clamav/docs/UserManual/development.md

# ClamAV Development
This page aims to provide information useful when developing, debugging, or
profiling ClamAV.

## Building ClamAV for Development
Below are some recommendations for building ClamAV so that it's easy to debug.

### Satisfying Build Dependencies
To satisify all build dependencies:

#### Debian/Ubuntu:
```
sudo apt-get install libxml2-dev libxml2 libbz2-dev bzip2 check make libssl-dev openssl zlib1g zlib1g-dev gcc gettext autoconf automake libtool cmake autoconf-archive pkg-config g++-multilib libmilter1.0.1 libmilter-dev valgrind libcurl4-openssl-dev libjson-c-dev ncurses-dev libpcre3-dev
```

#### CentOS/RHEL/Fedora
```
sudo yum install libxml2-devel libxml2 bzip2-devel bzip2 check make openssl-devel openssl zlib zlib-devel gcc gettext autoconf automake libtool cmake autoreconf pkg-config g++-multilib sendmail sendmail-devel libtool-ltdl-devel valgrind

sudo yum groupinstall "Development Tools"
```

#### Solaris (using OpenCSW)
```
sudo /opt/csw/bin/pkgutil -y -i common coreutils automake autoconf libxml2_2 libxml2_dev bzip2 libbz2_dev libcheck0 libcheck_dev gmake cmake libssl1_0_0 libssl_dev openssl_utilslibgcc_s1 libiconv2 zlib1 libstdc++6 libpcre1 libltdl7 lzlib_stub zlib_stub libmilter libtool ggrep gsed pkgconfig ggettext gcc4core gcc4g++ libgcc_s1 libgccpp1

sudo pkg install system/header

sudo ln -sf /opt/csw/bin/gnm /usr/bin/nm
sudo ln -sf /opt/csw/bin/gsed /usr/bin/sed
sudo ln -sf /opt/csw/bin/gmake /usr/bin/make
```
If you receive an error message like
`gcc: error: /opt/csw/lib/libstdc++.so: No such file or directory`,
change versions with `/opt/csw/sbin/alternatives --config automake`

#### FreeBSD
The easiest way to install dependencies for FreeBSD is to just rely on ports:
```
cd /usr/ports/security/clamav
make
```

### Download the Source
```
git clone https://github.com/Cisco-Talos/clamav-devel.git
cd clamav-devel
```

If you intend to make changes and submit a pull request, fork the clamav-devel
repo first and then clone your fork of the repository.

### Running ./configure
Suggestions:

- Modify the CFLAGS variable as follows (assuming you're build with gcc):

    - Include gdb debugging information (`-ggdb`).  This will make it easier
      to debug with gdb

    - Disable optimizations (`-O0`).  This will ensure the line numbers you
      see in gdb match up with what is actually being executed.

- Run configure with the following options:

    - ``--prefix=`pwd`/build``: This will cause `make install` to install
      into the specified directory to avoid potentially tainting a release
      install of ClamAV that you may have.

    - `--enable-debug`: This will define *CL_DEBUG*, which mostly just enables
      additional print statements that are useful for debugging

    - `--enable-check`: Enables the unit tests, which can be run with 'make
      check'

    - `--enable-coverage`: If using gcc, sets `-fprofile-arcs -ftest-coverage`
      so that code coverage metrics will get generated when the program is run.
      Note that the code inserted to store program flow data may show up in
      any generated flame graphs or profiling output, so if you don't care
      about code coverage, omit this

    - `--enable-libjson`: Enables libjson, which enables the `--gen-json` option.
      The json output contains additional metadata that might be helpful when
      debugging.

    - `--with-systemdsystemunitdir=no`: Don't try to register clamd as a
      systemd service (on systems that use systemd). You likely don't want this
      development build of clamd to register as a service, and this eliminates
      the need to run `make install` with `sudo`.

    - You might want to include the following flags also so that the optional
      functionality is enabled: `--enable-experimental --enable-clamdtop
      --enable-libjson --enable-milter --enable-xml --enable-pcre`.
      Note that this may require you to install additional development
      libraries.

    - `--disable-llvm`: When enabled, LLVM provides the capability to
      just-in-time compile ClamAV bytecode signatures. Without LLVM, ClamAV
      uses a built-in bytecode interpreter to execute bytecode signatures.
      The mechanism is different, but the results are same and the performance
      overall is comparable.  At present only LLVM versions up to LLVM 3.6.2
      are supported by ClamAV, and LLVM 3.6.2 is old enough that newer
      distributions no longer provide it. Therefore, we recommend using
      the `--disable-llvm` configure option.

