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@xxxxxxxxxxxx> 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@xxxxxx> > > --- > > 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/
