Debugging tips

Debug pre-auth IMAP with libexec/dovecot/imap

The imap executable can be started in IMAP PREAUTH mode. This circumvents login and authorization and lets you connect to the imap process directly, without having to go through the imap-login process. Start imap in PREAUTH with one of:

/usr/libexec/dovecot/imap -u <username>
src/imap/imap -u <username>
doveadm exec imap -u <username>

This also works for pop3 and managesieve. lmtp can also be started from command line without the -u parameter (because the recipient is given with RCPT TO command).

Debug pre-auth IMAP with a real client

  1. Run the following script

rm -f /tmp/f && mkfifo /tmp/f && \
cat /tmp/f | ./imap -u testuser | nc -l 143 > /tmp/f
  1. Connect to port 143 with the imap client of your choice

  2. Attach with gdb to the running imap process, if you so wish. This can also be used together with imaptest, if you define imaptest to use only one username. Some configurations or use cases might require dovecot to be running (e.g. for authentication). In those cases, select a different port for “nc” or change dovecot not to listen on 143.

Debug most/some dovecot executables inside the source tree.

To avoid having to make install dovecot before running an executable in gdb or valgrind, use libtool. For example:

libtool e gdb --args src/imap/imap

Maybe add a helpful alias to your .bashrc or similar:

alias lgdb='libtool --mode=execute gdb'

Debug dovecot master

GDB=1 gdb --args dovecot -F

Start with -F = foreground mode.

If you’ve compiled –with-devel-checks, GDB=1 environment needs to be set so it doesn’t fail when it detects extra “leaked” file descriptors.

Standalone non-root debugging environments

Dovecot can be instructed to run the imap handler as a non-root user, and therefore that binary can be debugged by that same non-root user. At the moment, only manual (telnet) interaction is possible. This setup is documented in Rootless Installation.

Disabling optimizations

gdb can be difficult if code is compiled with optimizations (-O2). Printing variables may fail and stepping over code can jump confusingly all over the place. The easiest way to avoid this is to just disable optimizations either for Dovecot entirely or just some files:

  • Globally: EXTRA_CFLAGS=-O0 ./configure ...

  • Within a directory: Remove -O2 from CFLAGS in Makefile

    • Edit/touch some .c file(s) in the directory to recompile, or make clean to rebuild everything in the directory

If you want to just select a few functions to be built with -O0, then create 2 files, src/lib/optimise-off.h and src/lib/options-restore.h as follows:

#if defined(__GNUC__)
#if (__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ > 4))
#pragma GCC push_options
#pragma GCC optimize (0)
#warning GCC not new enough to support local optimization pragmas


#if defined(__GNUC__)
#if (__GNUC__ > 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ > 4))
#pragma GCC pop_options
/* He already knows */

Then just use #include those files around just the functions you want un-optimised.

Another dirty and quick way to opt out the optimization is to use pragma to quote the code.

#pragma GCC push_options
#pragma GCC optimize ("O0")
 .... code
#pragma GCC pop_options

Debugging core dumps in other systems

You need the core dump, the binary that produced it and ALL the shared libraries on the system. For example:

(echo "info shared"; sleep 1) |
gdb $binary $core |
grep '^0x.*/' | sed 's,^[^/]*,,' |
xargs tar czf $dest --dereference $binary $core

In v2.3.10+ there is dovecot-sysreport <> that can do the same (and more):

dovecot-sysreport --core <binary> <core>

There’s also

Usage: ./ <binary> <core> <dest.tar.gz>

Debugging on the test server then ideally would have all the debuginfo packages (for exactly the same binaries). You can run gdb there with:

mkdir coretest
cd coretest
tar xzf ../core.tar.gz
gdb imap
set solib-absolute-prefix .
core imap.core
bt full

Debugging core dumps in other systems: Installing rpms in different distribution

If you have a tar.gz generated from dovecot-sysreport, you can debug it in any Linux distribution. But you still need to have the Dovecot debuginfo packages installed globally, which could be a bit tricky. With yum based systems you can setup /etc/yum.repos.d/dovecot.repo pointing to the repository you want according to Then you can install the packages easily with:

rpm -Uvh --nodeps $(repoquery --location dovecot-ee-debuginfo)

Scripting gdb for getting backtrace from many core dumps

When you have tens of core dumps, it’s getting a bit troublesome to manually get the backtraces. Here’s a script that takes a number of dovecot-sysreport-.tar.gz files as parameters and creates output files for them:

#!/bin/bash -e

for fname in $*; do
  mkdir tmp-gdb
  cd tmp-gdb
  tar xzf ../$fname
  core_path=$(find . -name '*core*')
  # FIXME: handles only libexec files - should also support doveadm at least
  binary_name=$(file $core_path \| grep "dovecot/" \| sed "s/^.*from 'dovecot\/\([^']*\).*$/\1/")
  cat <<EOF | gdb usr/libexec/dovecot/$binary_name > ../$
set pagination off
set solib-absolute-prefix .
core $core_path
bt full
  cd ..
  rm -rf tmp-gdb

Following deep inside structs

Dovecot implements classes/objects using C structs. For example there is the struct connection base object, which is extended with:

struct dict_connection {
  struct connection conn;

However, many places still refer to these extended objects using their base classes, so you need to cast them to get all of their wanted fields visible. For example:

(gdb) p dict_connections
$1 = (struct connection_list *) 0x55823025e9a0
(gdb) p dict_connections.connections
$2 = (struct connection *) 0x55823025c160
(gdb) p *dict_connections.connections
$3 = {prev = 0x0, next = 0x0, list = 0x55823025e9a0,
... the rest of struct connection
(gdb) p *(struct dict_connection *)dict_connections.connections
$4 = {conn = {prev = 0x0, next = 0x0, list = 0x55823025e9a0,
... the rest of struct dict_connection

It’s a bit more tricky to look inside dynamic array types. As an example lets consider ARRAY(struct dict_connection_cmd *) cmds. This ends up being expanded into:

struct array {
  buffer_t *buffer;
  size_t element_size;
union {
  struct array arr;
  struct dict_connection_cmd *const *v;
  struct dict_connection_cmd **v_modifiable;
} cmds;

You can find out the size of the array with:

p cmds.arr.buffer.used / cmds.arr.element_size

You can access the elements of the array with:

p *(*cmds.v)[0]
p *(*cmds.v)[1]
p *(*cmds.v)[...]