Altogether, the following configure command can be used:

```
CFLAGS="-ggdb -O0" ./configure --prefix=`pwd`/installed --enable-debug --enable-check --enable-coverage --enable-libjson --with-systemdsystemunitdir=no --enable-experimental --enable-clamdtop --enable-libjson --enable-xml --enable-pcre --disable-llvm
```

NOTE: It is possible to build libclamav as a static library and have it
statically linked into clamscan/clamd (to do this, run `./configure` with
`--enable-static --disable-shared`).  This is useful for using tools like gprof
that do not support profiling code in shared objects.  However, there are two
drawbacks to doing this:

 - clamscan/clamd will not be able to extract files from RAR archives.  Based
   on the software license of the unrar library that ClamAV uses, the library
   can only be dynamically loaded.  ClamAV will attempt to dlopen the unrar
   library shared object and will continue on without RAR extraction support
   if the library can't be found (or if it doesn't get built, which is what
   happens if you indicate that shared libraries should not be built).

 - If you make changes to libclamav, you'll need to `make clean`, `make`, and
   `make install` again to have clamscan/clamd rebuilt using the new
   libclamav.a.  The makefiles don't seem to know to rebuild clamscan/clamd
   when libclamav.a changes (TODO, fix this).

### Running make
Run the following to finishing building.  `-j2` in the code below is used to
indicate that the build process should use 2 cores.  Increase this if your
machine is more powerful.
```
make -j2
make install
```
Also, you can run 'make check' to run the unit tests

### Downloading the Official Ruleset
If you plan to use custom rules for testing, you can invoke clamscan via
`./installed/bin/clamscan`, specifying your custom rule files via `-d` parameters.

If you want to download the official ruleset to use with clamscan, do the
following:
1. Run `mkdir -p installed/share/clamav`
2. Comment out line 8 of etc/freshclam.conf.sample
3. Run `./installed/bin/freshclam --config-file etc/freshclam.conf.sample`

## General Debugging
NOTE: Some of the debugging/profiling tools mentioned in the sections below are
specific to Linux

### Useful clamscan Flags
The following are useful flags to include when debugging clamscan:

- `--debug --verbose`: Print lots of helpful debug information

- `--gen-json`: Print some additional debug information in a JSON format

- `--statistics=pcre --statistics=bytecode`:  Print execution statistics on any
  PCRE and bytecode rules that were evaluated

- `--dev-performance`: Print per-file statistics regarding how long scanning
  took and the times spent in various scanning stages

- `--detect-broken`: This will attempt to detect broken executable files.  If
  an executable is determined to be broken, some functionality might not get
  invoked for the sample, and this could be an indication of an issue parsing
  the PE header or file.  This causes those binary to generate an alert instead
  of just continuing on.  NOTE: This will be renamed to `--alert-broken`
  starting in ClamAV 0.101.

- `--max-filesize=2000M --max-scansize=2000M --max-files=2000000
   --max-recursion=2000000 --max-embeddedpe=2000M --max-htmlnormalize=2000000
   --max-htmlnotags=2000000 --max-scriptnormalize=2000000
   --max-ziptypercg=2000000 --max-partitions=2000000 --max-iconspe=2000000
   --max-rechwp3=2000000 --pcre-match-limit=2000000
   --pcre-recmatch-limit=2000000 --pcre-max-filesize=2000M`:
  Effectively disables all file limits and maximums for scanning.  This is
  useful if you'd like to ensure that all files in a set get scanned, and would
  prefer clam to just run slowly or crash rather than skip a file because it
  encounters one of these thresholds

The following are useful flags to include when debugging rules that you're
writing:

- `-d`: Allows you to specify a custom ClamAV rule file from the command line

- `--bytecode-unsigned`: If you are testing custom bytecode rules, you'll need
  this flag so that clamscan actually runs the bytecode signature

- `--all-match`: Allows multiple signatures to match on a file being scanned

- `--leave-temps --tmpdir=/tmp`: By default, ClamAV will attempt to extract
  embedded files that it finds, normalize certain text files before looking
  for matches, and unpack packed executables that it has unpacking support for.
  These flags tell ClamAV to write these intermediate files out to the
  directory specified.  Usually when a file is written, it will mention the
  file name in the --debug output, so you can have some idea at what stage in
  the scanning process a tmp file was created.

