compile/2: add limit option

By default, limit filter length to an arbitrary value of 8192 bytes.

Using very large filters with pcap 1.7.4 can result in a segmentation
fault:

    epcap_compile:compile(string:copies("ip and ", 50000) ++ "ip").

The most likely cause is a stack overflow. pcap_compile() in pcap 1.8.1
is thread-safe. Presumably this means memory is allocated on the heap so
as a consequence, the stack overflow may be fixed. I haven't debugged
this further yet.

The limit can be disabled by using a value of -1 as the limit option.

Some notes:

    % {ok, _}
    epcap_compile:compile(string:copies("ip or ", 50000) ++ "ip").

    % segfault
    epcap_compile:compile(string:copies("ip or ", 60000) ++ "ip").

    % {error, "out of memory"}
    epcap_compile:compile(string:copies("tcp and ", 50000) ++ "ip").

    # shell
    ulimit -s
    8192

    (seq 1 50000 | while read l; do printf "ip and "; done; echo "ip") > filter

    # works
    sudo tcpdump -d -F filter

    ulimit -s 1024
    sudo tcpdump -d -F filter
    Segmentation fault (core dumped)

README.md

@@ -11,10 +11,13 @@ Since the library passes the filter string to pcap\_compile(3PCAP)
 directly, any bugs in pcap\_compile() may cause the Erlang VM to crash. Do
 not use filters from untrusted sources.
 
-Also note very large filters may block the scheduler. For example:
+Also note very large filters may block the scheduler or cause a stack
+overflow. For example:
 
     epcap_compile:compile(string:copies("ip and ", 50000) ++ "ip").
 
+Filters larger than 8192 bytes will not be be allowed by default. See the
+`limit` option to compile/2.
 
 ## REQUIREMENTS
 
@@ -55,6 +58,7 @@ These libraries are not required but can be used with epcap\_compile:
                     | {netmask, IPaddr}
                     | {dlt, integer()}
                     | {snaplen, integer()}
+                    | {limit, integer()}
 
         Filter is a string in pcap-filter(7) format.
 
@@ -75,6 +79,10 @@ These libraries are not required but can be used with epcap\_compile:
 
             * a packet length of 65535 bytes
 
+            * a limit of 8192 bytes for filters. Filters larger than
+              this limit will return `{error, enomem}`. A limit less than
+              0 disables the length check.
+
         See pcap_compile(3PCAP) for information about each of these options.
 
 

src/epcap_compile.erl

@@ -1,4 +1,4 @@
-%% Copyright (c) 2012-2016, Michael Santos <michael.santos@gmail.com>
+%% Copyright (c) 2012-2017, Michael Santos <michael.santos@gmail.com>
 %% All rights reserved.
 %%
 %% Redistribution and use in source and binary forms, with or without
@@ -61,7 +61,8 @@ compile(Filter) ->
         {optimize, true | false} |
         {netmask, non_neg_integer()} |
         {dlt, integer()} |
-        {snaplen, integer()}
+        {snaplen, integer()} |
+        {limit, integer()}
     ].
 -spec compile(Filter :: iodata(), compile_options())
     -> {ok, [binary()]} | {error, string()}.
@@ -71,8 +72,14 @@ compile(Filter, Options) when is_binary(Filter); is_list(Filter) ->
             ?PCAP_NETMASK_UNKNOWN)),
     Linktype = proplists:get_value(dlt, Options, ?DLT_EN10MB),
     Snaplen = proplists:get_value(snaplen, Options, 16#ffff),
+    Limit = proplists:get_value(limit, Options, 8192),
 
-    pcap_compile(Filter, Optimize, Netmask, Linktype, Snaplen).
+    case iolist_size(Filter) < Limit orelse Limit < 0 of
+        true ->
+            pcap_compile(Filter, Optimize, Netmask, Linktype, Snaplen);
+        false ->
+            {error, enomem}
+    end.
 
 bool(true) -> 1;
 bool(false) -> 0.