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
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