So to actually access the dict_connection.cmds array for the first connection in dict_connections, the gdb print commands get a bit long:

(gdb) p ((struct dict_connection *)dict_connections.connections).cmds
$5 = {arr = {buffer = 0x55823026da80, element_size = 8}, v = 0x55823026da80,
  v_modifiable = 0x55823026da80}

(gdb) p ((struct dict_connection *)dict_connections.connections).cmds.arr.buffer.used / 8
$6 = 1

(gdb) p *(*((struct dict_connection *)dict_connections.connections).cmds.v)[0]
$7 = {cmd = 0x55822ecc8b00 <cmds+16>, conn = 0x55823025c160, start_timeval = {
    tv_sec = 1632257119, tv_usec = 530341}, event = 0x558230280b98,
  reply = 0x0, iter = 0x0, iter_flags = 0, async_reply_id = 0, trans_id = 0,
  rows = 0, uncork_pending = false}

There can of course be multiple dict connections, which you can access by following the linked list:

(gdb) p *
(gdb) p *
(gdb) p *

Debugging data stack growth

Dovecot uses Data Stack to implement its own secondary stack. This stack is intended to usually stay rather small, ideally within its initial 32 kB size. There are data_stack_grow events sent when it grows. To debug why data stack grows, you can have it panic:

log_core_filter = event=data_stack_grow
# Or have it panic later:
log_core_filter = event=data_stack_grow and alloc_size >= 10240

The core dump can then be analyzed:

(gdb) p *current_frame
$1 = {prev = 0x555555874e78, block = 0x555555910760, block_space_left = 15640,
  last_alloc_size = 96, marker = 0x5555557e726c "data-stack.c:514",
  alloc_bytes = 96, alloc_count = 1}
(gdb) p *current_frame.prev
$2 = {prev = 0x555555874e18, block = 0x5555558742a0, block_space_left = 7264,
  last_alloc_size = 744, marker = 0x5555557c011f "index-storage.c:1056",
  alloc_bytes = 7312, alloc_count = 71}
(gdb) p *current_frame.prev.block
$3 = {prev = 0x0, next = 0x555555910760, size = 10240, left = 696,
  left_lowwater = 696, canary = 0xbadbadd5badbadd5, data = 0x5555558742d0 "8"}

First look at the block variable for these frames, and note how it changes for the 2rd one. So the data stack is grown between the 1nd and the 2rd frame. And since block_space_left was about 7 kB while the block’s full size was 10240 bytes, most of the space is allocated sometimes after index-storage.c:1056. We can also look further into the data stack frames to see if there are any other frames that use up a lot of memory:

(gdb) p *current_frame.prev.prev
$4 = {prev = 0x555555874db8, block = 0x5555558742a0, block_space_left = 7360,
  last_alloc_size = 96, marker = 0x5555557aef7c "mail-storage.c:2818",
  alloc_bytes = 96, alloc_count = 1}
$5 = {prev = 0x5555558743e0, block = 0x5555558742a0, block_space_left = 8440,
  last_alloc_size = 560, marker = 0x5555557a5467 "cmd-copy.c:328",
  alloc_bytes = 984, alloc_count = 6}
(gdb) p *current_frame.prev.prev.prev.prev.prev
$6 = {prev = 0x555555874338, block = 0x5555558742a0, block_space_left = 9976,
  last_alloc_size = 112, marker = 0x5555557a720f "imap-client.c:1357",
  alloc_bytes = 1536, alloc_count = 14}

So there was also some 1.5 kB used between imap-client.c:1357 and cmd-copy.c:328 which might be worth looking into.

Once you start debugging, get a gdb backtrace and start inserting further data stack frames into the function calls that the gdb backtrace shows. For example:

(gdb) bt
#0  data_stack_send_grow_event (last_alloc_size=744) at data-stack.c:400
#1  t_malloc_real (size=<optimized out>, permanent=<optimized out>)
    at data-stack.c:523
#10 0x000055555565257c in index_list_get_metadata (box=0x5555558ee8b0,
    items=MAILBOX_METADATA_CACHE_FIELDS, metadata_r=0x7fffffffe180)
    at mailbox-list-index-status.c:343
#11 0x00005555555ea928 in mailbox_get_metadata (box=0x5555558ee8b0,
    metadata_r=metadata_r@entry=0x7fffffffe180) at mail-storage.c:2204
#12 0x0000555555672794 in index_copy_cache_fields (
    ctx=ctx@entry=0x5555559093b0, src_mail=src_mail@entry=0x555555904408,
    dest_seq=1) at index-storage.c:1068

Here you can see that #1 matches is inside the data-stack.c:514 data stack frame and #12 is inside the index-storage.c:1056 data stack frame. So you could start placing more T_BEGIN { .. } T_END frames between #2 and #11 frames shown by gdb to get more details where the data stack is being used.