Stripping binaries


#1

Hi, I’d like to get recommendations from the community regarding stripping binaries. My use case is a program (for Unix and Mac OS X), built with dune/ocamlopt, the source code of which does not rely on dynamic linking nor on the FFI. However it relies on several opam packages and on the Unix module too, so I wondered whether I should be wary of something here. Are there good practices to rely upon? Or can I just run the standard strip command on my binary?


#2

Stripping binaries was very important back in the days when disk space was very limited. These days, I think it’s an antipattern. Why do you want to do it?


#3

Because disk space is limited for some people, for socio-economical or regulatory reasons for instance. But please let’s not debate on this topic here; I’m only asking for technical advice.


#4

I haven’t tried to strip OCaml binaries recently, but the last time I tried everything was fine with ocamlopt-compiled binaries. They should act more or less like a stripped binary compiled from C.


#5

This is what I do. Sometimes I need to transport binaries over the network, and strip reduces roughly 33% of the binary size.


#6

In 2016 I did a quick benchmark comparing binary sizes and start-up time of a hello-world OCaml program. Compared the original, strip, as well as upx tool which also archives the executable to reduce the size even more. I’ve also compared a bare-bones hello-world with the one where Jane Street Core was linked.

Here’s what I’ve got:

Original strip upx --best
OCaml 201K, 12ms 113K, 12ms 64K, 14ms
OCaml+Core 14M, 35ms 8.1M, 35ms 2.3M, 75ms

#7

Oh yeah even better, I had forgotten about this one. Unfortunately, my upx’ed program core dumps when launched?! The stripped program runs fine OTOH.


#8

Binary sizes of 14M vs 201k are absurd. Can someone explain how linking in OCaml works? Does linking always include all modules independent whether they are used or not? Or is Core so entangled that it is impossible to exclude certain modules?


#9

The key issue here is that OCaml overapproximates dependencies at the module level, which means that lots of functionality that isn’t use can nonetheless be linked in. There are some plans for better native dead-code elimination, but there’s nothing usable for that yet.

It’s worth noting that Base is already considerably lighter. My experiments show:

  • Base: 3.9M
  • Core_kernel: 8.5M
  • Core: 9.9M

In any case, as a general matter, if you don’t need the extra functionality of Core_kernel or Core, I think Base is considerably preferable, both because it’s lighter, and because it enforces some better idioms, like hiding polymorphic comparison by default, and encouraging better more modular idioms for building things like comparison-based container types.

There are some changes coming to Base in the next release that should make it yet lighter, though I’m not sure how far those will go.

y


#10

To be clear, though: OCaml doesn’t include any module you don’t reference, but anything you do reference, and anything transitively referenced by the modules you directly reference, will be included, whether or not you ever touch that functionality in your code.

y


#11

That’s unfortunate. There’s no good way to trace out what’s live and what isn’t as things stand?


#12

Yes it’s possible: see https://github.com/ocaml/ocaml/pull/608


#13

The only case where strip should not be used is for bytecode executables produced by ocamlc in -custom mode. Tehse are hybrid files composed of an ELF executable followed by OCaml bytecode, which confuses the strip command. But -custom mode is rarely used nowadays.

If your executable is produced by ocamlopt, nothing bad will happen if you run strip on it.


#14

Thanks, good to know.


#15

This looks rather compelling!


#16

Just to chime in with a security angle: Stripped binaries also provide fewer useful gadgets for exploit writers, and can have a significant impact on the viability of ROP exploits, especially when the would-be stripped sections are mapped executable (? like if you’re using mirage on solo5, and maybe also the -xen target ?).


#17

Stripped sections aren’t generally marked executable. (I am not sure they’re ever marked executable – if they were, how could you strip them? Indeed, if you can load it, it isn’t stripped.)

On your “especially”, if a stripped section isn’t loaded, how could it possibly be used for things like ROP exploits?


#18

No, the sections that are affected by stripping are not generally (highly dependent on which parameters you pass to strip, for instnace whether you strip relocations or not) marked as executable.
That’s why I gave an example of a case where the protection markings are not respected (solo5), and with my question marks I intended to convey that I don’t know how mirage-xen maps the sections, but I have feeling it might be the same there.

Re: my “especially”: Sorry if I was bit unclear: I’d argue that the impact of strip'ing is usually beneficial to security, so a stripped section (that’s not loaded) is one less section to be used in ROP attacks (when the section is mapped as executable), or data-only attacks (when the section is not mapped as executable).
Essentially the fewer bytes or sections mapped R/R+W/R+E/R+W+E in your process space, the better.


#19

I see no evidence for that whatsoever. Stripped sections are not loaded. If a stripped section needs to be loaded, then the executable will stop working. If it isn’t loaded, ROP attacks can’t target it in the first place. If it is loaded, that’s probably because the binary needs it or it won’t work.