is the converse of process_vm_readv()—it transfers
data from the local process to the remote process. Other than the direction of the
transfer, the arguments liovcnt, local_iov, riovcnt, and remote_iov have the same meaning
as for process_vm_readv().

The local_iov and remote_iov arguments point to an array of iovec structures, described in
iovec(3type).

Buffers are processed in array order. This means that process_vm_readv() completely fills
local_iov[0] before proceeding to local_iov[1], and so on. Likewise, remote_iov[0] is
completely read before proceeding to remote_iov[1], and so on.

Similarly, process_vm_writev() writes out the entire contents of local_iov[0] before
proceeding to local_iov[1], and it completely fills remote_iov[0] before proceeding to
remote_iov[1].

The lengths of remote_iov[i].iov_len and local_iov[i].iov_len do not have to be the same.
Thus, it is possible to split a single local buffer into multiple remote buffers, or vice
versa.

The flags argument is currently unused and must be set to 0.

The values specified in the liovcnt and riovcnt arguments must be less than or equal to
IOV_MAX (defined in <limits.h> or accessible via the call sysconf(_SC_IOV_MAX)).

The count arguments and local_iov are checked before doing any transfers. If the counts
are too big, or local_iov is invalid, or the addresses refer to regions that are
inaccessible to the local process, none of the vectors will be processed and an error will
be returned immediately.

Note, however, that these system calls do not check the memory regions in the remote
process until just before doing the read/write. Consequently, a partial read/write (see
RETURN VALUE) may result if one of the remote_iov elements points to an invalid memory
region in the remote process. No further reads/writes will be attempted beyond that
point. Keep this in mind when attempting to read data of unknown length (such as C
strings that are null-terminated) from a remote process, by avoiding spanning memory pages
(typically 4 KiB) in a single remote iovec element. (Instead, split the remote read into
two remote_iov elements and have them merge back into a single write local_iov entry. The
first read entry goes up to the page boundary, while the second starts on the next page
boundary.)

Permission to read from or write to another process is governed by a ptrace access mode
PTRACE_MODE_ATTACH_REALCREDS check; see ptrace(2).

RETURN VALUE

       On  success,  process_vm_readv()  returns the number of bytes read and process_vm_writev()
       returns the number of bytes written.  This return value may be less than the total  number
       of  requested  bytes,  if  a partial read/write occurred.  (Partial transfers apply at the
       granularity of iovec elements.  These system calls won't perform a partial  transfer  that
       splits  a  single  iovec  element.)  The caller should check the return value to determine
       whether a partial read/write occurred.

       On error, -1 is returned and errno is set to indicate the error.

ERRORS

       EFAULT The memory described by local_iov is outside the caller's accessible address space.

       EFAULT The memory described by remote_iov is outside the accessible address space  of  the
              process pid.

       EINVAL The sum of the iov_len values of either local_iov or remote_iov overflows a ssize_t
              value.

       EINVAL flags is not 0.

       EINVAL liovcnt or riovcnt is too large.

       ENOMEM Could not allocate memory for internal copies of the iovec structures.

       EPERM  The caller does not have permission to access the address space of the process pid.

       ESRCH  No process with ID pid exists.

STANDARDS

       Linux.

HISTORY

       Linux 3.2, glibc 2.15.

NOTES

       The data  transfers  performed  by  process_vm_readv()  and  process_vm_writev()  are  not
       guaranteed to be atomic in any way.

       These system calls were designed to permit fast message passing by allowing messages to be
       exchanged with a single copy operation (rather than the double copy that would be required
       when using, for example, shared memory or pipes).

EXAMPLES

       The  following  code sample demonstrates the use of process_vm_readv().  It reads 20 bytes
       at the address 0x10000 from the process with PID 10 and writes the  first  10  bytes  into
       buf1 and the second 10 bytes into buf2.

       #define _GNU_SOURCE
       #include <stdlib.h>
       #include <sys/types.h>
       #include <sys/uio.h>

       int
       main(void)
       {
           char          buf1[10];
           char          buf2[10];
           pid_t         pid = 10;    /* PID of remote process */
           ssize_t       nread;
           struct iovec  local[2];
           struct iovec  remote[1];

           local[0].iov_base = buf1;
           local[0].iov_len = 10;
           local[1].iov_base = buf2;
           local[1].iov_len = 10;
           remote[0].iov_base = (void *) 0x10000;
           remote[0].iov_len = 20;

           nread = process_vm_readv(pid, local, 2, remote, 1, 0);
           if (nread != 20)
               exit(EXIT_FAILURE);

           exit(EXIT_SUCCESS);
       }