- `--dump-certs`: For signed PE files that match a rule, display information
  about the certificates stored within the binary.  Note - sigtool has this
  functionality as well and doesn't require a rule match to view the cert data

### Using gdb
Given that you might want to pass a lot of arguments to gdb, consider taking
advantage of the `--args` parameter.  For example:
```
gdb --args ./installed/bin/clamscan -d /tmp/test.ldb -d /tmp/blacklist.crb -d --dumpcerts --debug --verbose --max-filesize=2000M --max-scansize=2000M --max-files=2000000 --max-recursion=2000000 --max-embeddedpe=2000M --max-iconspe=2000000 f8f101166fec5785b4e240e4b9e748fb6c14fdc3cd7815d74205fc59ce121515
```

When using ClamAV without libclamav statically linked, if you set breakpoints
on libclamav functions by name, you'll need to make sure to indicate that
the breakpoints should be resolved after libraries have been loaded.

For other documentation about how to use gdb, check out the following
resources:
 - [A Guide to gdb](http://www.cabrillo.edu/~shodges/cs19/progs/guide_to_gdb_1.1.pdf)
 - [gdb Quick Reference](http://users.ece.utexas.edu/~adnan/gdb-refcard.pdf)

## Hunting for Memory Leaks
You can easily hunt for memory leaks with valgrind.  Check out this guide to
get started:
 - [Valgrind Quick Start](http://valgrind.org/docs/manual/quick-start.html)
If checking for leaks, be sure to run clamscan with samples that will hit as
many of the unique code paths in the code you are testing.  An example
invocation is as follows:
```
valgrind --leak-check=full ./installed/bin/clamscan -d /tmp/test.ldb --leave-temps --tempdir /tmp/test --debug --verbose /tmp/upx-samples/ > /tmp/upx-results-2.txt 2>&1
```
Alternatively, on Linux, you can use glibc's built-in leak checking
functionality:
```
MALLOC_CHECK_=7 ./installed/bin/clamscan
```
See the [mallopt man page](http://manpages.ubuntu.com/manpages/trusty/man3/mallopt.3.html) for more details

## Computing Code Coverage
gcov/lcov can be used to produce a code coverage report indicating which lines
of code were executed on a single run or by multiple runs of clamscan.  NOTE:
for these metrics to be collected, ClamAV needs to have been configured with
the `--enable-coverage` option.

First, run the following to zero out all of the performance metrics:
```
lcov -z --directory . --output-file coverage.lcov.data
```
Next, run ClamAV through whatever test cases you have.  Then, run lcov again
to collect the coverage data as follows:
```
lcov -c --directory . --output-file coverage.lcov.data
```
Finally, run the genhtml tool that ships with lcov to produce the code coverage
report:
```
genhtml coverage.lcov.data --output-directory report
```
The report directory will have an index.html page which can be loaded into any
web browser.

For more information, visit the [lcov webpage](http://ltp.sourceforge.net/coverage/lcov.php)

## Profiling - Flame Graphs
[FlameGraph](https://github.com/brendangregg/FlameGraph) is a great tool for
generating interactive flame graphs based collected profiling data.  The github
page has thorough documentation on how to use the tool, but an overview is
presented below:

First, install perf, which on Linux can be done via:
```
apt-get install linux-tools-common linux-tools-generic linux-tools-`uname -r`
```

Modify the system settings to allow perf record to be run by a standard user:
```
$ sudo su
# cat /proc/sys/kernel/perf_event_paranoid 
# echo "1" > /proc/sys/kernel/perf_event_paranoid 
# exit
```

Invoke clamscan via perf record as follows, and run perf script to collect the
profiling data:
```
perf record -F 100 -g -- ./installed/bin/clamscan -d /tmp/test.ldb /tmp/2aa6b18d509090c60c3e4ecdd8aeb16e5f149807e3404c86892112710eab576d
perf script > out.perf
```
The '-F' parameter indicates how many samples should be collected during
program execution.  If your scan will take a long time to run, a lower value
should be sufficient.  Otherwise, consider choosing a higher value (on Ubuntu
18.04, 7250 is the max frequency, but it can be increased via
/proc/sys/kernel/perf_event_max_sample_rate.

Check out the FlameGraph project and run the following commands to generate
the flame graph:
```
perl stackcollapse-perf.pl ../clamav-devel/out.perf > /tmp/out.folded
perl flamegraph.pl /tmp/out.folded > /tmp/test.svg
```

The SVG that is generated is interactive, but some viewers don't support this.
Be sure to open it in a web browser like Chrome to be able to take full
advantage of it.

