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Using gdb
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If you use the gcc or egcs compilers, it is likely that the best
debugger for your system is gdb. This is only a brief summary of how
to run gdb on Apache -- you should look at the info and man files for
gdb to get more information on gdb commands and common debugging techniques.
Before running gdb, be sure that the server is compiled with the
-g option in EXTRA_CFLAGS to include the
symbol information in the object files.
The only tricky part of running gdb on Apache is forcing the server
into a single-process mode so that the parent process being debugged
does the request-handling work instead of forking child processes.
We have provided the -X option for that purpose, which will
work fine for most cases. However, some modules don't like starting
up with -X , but are happy if you force only one child to run
(using "MaxClients 1 "); you can then use gdb's attach command
to debug the child server.
The following example, with user input in green,
shows the output of gdb run on a server executable (httpd) in the current
working directory and using the server root of
/usr/local/apache :
% gdb httpd
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.16.gnat.1.13 (sparc-sun-solaris2.5),
Copyright 1996 Free Software Foundation, Inc...
(gdb) b ap_process_request
Breakpoint 1 at 0x49fb4: file http_request.c, line 1164.
(gdb) run -X -d /usr/local/apache
Starting program: /usr/local/apache/src/httpd -X -d /usr/local/apache
[at this point I make a request from another window]
Breakpoint 1, ap_process_request (r=0x95250) at http_request.c:1164
1164 if (ap_extended_status)
(gdb) s
1165 ap_time_process_request(r->connection->child_num, ...
(gdb) n
1167 process_request_internal(r);
(gdb) s
process_request_internal (r=0x95250) at http_request.c:1028
1028 if (!r->proxyreq && r->parsed_uri.path) {
(gdb) s
1029 access_status = ap_unescape_url(r->parsed_uri.path);
(gdb) n
1030 if (access_status) {
(gdb) s
1036 ap_getparents(r->uri); /* OK ...
(gdb) n
1038 if ((access_status = location_walk(r))) {
(gdb) n
1043 if ((access_status = ap_translate_name(r))) {
(gdb) n
1048 if (!r->proxyreq) {
(gdb) n
1053 if (r->method_number == M_TRACE) {
(gdb) n
1062 if (r->proto_num > HTTP_VERSION(1,0) && ap_ ...
(gdb) n
1071 if ((access_status = directory_walk(r))) {
(gdb) s
directory_walk (r=0x95250) at http_request.c:288
288 core_server_config *sconf = ap_get_module_ ...
(gdb) b ap_send_error_response
Breakpoint 2 at 0x47dcc: file http_protocol.c, line 2090.
(gdb) c
Continuing.
Breakpoint 2, ap_send_error_response (r=0x95250, recursive_error=0)
at http_protocol.c:2090
2090 BUFF *fd = r->connection->client;
(gdb) where
#0 ap_send_error_response (r=0x95250, recursive_error=0)
at http_protocol.c:2090
#1 0x49b10 in ap_die (type=403, r=0x95250) at http_request.c:989
#2 0x49b60 in decl_die (status=403, phase=0x62db8 "check access", r=0x95250)
at http_request.c:1000
#3 0x49f68 in process_request_internal (r=0x95250) at http_request.c:1141
#4 0x49fe0 in ap_process_request (r=0x95250) at http_request.c:1167
#5 0x439d8 in child_main (child_num_arg=550608) at http_main.c:3826
#6 0x43b5c in make_child (s=0x7c3e8, slot=0, now=907958743)
at http_main.c:3898
#7 0x43ca8 in startup_children (number_to_start=6) at http_main.c:3972
#8 0x44260 in standalone_main (argc=392552, argv=0x75800) at http_main.c:4250
#9 0x449fc in main (argc=4, argv=0xefffee8c) at http_main.c:4534
(gdb) s
2091 int status = r->status;
(gdb) p status
$1 = 403
(gdb)
There are a few things to note about the above example:
- the "
gdb httpd " command does not include any command-line
options for httpd: those are provided when the "run " command
is done within gdb;
- I set a breakpoint before starting the run so that execution would stop
at the top of ap_process_request();
- the "
s " command steps through the code and into called
procedures, whereas the "n " (next) command steps through
the code but not into called procedures.
- additional breakpoints can be set with the "
b " command,
and the run continued with the "c " command.
- use the "
where " command (a.k.a. "bt ") to see
a stack backtrace that shows the order of called procedures and their
parameter values.
- use the "
p " command to print the value of a variable.
A file in the src/ directory (1.3) or the root directory
(2.x) called .gdbinit provides useful macros for printing
out various internal structures of httpd like tables (dump_table ),
brigades (dump_brigade ) and filter chains
(dump_filters ).
If you are debugging a repeatable crash, simply run gdb as above
and make the request -- gdb should capture the crash and provide a
prompt where it occurs.
