[Wayland] plasma crashes and cannot attach gdb to kwin_wayland

[Germano Massullo]

Hi there, I am testing Plasma 5.9.4 on Wayland (on Fedora 25) and I am
filling various bugreports.
If I run plasmashell from bash, when it crashes I obtain message
<The Wayland connection broke. Did the Wayland compositor die?>
so I started trying to attach gdb to kwin_wayland process. I followed
https://community.kde.org/KWin/Debugging
and I tried to run
$ gdb --pid 1450  2>&1 | tee kwin.gdb
but I still obtained message
ptrace: Operation not permitted.

What can I do to retrieve useful informations?
Thank you