Compiling with '-pack' and '-a x.ml -o x.cma' succeeds and but the executable segfaults

[vicuna]

Original bug ID: 7479
Reporter: etc
Status: resolved (set by @xavierleroy on 2017-02-19T16:50:01Z)
Resolution: fixed
Priority: normal
Severity: minor
Platform: Linux
OS: arch linux
OS Version: 4.7.2-1
Version: 4.02.3
Target version: 4.05.0 +dev/beta1/beta2/beta3/rc1
Fixed in version: 4.05.0 +dev/beta1/beta2/beta3/rc1
Category: back end (clambda to assembly)

Bug description

Compiling with -pack and -a x.ml -o x.cma will succeed but when it's linked and used it will segfault.

I found that by changing the makefile to use -c x.ml -o x.cmo instead is how it's supposed to be done. Maybe compiling with -a shouldn't succeed when it's also -pack?

Steps to reproduce

Clone this git repository: https://github.com/Charlesetc/ocaml-segfault/

$ make
$ ./test.byte

Additional information

On the same git repo, there's a "success" branch https://github.com/Charlesetc/ocaml-segfault/tree/success, which has a change in the command that actually works. Thanks for taking a look!

[vicuna]

Comment author: @xavierleroy

As far as I see the problem comes from
ocamlc -I pear -pack pear/grape.cma -o pear.cma
i.e. an attempt to build a pack from multiple compilation units found in grape.cma.

(By the way, -pack always produces a single object file, so the output should be "-o pear.cmo". But .cmo and .cma have different magic numbers so ocamlc is not confused.)

Here we get a pear.cmi that contains
module Pear : sig module Grape : sig ... end end
but a pear.cmo/pear.cma that contains
0 CONST0
1 MAKEBLOCK1 0
3 SETGLOBAL Pear
i.e. that sets Pear = struct end (the empty module). Hence the segfault when Pear is used in Main.

[vicuna]

Comment author: @xavierleroy

Also: you write "Compiling with -pack and -a x.ml -o x.cma... will segfault", but this is not what your repro is testing. Are there other problems with the combination you describe but do not reproduce?

[vicuna]

Comment author: @xavierleroy

Fixed the easy way in 4.05: ocamlc -pack rejects arguments that are .cma files, and ocamlopt -pack rejects arguments that are .cmxa files.