Re: [PATCH 2/2] GDB test suite: Get core files on targets with systemd-coredump

[Andreas Arnez <arnez@linux.vnet.ibm.com>]

On Tue, Oct 17 2017, Pedro Alves wrote:

> On 09/18/2017 07:41 PM, Andreas Arnez wrote:
>> So far the test suite skips tests if they need system-generated core files
>> and the core files can not be found.  In particular this is usually the
>> case on systems with an active systemd-coredump service.  On such systems,
>> core files are not written into the local directory, but made accessible
>> via a command-line utitily "coredumpctl" instead.
>> 
>> This patch enables processing core files on such systems as well.  Note
>> that there are a few quirks:
>> 
>> * In my tests, after invoking a command that dumps core, it could happen
>>   that "coredumpctl" did not find the dump immediately afterwards.  After
>>   waiting a bit, the dump was found and could be accessed.  Thus the patch
>>   performs a single wait-and-retry in case of failure.
>> 
>> * There does not seem to be a way for a user to remove specific core dumps
>>   from the journal.  Thus it can happen that "coredumpctl" returns an old
>>   dump, and the test case continues with that instead of the new one.  It
>>   might be possible to improve the logic here, by considering the time
>>   stamps as well.  I leave that for a future patch.
>> 
>> * On the system I've tested it on, the bigcore.exp test case still failed
>>   because coredumpctl truncated the core file after 4G for some reason.
>
> I'm a bit unsure about whether this is the right approach,
> expecially given the caveats above.  Also, this seems to mean that
> running the testsuite on such a system clutters the system log on and on,
> maybe even triggers dispatch of notifications to admins, etc.

The caveats above are really bugs/design flaws of systemd-coredump.  If
nothing else, maybe this discussion helps addressing them.  Offering no
way to prevent system log cluttering could be viewed as another flaw,
see also below.

> I wonder whether there's a way to tell systemd-coredump to
> let the core dumps be generated on the file system for the current
> shell environment?  Like we try to run "ulimit -c unlimited".

I agree that there *should* be such a way -- but I haven't found any.
And such a mechanism should probably allow suppressing the log entries
as well.

> Failing that, it may be better to instead make the testsuite skip
> the tests gracefully, and display a big and visible warning
> if systemd-coredump is detected as active.

This might be the right trade-off if we expect test cases to be executed
only on systems that the user has full control over.  But I consider
this restriction too tight and would prefer a "best effort" approach
instead.  Maybe we should emit a warning *and* try our best to execute
the test?

> I mean, you already have to tweak other things in the system in
> order to be able to run the testsuite correctly.  For example,
> you have to tweak /proc/sys/kernel/yama/ptrace_scope to make
> attach tests work at all, for example.  systemd-coredump kind of
> seems like more of the same.

So should we document a sequence of admin commands that makes a system
debug-ready, or in particular ready for the GDB test suite?

But I'm not so sure about this.  IMHO a default mainstream Linux
installation should be suited for development- and debugging purposes
*without* any tweaking.  Also, if there are good reasons for a security
measure, we shouldn't rely on disabling it globally.

With respect to Yama's ptrace scope, the distributions seem to differ.
For instance, Fedora does not activate it by default
(https://fedoraproject.org/wiki/Security_Features_Matrix), while Ubuntu
does (https://wiki.ubuntu.com/Security/Features).  And I wonder whether
this feature couldn't be adjusted to be more debug-friendly either.

--
Andreas