## Profiling - Callgrind
Callgrind is a profiling tool included with valgrind.  This can be done by
prepending `valgrind --tool=callgrind ` to the clamscan command.
[kcachegrind](https://kcachegrind.github.io/html/Home.html)
is a follow-on tool that will graphically present the
profiling data and allow you to explore it visually, although if you don't
already use KDE you'll have to install lots of extra packages to use it.

## System Call Tracing / Fault Injection
strace can be used to track the system calls that are performed and provide the
number of calls / time spent in each system call.  This can be done by
prepending `strace -c ` to a clamscan command.  Results will look something
like this:
```
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 95.04    0.831430          13     62518           read
  3.22    0.028172          14      2053           munmap
  0.69    0.006005           3      2102           mmap
  0.28    0.002420           7       344           pread64
  0.16    0.001415           5       305         1 openat
  0.13    0.001108           3       405           write
  0.11    0.000932          23        40           mprotect
  0.07    0.000632           2       310           close
  0.07    0.000583           9        67        30 access
  0.05    0.000395           1       444           lseek
  0.04    0.000344           2       162           fstat
  0.04    0.000338           1       253           brk
  0.03    0.000262           1       422           fcntl
  0.02    0.000218          16        14           futex
  0.01    0.000119           1       212           getpid
  0.01    0.000086          14         6           getdents
  0.00    0.000043           7         6           dup
  0.00    0.000040           1        31           unlink
  0.00    0.000038          19         2           rt_sigaction
  0.00    0.000037          19         2           rt_sigprocmask
  0.00    0.000029           1        37           stat
  0.00    0.000022          11         2           prlimit64
  0.00    0.000021          21         1           sysinfo
  0.00    0.000020           1        33           clock_gettime
  0.00    0.000019          19         1           arch_prctl
  0.00    0.000018          18         1           set_tid_address
  0.00    0.000018          18         1           set_robust_list
  0.00    0.000013           0        60           lstat
  0.00    0.000011           0        65           madvise
  0.00    0.000002           0        68           geteuid
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         1           uname
  0.00    0.000000           0         1           getcwd
------ ----------- ----------- --------- --------- ----------------
100.00    0.874790                 69970        31 total
```

strace can also be used for cool things like system call fault injection.  For
instance, I was curious whether the 'read' bytecode API call was implemented
in such a way that the underlying read system call could handle EINTR being
returned (which can happen periodically).  To test this, I wrote the following
bytecode rule:
```
VIRUSNAME_PREFIX("BC.Heuristic.Test.Read.Passed")
VIRUSNAMES("")
TARGET(0)

SIGNATURES_DECL_BEGIN
DECLARE_SIGNATURE(zeroes)
SIGNATURES_DECL_END

SIGNATURES_DEF_BEGIN
DEFINE_SIGNATURE(zeroes, "0:0000")
SIGNATURES_DEF_END

bool logical_trigger()
{
    return matches(Signatures.zeroes);
}

#define READ_S(value, size) if (read(value, size) != size) return 0;

int entrypoint(void)
{
    char buffer[65536];
    int i;

    for (i = 0; i < 256; i++)
    {
        debug(i);
        debug("\n");
        READ_S(buffer, sizeof(buffer));
    }

    foundVirus("");
    return 0;
}
```
I compiled the rule, made a test file to match against, and ran it under
strace to determine what underlying read system call was used for the bytecode
read function:
```
clambc-compiler read_test.bc
dd if=/dev/zero of=/tmp/zeroes bs=65535 count=256
strace clamscan -d read_test.cbc --bytecode-unsigned /tmp/zeroes
```
It uses pread64 under the hood, so the following command could be used for fault
injection:
```
strace -e fault=pread64:error=EINTR:when=20+10 clamscan -d read_test.cbc --bytecode-unsigned /tmp/zeroes 
```
This command tells strace to skip the first 20 pread64 calls (these appear to
be used by the loader, which didn't seem to handle EINTR correctly) but to
inject EINTR for every 10th call afterward.  We can see the injection in action
and that the system call is retried successfully:
```
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15007744) = 65536
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15073280) = 65536
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15138816) = 65536
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15204352) = 65536
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15269888) = 65536
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15335424) = 65536
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15400960) = 65536
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15466496) = 65536
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15532032) = 65536
pread64(3, 0x7f6a7ff43000, 65536, 15597568) = -1 EINTR (Interrupted system call) (INJECTED)
pread64(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65536, 15597568) = 65536
```
More documentation on this feature can be found in
[this presentation](https://archive.fosdem.org/2017/schedule/event/failing_strace/attachments/slides/1630/export/events/attachments/failing_strace/slides/1630/strace_fosdem2017_ta_slides.pdf)
from FOSDEM 2017.