On Thu, Mar 22, 2012 at 7:29 AM, Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
> On Thu, Mar 22, 2012 at 4:43 AM, Mike Frysinger <vapier@xxxxxxxxxx> wrote:
>> Michael: should i fold in the updates that came from this thread and send a
>> v2, or are you going to tackle it ?
>
> I'll work on a draft, and have it out for review in a few days.
Mike, Chris,
Here's a heavily edited version of the page that Mike sent in. Since I
might have messed something up, could I ask you two to take a good
look at this page?
Thanks,
Michael
.\" Copyright (C) 2011 Christopher Yeoh <cyeoh@xxxxxxxxxxx>
.\" and Copyright (C) 2012 Mike Frysinger <vapier@xxxxxxxxxx>
.\" and Copyright (C) 2012 Michael Kerrisk <mtk.man-pages@xxxxxxxxx>
.\"
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.\" Commit fcf634098c00dd9cd247447368495f0b79be12d1
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.TH PROCESS_VM_READV 2 2012-03-25 "Linux" "Linux Programmer's Manual"
.SH NAME
process_vm_readv, process_vm_writev \- transfer data between process
address spaces
.SH SYNOPSIS
.B #include <sys/uio.h>
.nf
.BI "ssize_t process_vm_readv(pid_t " pid ,
.BI " const struct iovec *" local_iov ,
.BI " unsigned long " liovcnt ,
.BI " const struct iovec *" remote_iov ,
.BI " unsigned long " riovcnt ,
.BI " unsigned long " flags ");"
.BI "ssize_t process_vm_writev(pid_t " pid ,
.BI " const struct iovec *" local_iov ,
.BI " unsigned long " liovcnt ,
.BI " const struct iovec *" remote_iov ,
.BI " unsigned long " riovcnt ,
.BI " unsigned long " flags ");"
.fi
.SH DESCRIPTION
These system calls transfer data between the address space
of the calling process ("the local process") and the process identified by
.IR pid
("the remote process").
The data moves directly between the address spaces of the two processes,
without passing through kernel space.
The
.BR process_vm_readv ()
system call transfers data from the process
.I pid
to the calling process.
The data to be transferred is identified by
.IR remote_vec
and
.IR riovcnt :
.IR remote_vec
is a pointer to an array describing address ranges in the process
.IR pid ,
and
.IR riovcnt
specifies the number of items in
.IR remote_vec .
The data is transferred to the locations specified by
.IR local_vec
and
.IR liovcnt :
.IR local_vec
is a pointer to an array describing address ranges in the calling process,
and
.IR liovcnt
specifies the specifies the number of items in
.IR local_vec .
The
.BR process_vm_writev ()
system call is the converse of
.BR process_vm_readv ()\(emit
transfers data from the calling process to the process
.IR pid .
Other than the direction of the transfer, the arguments
.IR liovcnt ,
.IR local_vec ,
.IR liovcnt ,
and
.IR remote_vec
have the same meaning as for
.BR process_vm_readv ().
The
.I local_iov
and
.I remote_iov
arguments point to an array of
.I iovec
structures, defined in
.IR <sys/uio.h>
as:
.in +4n
.nf
struct iovec {
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes to transfer */
};
.fi
.in
Buffers are processed in array order.
This means that
.BR process_vm_readv ()
completely fills
.I local_iov[0]
before proceeding to
.IR local_iov[1] ,
and so on.
Likewise,
.I remote_iov[0]
is completely read before proceeding to
.IR remote_iov[1] ,
and so on.
Similarly,
.BR process_vm_writev ()
writes out the entire contents of
.I local_iov[0]
before proceeding to
.IR local_iov[1] ,
and it completely fills
.I remote_vec[0]
before proceeding to
.IR remote_iov[1] .
The lengths of
.I remote_iov[i].iov_len
and
.I 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
.I flags
argument is currently unused and must be set to 0.
The values specified in the
.I liovcnt
and
.I riovcnt
arguments must be less than or equal to
.BR IOV_MAX
(defined in
.I <limits.h>
or accessible via the call
.IR sysconf(_SC_IOV_MAX) ).
.\" In time, glibc might provide a wrapper that works around this limit,
.\" as is done for readv()/writev()
The count arguments and
.IR local_vec
are checked before doing any transfers.
If the counts are too big, or
.I local_vec
is invalid,
or the addresses refer to regions that are inaccessible in the local process,
none of the vectors will be processed and an
error will be returned immediately.
.\" FIXME: What does the following sentence mean?
Keep this in mind when attempting to
extract data of unknown length (such as C strings which are null-terminated)
by avoiding spanning memory pages (typically 4KiB).
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 may result if one of the
.I remote_vec
elements points to an invalid memory region in the remote process.
No further reads/writes will be attempted beyond that point.
In order to read from or write to another process,
either the caller must have the capability
.BR CAP_SYS_PTRACE ,
or
the real, effective, and saved set user IDs
of the target process must match the real user ID of the caller
.I and
the real, effective, and saved set group IDs
of the target process must match the real group ID of the caller.
(The permission required is exactly the same as that required to perform a
.BR ptrace (2)
.BR PTRACE_ATTACH
on the target process.)
.SH "RETURN VALUE"
On success,
.BR process_vm_readv ()
returns the number of bytes read and
.BR 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.
The caller should check the return value to determine whether
a partial read/write occurred.)
On error, \-1 is returned and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EINVAL
The sum of the
.I iov_len
values of either
.I local_iov
or
.I remote_iov
overflows a
.I ssize_t
value.
.TP
.B EINVAL
.I flags
is not 0.
.TP
.B EINVAL
.I liovcnt
or
.I riovcnt
is too large.
.TP
.B EFAULT
The memory described by
.I local_iov
is outside the caller's accessible address space.
.TP
.B EFAULT
The memory described by
.I remote_iov
is outside the accessible address space of the process
.IR pid .
.TP
.B ENOMEM
Out of memory.
.TP
.B EPERM
The caller does not have permission to access the address space of the process
.IR pid .
.TP
.B ESRCH
No process with ID
.I pid
exists.
.SH VERSIONS
These system calls were added in Linux 3.2.
Support is provided in glibc since version 2.15.
.SH "CONFORMING TO"
These system calls are nonstandard Linux extensions.
.SH NOTES
The data transfers performed by
.BR process_vm_readv ()
and
.BR 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).
.\" Original user is MPI, http://www.mcs.anl.gov/research/projects/mpi/
.\" See also some benchmarks at http://lwn.net/Articles/405284/
.\" and http://marc.info/?l=linux-mm&m=130105930902915&w=2
.SH EXAMPLE
The following code sample demonstrates the use of
.BR process_vm_readv ().
It reads 20 bytes at the address 0x10000 from the process with PID 10
and writes the first 10 bytes into
.I buf1
and the second 10 bytes into
.IR buf2 .
.sp
.nf
#include <sys/uio.h>
int
main(void)
{
struct iovec local[2];
struct iovec remote[1];
char buf1[10];
char buf2[10];
ssize_t nread;
pid_t pid = 10; /* PID of target process */
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[1].iov_len = 20;
nread = process_vm_readv(pid, local, 2, remote, 1, 0);
if (nread != 20)
return 1;
else
return 0;
}
.fi
.SH "SEE ALSO"
.BR readv (2),
.BR writev (2)
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process_vm_readv.2
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