slow log pread() failed - no stack trace

[Michael Andersen]

Hi FPM folks,

We're running php-fpm 7.1.10 with Apache 2.4 on RedHat Enterprise 6.10.

I'm trying to use the slow log in FPM to capture a process that's hanging in PHP - we don' t know where the problem is, but the back trace from the slow log could explain everything.

To test the slow log, we set it up in a test system and set the timeout to 1 second.

But we can't get a back trace in the slow log. We just get:

+++ dump failed

In the php-fpm.log file, we see this error:

[06-Jan-2021 17:41:07] WARNING: [pool www] child 21184, script '<script_path_redacted' (request: "POST /<script_path_redacted) executing too slow (1.332840 sec), logging
[06-Jan-2021 17:41:07] NOTICE: child 21184 stopped for tracing
[06-Jan-2021 17:41:07] NOTICE: about to trace 21184
[06-Jan-2021 17:41:07] ERROR: pread() failed: Operation not permitted (1)
[06-Jan-2021 17:41:07] NOTICE: finished trace of 21184

Does anyone here know why we would get that error in place of a back trace? I'm sure this isn't an FPM problem, it's a Linux permission setting problem or a user group membership problem or some security issue, but I'm stumped about where to look for it or even what sort of questions to ask.

fpm runs as the same user as apache, and has read-write access to the log directory (it's already writing there, it's just not writing the stack trace). The fpm user of course is the same user who owns all of the fpm child processes, so I can't imagine why it doesn't have the necessary permissions.

I've tried googling this problem, but nothing useful comes up.

I'm open to any suggestions - thanks in advance!

Mike Andersen