[Bug runtime/27224] New: stap 4.4 'cannot attach to module' in debugfs mode as non-root

["craig.ringer at 2ndquadrant dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27224

            Bug ID: 27224
           Summary: stap 4.4 'cannot attach to module' in debugfs mode as
                    non-root
           Product: systemtap
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: runtime
          Assignee: systemtap at sourceware dot org
          Reporter: craig.ringer at 2ndquadrant dot com
  Target Milestone: ---

On Fedora 33 with kernel 5.9.16-200.fc33.x86_64 and stap 4.4/0.182, rpm
4.4-2.fc33 I noticed that staprun can't seem to attach to module I/O anymore.
It fails with

    ERROR: Cannot attach to module stap_nnn control channel; not running?
    ERROR: 'stap_nnn' is not a zombie systemtap module.

The issue arises with no explicit transport selection or with
-DSTAP_TRANS_DEBUGFS explicitly requested.

This has also been reported by a colleague on Debian who is using "Systemtap
translator/driver (version 4.4/0.176, Debian version 4.4-1~bpo10+1
(buster-backports))"

Running stap with sudo has no effect.

WORKAROUND
========== 

I have been able to work around the error with:

    sudo mount /sys/kernel/debug -o remount,mode=755

DETAIL
======

I found the above info the ancient Debian bug report
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=706817 from 2013, and it
references a 2012 Linux kernel patch
http://lkml.iu.edu/hypermail/linux/kernel/1201.3/00626.html . Also
https://github.com/torvalds/linux/commit/82aceae4f0d42f03d9ad7d1e90389e731153898f
 .

But remounting debugfs solves this issue despite the last relevant changes I
can find related to debugfs being 7 or 8 years old.

It might be related to the changes in systemtap intended to work around the
kernel lockdown issues and add procfs transport support, per
https://bugzilla.redhat.com/show_bug.cgi?id=1873492 in commit 7615cae79 ?

Running with -DSTAP_TRANS_PROCFS output instead works, but only when stap
itself is run as root, even though staprun is setuid root. Unclear why, perhaps
a separate issue.

I don't see the same problem when running on a self-compiled version of git
master @ HEAD = d7ea535c6 but I don't yet know if that's due to differences in
configuration and installation for packages vs source builds, or whether it's
down to a code change since the 4.4 release. I'll compile 4.4 to see.

Sample output for various runs is attached.

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