If you are debugging an apparent infinite loop, simply run gdb as above
and type a Control-C -- gdb will interrupt the process and provide a
prompt where it was stopped.
If you are debugging a system crash and you have a core file from
the crash, then do the following:
% gdb httpd -c core
(gdb) where
and it will (hopefully) print a stack backtrace of where the core dump
occurred during processing.
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Getting a live backtrace on unix
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A backtrace will let you know the hierarchy of procedures that
were called to get to a particular point in the process. On some platforms
you can get a live backtrace of any process.
For SVR4-based variants of Unix, the pstack command for proc can
be used to display a a live backtrace. For example, on Solaris it looks
like
% /usr/proc/bin/pstack 10623
10623: httpd -d /usr/local/apache
ef5b68d8 poll (efffcd08, 0, 3e8)
ef5d21e0 select (0, ef612c28, 0, 0, 3e8, efffcd08) + 288
00042574 wait_or_timeout (0, 75000, 75000, 7c3e8, 60f40, 52c00) + 78
00044310 standalone_main (5fd68, 75800, 75c00, 75000, 2, 64) + 240
000449f4 main (3, efffeee4, efffeef4, 75fe4, 1, 0) + 374
000162fc _start (0, 0, 0, 0, 0, 0) + 5c
Another technique is to use gdb to attach to the running process
and then using "where" to print the backtrace, as in
% gdb httpd 10623
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.16.gnat.1.13 (sparc-sun-solaris2.5),
Copyright 1996 Free Software Foundation, Inc...
/usr/local/apache/src/10623: No such file or directory.
Attaching to program `/usr/local/apache/src/httpd', process 10623
Reading symbols from /usr/lib/libsocket.so.1...done.
Reading symbols from /usr/lib/libnsl.so.1...done.
Reading symbols from /usr/lib/libc.so.1...done.
Reading symbols from /usr/lib/libdl.so.1...done.
Reading symbols from /usr/lib/libintl.so.1...done.
Reading symbols from /usr/lib/libmp.so.1...done.
Reading symbols from /usr/lib/libw.so.1...done.
Reading symbols from /usr/platform/SUNW,Ultra-1/lib/libc_psr.so.1...done.
0xef5b68d8 in ()
(gdb) where
#0 0xef5b68d8 in ()
#1 0xef5d21e8 in select ()
#2 0x4257c in wait_or_timeout (status=0x0) at http_main.c:2357
#3 0x44318 in standalone_main (argc=392552, argv=0x75800) at ...
#4 0x449fc in main (argc=3, argv=0xefffeee4) at http_main.c:4534
(gdb)
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Getting a live backtrace on Windows
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- Unzip the
-symbols.zip files (obtained from the
Apache download site) in the root Apache2 directory tree (where
bin\, htdocs\, modules\ etc. are usually found.) These .pdb files
should unpack alongside the .exe, .dll, .so binary files they
represent, e.g., mod_usertrack.pdb will unpack alongside
mod_usertrack.so.
- Invoke
drwtsn32 and ensure you are creating a crash
dump file, you are dumping all thread contexts, your log and crash
dump paths make sense, and (depending on the nature of the bug) you
pick an appropriate crash dump type. (Full is quite large, but
necessary sometimes for a programmer-type to load your crash dump
into a debugger and begin unwinding exactly what has happened.
Mini is sufficient for your first pass through the process.)
- Note that if you previously installed and then uninstalled other debugging
software, you may need to invoke
drwtsn32 -i in order to make
Dr Watson your default crash dump tool. This will replace the 'report
problem to MS' dialogs. (Don't do this if you have a full debugger such
as Visual Studio or windbg installed on the machine, unless you back up the
registry value for Debugger under the
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug
registry tree. Developers using multiple tools might want to keep copies
of their different tools Debugger entries there, for fast switching.)
- Invoke the Task Manager, Choose 'show processes from all users',
and modify the
View -> Select Columns to include
at least the PID and Thread
Count . You can change this just once and Task Manager
should keep your preference.
- Now, track down the errant Apache that is hanging. The parent
process has about three threads, we don't care about that one. The
child worker process we want has many more threads (a few more than
you configured with the ThreadsPerChild directive.) The process
name is Apache (for 1.3 and 2.0) or httpd (for 2.2). Make note of
the child worker's PID.
- Using the {pid} number you noted above, invoke the command
drwtsn32 -p {pid}
Voila, you will find in your 'log file path' a
drwtsn32.log file, and if you choose to 'append to
existing log file', jump through the 'App:' sections until you find
the one for the process you just killed. Now you can identify
about where 'Stack Back Trace' points to help identify what the server
is doing.
You will note that many threads look identical, almost all of them
polling for the next connection, and you don't care about those. You will
want to see the ones that are deep inside of a request at the time you
kill them, and only the stack back trace entries for those. This can
give folks a clue of where that request is hanging, which handler
module picked up the request, and what filter it might be stuck in.
