makes bap-extra packages optional to facilitate local development

Eases local dune-base developement, so that you can easily start local
development without too much external dependencies hassle. Allows you
to disable bap-extra packages and just do `dune build` and everything
should work.

For the context, Dune doesn't provide nice options for multi-project
monorepos. The dune files represent the static information about the
project and it is presumed that there is an external tool that
generates them. Cf., tezos manifest that generates dune files from the
OCaml EDSL or whatever they use in Janestreet (jenga?). Originally,
the plan was to implement the same mechanism, e.g., generate the
dune-project file during the configuration phase. But right now I
don't have enough time for that, so I am pushing a simple
workaround. All packages in bap-extra are now marked as allow_empty,
indicating that they can produce now artifacts. Their corresponding
libraries are now optional (if a dependency is not met, then the
library is silently not built). The only special case is ghidra that
doesn't have dependencies trackable by Dune (i.e., OCaml libraries,
besides base and bap), so to prevent it from building I added a small
check that ghidra library is installed.

Author: ivg

README.md

@@ -71,9 +71,15 @@ git clone git@github.com:BinaryAnalysisPlatform/bap.git && cd bap
 opam install . --deps-only
 dune build && dune install
 ```
-
 to install bap and its dependencies into the currently selected switch.
 
+The above command will build all packages available in bap, including experimental packages like Ghidra or Radare (included in the bap-extra) meta-package. To prepare development envionment that excludes these packages (or any other packages) just remove corresponding .opam files (don't worry, opam will regenerate them later), before creating a switch e.g.,
+```
+rm bap-{extra,radare2,ghidra,primus-symbolic-executor,ida}.opam
+opam switch create . --deps-only
+dune build && dune install
+```
+
 ## Using
 
 BAP, like Docker or Git, is driven by a single command-line utility called [bap][man-bap]. Just type `bap` in your shell and it will print a message which shows BAP capabilities. The `disassemble` command will take a binary program, disassemble it, lift it into the intermediate architecture agnostic representation, build a control flow graph, and finally apply staged user-defined analysis in a form of disassembling passes. Finally, the `--dump` option (`-d` in short) will output the resulting program in the specified format. This is the default command, so you don't even need to specify it, e.g., the following will disassembled and dump the `/bin/echo` binary on your machine:

dune-project

@@ -565,6 +565,7 @@ be seen as a common denominator between Lwt and Async libraries.")
  (name bap-ghidra)
  (synopsis "The BAP NSA Ghidra backend")
  (tags (bap ghidra disassembler))
+ (allow_empty)
  (depends
   (core_kernel (and (>= v0.14) (< v0.16)))
   (ppx_bap (= :version))
@@ -996,6 +997,7 @@ be seen as a common denominator between Lwt and Async libraries.")
  (name bap-primus-symbolic-executor)
  (synopsis "A BAP Primus symbolic executor")
  (tags (bap bap-primus))
+ (allow_empty)
  (depends
   (bap-common (= :version))
   (bap-primus (= :version))
@@ -1096,6 +1098,7 @@ be seen as a common denominator between Lwt and Async libraries.")
  (name bap-radare2)
  (synopsis "BAP Radare2 integration")
  (tags (bap bap-plugin radare2))
+ (allow_empty)
  (depends
   (bap-abi (= :version))
   (bap-arm (= :version))

lib/bap_ghidra/dune

@@ -1,3 +1,6 @@
+(* -*- tuareg -*- *)
+
+let () = Format.kasprintf Jbuild_plugin.V1.send {|
 (library
  (name bap_ghidra)
  (public_name bap-ghidra)
@@ -12,4 +15,6 @@
  (foreign_stubs
   (language c)
   (names ghidra_stubs))
- (c_library_flags :standard -L/usr/lib/ghidra -ldecomp))
+ (c_library_flags :standard -L/usr/lib/ghidra -ldecomp)
+ (enabled_if %b))
+|} (Sys.file_exists "/usr/lib/ghidra")

plugins/ghidra/dune

@@ -1,9 +1,11 @@
 (library
+ (optional)
  (name bap_ghidra_plugin)
  (public_name bap-ghidra.plugin)
  (libraries bap bap-ghidra bap-main core_kernel))
 
 (plugin
+ (optional)
  (name ghidra)
  (package bap-ghidra)
  (libraries bap-ghidra.plugin)

plugins/primus_symbolic_executor/dune

@@ -1,4 +1,5 @@
 (library
+ (optional)
  (name bap_primus_symbolic_executor_plugin)
  (public_name bap-primus-symbolic-executor.plugin)
  (preprocess (pps ppx_bap))
@@ -16,6 +17,7 @@
    bap-core-theory))
 
 (plugin
+ (optional)
  (name primus-symbolic-executor)
  (package bap-primus-symbolic-executor)
  (libraries bap-primus-symbolic-executor.plugin)

plugins/radare2/dune

@@ -1,4 +1,5 @@
 (library
+ (optional)
  (name bap_radare2_plugin)
  (public_name bap-radare2.plugin)
  (libraries
@@ -17,6 +18,7 @@
    zarith))
 
 (plugin
+ (optional)
  (name radare2)
  (package bap-radare2)
  (libraries bap-radare2.plugin)