Documentation

You can view the documentation below, or browse our GitHub Repository, where you can contribute to FAQ Documents. For complete documentation, download the ClamAV Manual.

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:

Hunting for Memory Leaks

You can easily hunt for memory leaks with valgrind. Check out this guide to get started:

  • Valgrind Quick Start 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 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

Profiling - Flame Graphs

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 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 from FOSDEM 2017.