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Debugging intermittent crashes
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For situations where a child process is crashing intermittently,
the server must be configured and started such that it produces core
dumps which can be analyzed further.
To ensure that a core dump is written to a directory which is
writable by the user which child processes run as (such as
apache ), the CoreDumpDirectory
directive must be added to httpd.conf ; for example:
CoreDumpDirectory /tmp
Before starting up the server, any process limits on core dump file
size must be lifted; for example:
# ulimit -c unlimited
# apachectl start
On some platforms, further steps might be needed to enable core
dumps - see Solaris and coredumps below.
When a child process crashes, a message like the following will be
logged to the error_log:
[Mon Sep 05 13:35:39 2005] [notice] child pid 2027 exit signal Segmentation fault (11), possible coredump in /tmp
If the text "possible coredump in /tmp" does not appear in the
error line, check that the ulimit was set correctly, that the
permissions on the configured CoreDumpDirectory are
suitable and that platform specific steps
(Solaris and coredumps) have been done if needed.
To analyse the core dump, pass the core dump filename on the gdb
command-line, and enter the command bt full at the gdb
prompt:
% gdb /usr/local/apache2/bin/httpd /tmp/core.2027
...
Core was generated by `/usr/local/apache2/bin/httpd -k start'
...
(gdb) bt full
If attempting to debug a threaded server, for example when using
the worker MPM, use the following gdb command:
(gdb) thread apply all bt full
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Using 'truss/trace/strace' to trace system calls and
signals
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Most Unix-based systems have at least one command for displaying
a trace of system calls and signals as they are accessed by a running
process. This command is called truss on most SVR4-based
systems and either trace or strace on many
other systems.
A useful tip for using the truss command on Solaris is
the -f option (often also works with strace ); it tells
truss to follow and continue tracing any child processes forked by the main
process. The easiest way to get a full trace of a server is to do something
like:
% truss -f httpd -d /usr/local/apache >& outfile
% egrep '^10698:' outfile
to view just the trace of the process id 10698.
If attempting to truss a threaded server, for example when using
the worker MPM, the truss option -l
is very useful as it prints also the LWP id after the process id. You can use
something like
% egrep '^10698/1:' outfile
to view just the trace of the process id 10698 and LWP id 1.
Other useful options for truss are
-
-a to print all command line parameters used for this executable.
-
-e to print all environment variables used for this executable.
-
-d to print timestamps.
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Getting the server to dump core
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Strangely enough, sometimes you actually want to force the server
to crash so that you can get a look at some nutty behavior. Normally
this can be done simply by using the gcore command.
However, for security reasons, most Unix systems do not allow a setuid
process to dump core, since the file contents might reveal something
that is supposed to be protected in memory.
Here is one way to get a core file from a setuid Apache httpd process
on Solaris, without knowing which httpd child might be the one to die
[note: it is probably easier to use the MaxClients trick in the first
section above].
# for pid in `ps -eaf | fgrep httpd | cut -d' ' -f4`
do
truss -f -l -t\!all -S SIGSEGV -p $pid 2>&1 | egrep SIGSEGV &
done
The
undocumented '-S' flag to truss will halt the process in place
upon receipt of a given signal (SIGSEGV in this case).
At this point you can use:
# gcore PID
and then look at the backtrace as discussed above for
gdb.
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Solaris and coredumps
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On Solaris (at least 2.7 and up) use coreadm to make
setuid() processes actually dump core. By default a setuid()
process does not dump core. This is the reason why httpd servers started as
root with child processes running as a different user (such as
apache ) do not coredump even when the
CoreDumpDirectory
directive had been set to an appropriate and writable directory and
ulimit -c has a sufficient size. See also
Debugging intermittent crashes above.
Jens-Uwe Mager wrote:
For example I am using:
# coreadm
global core file pattern: /var/core/core.%f.%p.u%u
init core file pattern: core
global core dumps: enabled
per-process core dumps: enabled
global setid core dumps: enabled
per-process setid core dumps: enabled
global core dump logging: disabled
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Getting and analyzing a TCP packet trace
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This is more difficult than I have time to describe at the moment.
Here are some pointers to useful discussions and tools:
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snoop is a packet sniffer that is part of Solaris.
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tcpdump is a packet sniffer that is
available for Unix-based systems and Windows
(windump).
It is part of many free Unix-based distributions.
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Wireshark is another packet sniffer
that is available for Unix-based systems and Windows. It has a nice GUI and
allows the analysis of the sniffed data.
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tcptrace is a TCP dump file analysis tool.
- tcpshow is
another one.
There is also a simple ASCII viewer for TCP dump traces in the Apache
repository in the file
src/test/tcpdumpscii.txt .
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