Understanding proc_rwmem

[John Baldwin]

On Friday 16 April 2010 8:11:25 am Fernando Apesteguía wrote:
> 2010/4/14 John Baldwin <jhb at freebsd.org>:
> > On Wednesday 14 April 2010 4:22:56 pm Fernando Apesteguía wrote:
> >> Hi all,
> >>
> >> I'm trying to read process memory other than the current process in
> >> kernel. I was told to use the proc_rwmem function, however I can't get
> >> it working properly. At first, I'm trying to read how many elements
> >> the environment variables vector has. To do this I tried this from a
> >> linprocfs filler function:
> >>
> >>
> >>         struct iovec iov;
> >>       struct uio tmp_uio;
> >>       struct ps_strings *pss;
> >>       int ret_code;
> >>
> >>       buff = malloc(sizeof(struct ps_strings), M_TEMP, M_WAITOK);
> >>       memset(buff, 0, sizeof(struct ps_strings));
> >>
> >>       PROC_LOCK_ASSERT(td->td_proc, MA_NOTOWNED);
> >>       iov.iov_base = (caddr_t) buff;
> >>       iov.iov_len = sizeof(struct ps_strings);
> >>       tmp_uio.uio_iov = &iov;
> >>       tmp_uio.uio_iovcnt = 1;
> >>       tmp_uio.uio_offset = (off_t)(p->p_sysent->sv_psstrings);
> >>       tmp_uio.uio_resid = sizeof(struct ps_strings);
> >>       tmp_uio.uio_segflg = UIO_USERSPACE;
> >>       tmp_uio.uio_rw = UIO_READ;
> >>       tmp_uio.uio_td = td;
> >>       ret_code = proc_rwmem(td->td_proc, &tmp_uio);
> >
> > I think you want to use 'p' instead of 'td->td_proc' here.  As it is you 
are
> > reading from the current process instead of the target process I believe.
> 
> Thank you. You are right.
> 
> I made the changes suggested by both you and Kostik. I still have
> random data when reading.
> I'm trying to to the same thing using kern/sys_generic.c::read and
> kern/sys_process.c::kern_ptrace
> as examples, but I'm missing something...
> After reading with proc_rwmem, is it possible to do something like the
> following?
> 
> if (ret_code == 0) {
> 		sbuf_printf(sb, "proc_rwmem successfully executed: %d\n", ret_code);
> } else {
> 		sbuf_printf(sb, "Error in proc_rwmem: %d\n", ret_code);
> }
> 
> pss = (struct ps_strings *)(iov.iov_base);
> sbuf_printf(sb, "ps_nargvstr = %d\nps_nenvstr = %d\n",
> pss->ps_nargvstr, pss->ps_nenvstr);
> 
> Thanks in advance.

No, functions like uiomove() modify the iovec structures.  Just use 'buff' 
instead of iov.iov_base.

-- 
John Baldwin