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List:       apache-httpd-bugs
Subject:    [Bug 57268] New: apache process crashes when downloading large file
From:       bugzilla () apache ! org
Date:       2014-11-26 14:46:53
Message-ID: bug-57268-7868 () https ! issues ! apache ! org/bugzilla/
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https://issues.apache.org/bugzilla/show_bug.cgi?id=57268

            Bug ID: 57268
           Summary: apache process crashes when downloading large file
           Product: Apache httpd-2
           Version: 2.4.10
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: mpm_event
          Assignee: bugs@httpd.apache.org
          Reporter: jaroslav@thinline.cz

When using event mpm, Apache processes randomly crash when serving request to
download large file. It seems the crash occurs when file download takes long
time (downloading 256MB file on 10Mbit line triggers the crash, but downloading
the same file on gbit LAN works fine.) It also seems the crash only occurs when
the server is busy serving other requests.

This is how I am able to reproduce the issue on my test server:

- create small file in document root (index.html with content "index") and
large file large.bin (dd if=/dev/zero of=large.bin bs=$((1024**2)) count=256)
- run ab -q -n 2000000 -c 20 'http://[a:b:c:d::1]/index.html' on the server
- run wget 'http://[a:b:c:d::1]/large.bin' on my workstation (~5 minutes long
download)

The server transmits some data and then wget reports "Connection closed at byte
xxx. Retrying." The amount of transmitted data changes every try, sometimes the
crash occurs after few seconds, sometimes it takes few minutes, in few cases
the file downloaded without causing Apache to crash.

When the server was idle - only wget running, no ab - server didn't crash in
any of my tries.

When the crash occurs, Apache logs this into error log:

[Wed Nov 26 14:38:14.982708 2014] [core:notice] [pid 12215:tid 140446535845760]
AH00052: child pid 16079 exit signal Segmentation fault (11).

In some cases kernel logs message like this one into dmesg (message from
different server, therefore different pid):

[1943221.021819] apache2[31073]: segfault at 7f3c380c52d8 ip 00007f3c59d66ab2
sp 00007f3c3afece30 error 4 in mod_mpm_event.so[7f3c59d60000+e000]

We are using Apache downloaded from Debian Jessie repository (apache2-bin),
together with libapache2-mod-fcgid, php5-cgi, suexec (apache2-suexec-custom)
and libapache2-mod-geoip. (PHP is not involved in processing any of the files
mentioned above.) List of active modules follows (from server-info): core.c,
event.c, http_core.c, mod_access_compat.c, mod_alias.c, mod_auth_basic.c,
mod_authn_core.c, mod_authn_file.c, mod_authz_core.c, mod_authz_host.c,
mod_authz_user.c, mod_autoindex.c, mod_deflate.c, mod_dir.c, mod_env.c,
mod_fcgid.c, mod_filter.c, mod_geoip.c, mod_info.c, mod_log_config.c,
mod_logio.c, mod_mime.c, mod_negotiation.c, mod_setenvif.c, mod_so.c,
mod_status.c, mod_unique_id.c, mod_unixd.c, mod_version.c, mod_watchdog.c

If I recall correctly, testing server is running in default configuration from
Debian package - let me know if you need some configuration details.

At the moment it seems switching to mpm_worker works around the problem.

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