Segfault, "No protocol specified"

[ChristianRomberg]

Hi, I recently upgraded to Ubuntu 17.10 which uses Wayland instead of X11. cnping used to work before the upgrade, and now it doesn't. I freshly installed cnping from source to make sure I got the latest version, but it segfaults after saying "No protocol specified". Screenshot below.

If you need anything, my grahpics settings etc, just leave a comment. Thanks!

[ChristianRomberg]

Done a little debugging (courtesy of some printf) and it turns out XOpenDisplay(NULL) returns NULL. Not sure why, I thought XWayland would take care of it. Other X Windows (such as xeyes) work fine.

[ChristianRomberg]

Ok. Got it. It's the setuid causing trouble. If I follow the instructions outlined here, everything works.

[cnlohr]

hmm, that is annoying it doesn't "just" work. But there's really no other way around it

[ChristianRomberg]

I guess one way to deal with it would be to build 2 seperate applications, one with elevated permissions (which does the pinging) and one for plotting. Having to execute those commands before using this awesome program is just a pita

[cnlohr]

indeed! Sounds like something that needs to be complained about for the people XWayland ppl to address.

[dreua]

Maybe we should try using capabilities instead of setuid before doing anything more complex. See this article for a short overview: https://www.insecure.ws/linux/getcap_setcap.html

[ChristianRomberg]

That works brilliantly! Thanks. If there aren't any major drawbacks to this I'm gonna do a PR

[cnlohr]

@dreua thank you so much for that. I had no idea it existed.

[dreua]

I experienced some really strange behavior when upgrading from the old (setuid) to the new (getcap) version:

1. On linuxinstall the file /usr/local/bin/cnping is overwritten but the setuid-bit from the file being previously at that location (from the old version) is for some reason (bug or feature in cp?) still present in the new version.
2. Now my cnping has both, the setuid bit and the cap_net_raw capability, and I get the same "No protocol specified" error as @ChristianRomberg. If I remove either the setuid bit or the cap_net_raw capability it works fine.

Can someone explain 1 or 2? Does someone experience the same?

This happens both on Fedora 26 and 27 in X11 mode. (Wayland is available but not activated.)

Proposed workaround: Add sudo rm /usr/local/bin/cnpingor sudo chmod -s /usr/local/bin/cnpingto the Makefile.

[cnlohr]

My recommendation would be to do the first option of removing cnping.

[ChristianRomberg]

We could just install it with the coreutils install which clears all the permissions and file flags. Its really only a cosmetic difference which archieves the same goal of removing the setuid.

[ChristianRomberg]

Kinda odd though that removing the file capability fixes the issue for @dreua . Not sure whats up with that. But can you confirm that it works when you give it only the capability and not the setuid-flag?

[dreua]

Yes, it works fine with the capability only and with the setuid bit only, but both is a problem. (I have no clue why, I guess it's a bug in Fedora.) I just tried your PR and it works fine for me, thanks 👍