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


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