Re: [PATCH] ptrace.2: Improve clarity for multi-threaded tracers

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Niklas,

Do you plan to revise this patch in the light of Dmitry's comments?

Thanks,

Michael

On Sun, 17 Feb 2019 at 23:15, Dmitry V. Levin <ldv@altlinux.org> wrote:
>
> Hi,
>
> On Sun, Feb 17, 2019 at 05:34:46PM +0100, Niklas Hambüchen wrote:
> > Until now, the man page said:
> >
> >     Attachment and subsequent commands are per thread:
> >     in a multi‐ threaded process, every thread can be individually attached to a
> >     (potentially different) tracer, or left not attached and thus not debugged.
> >     Therefore, "tracee" always means "(one) thread", never "a (possibly
> >     multithreaded) process".
> >
> > While the first sentence "Attachment ... [is] per thread" might be interpreted
> > as holding for both tracer and tracee, the rest talks only about the
> > multi-threadedness of the *tracee*, leaving some uncertainty in the reader on
> > whether the tracer may issue `ptrace()` from different threads.
> >
> > This patch adds more explicitness, removing any doubt.
>
> Thanks for making an attempt to remove any doubt.
>
> Yes, ptrace'ing is per task_struct on both sides.
>
> > Relevant resources:
> >
> > * LKML thread https://marc.info/?l=linux-kernel&m=155036848808748&w=2
> >   "ptrace() with multithreaded tracer"
> >   where I asked about this behaviour, in case anybody disagrees with my
> >   understanding
> > * https://stackoverflow.com/questions/18737866/can-a-thread-trace-a-process/
> >   where the previous ambiguity of the man page confused some users, and where
> >   and example program is given that confirms the behaviour I mention in this
> >   patch
> > * A program of mine, in which I have independently confirmed that using
> >   `ptrace()` from a thread that's not the tracer thread (a sibling thread in
> >   the process is the tracer instead) results in `ESRCH`
> > * https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/ptrace.c?id=96d4f267e40f9509e8a66e2b39e8b95655617693#n207
> >   where the comment on `ptrace_check_attach()` talks about `%current`, which
> >   is a thread
> >
> > Signed-off-by: Niklas Hambüchen <mail@nh2.me>
> > ---
> >  man2/ptrace.2 | 14 ++++++++++----
> >  1 file changed, 10 insertions(+), 4 deletions(-)
> >
> > diff --git a/man2/ptrace.2 b/man2/ptrace.2
> > index 3b6b6ea84..4058abe94 100644
> > --- a/man2/ptrace.2
> > +++ b/man2/ptrace.2
> > @@ -122,12 +122,18 @@ It is primarily used to implement breakpoint debugging and system
> >  call tracing.
> >  .PP
> >  A tracee first needs to be attached to the tracer.
> > -Attachment and subsequent commands are per thread:
> > -in a multithreaded process,
> > +Attachment and subsequent commands are per thread,
> > +on both the tracer and tracee side.
> > +Issuing a tracing command from a thread that is not the tracer of the given
> > +.I pid
> > +will result in an
> > +.B ESRCH
> > +error.
>
> This is confusing.  What do you mean by a tracing command?
> Is PTRACE_TRACEME a tracing command?  PTRACE_ATTACH?  PTRACE_SEIZE?
>
> I suggest leaving the explanation of ptrace return code to "ERRORS"
> section.
>
> > +In a multithreaded process to be traced,
> >  every thread can be individually attached to a
> >  (potentially different) tracer,
> >  or left not attached and thus not debugged.
> > -Therefore, "tracee" always means "(one) thread",
> > +Therefore, "tracer" or "tracee" always mean "(one) thread",
> >  never "a (possibly multithreaded) process".
> >  Ptrace commands are always sent to
> >  a specific tracee using a call of the form
> > @@ -2259,7 +2265,7 @@ or (on kernels before 2.6.26) be
> >  .TP
> >  .B ESRCH
> >  The specified process does not exist, or is not currently being traced
> > -by the caller, or is not stopped
> > +by the calling thread, or is not stopped
> >  (for requests that require a stopped tracee).
> >  .SH CONFORMING TO
> >  SVr4, 4.3BSD.
>
> I agree the current text can be made more clear on the subject,
> but, unfortunately, proposed change makes the description more confusing.
>
>
> --
> ldv



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/