ghc, rts/Linker.c, OpenBSD and WXNEEDED and NOPIE

[Ben Gamari]

Matthias Kilian <kili at outback.escape.de> writes:

> Hi,
>
Hi Matthias,

...
>
> However, after building ghc-8.6 with all the WXNNEDED stuff removed,
> and running ghci from this build, I still get an abort because pages
> are mapped both writable and executable (I've configured my system
> to abort processes when a W^X violation happens). Here's a backtrace
> in case it's helpfull at all:
>
> #0  _thread_sys___syscall () at -:3
> #1  0x00000002010bb179 in _libc_mmap (addr=Variable "addr" is not available.) at /usr/src/lib/libc/sys/mmap.c:47
> #2  0x000000021be22374 in mmapForLinker (bytes=Variable "bytes" is not available.) at rts/Linker.c:1029
> #3  0x000000021be3e99b in m32_allocator_init () at rts/linker/M32Alloc.c:160
> #4  0x00000002fd518be1 in cimc_info () from /usr/local/lib/ghc/bin/../ghci-8.6.4/libHSghci-8.6.4-ghc8.6.4.so
> #5  0x0000000000000000 in ?? ()
>
> Any thougts about this?
>
I suspect what you ran into here is GHCi loading a static archive:


I tried running GHCi compiled with debug information under GDB, set a
breakpoint on m32_allocator_init and found the following:

    >>> bt
    #0  m32_allocator_init () at rts/linker/M32Alloc.c:155
    #1  0x00007ffff2d680b7 in ghcizm8zi9zi0zi20191022_GHCiziRun_run1_info () from _build/stage1/lib/../lib/x86_64-linux-ghc-8.9.0.20191022/libHSghci-8.9.0.20191022-ghc8.9.0.20191022.so                                                                                          
    #2  0x0000000000000000 in ?? ()

While the backtrace doesn't reveal this, the caller appears to be
initLinker_ which I suspect is called by GHCi.ObjLink.initObjLinker
(which is in turn called by GHCi.Run.run).

I have opened #17407 to track this.

> And, since W|X (and nopie) isn't good on any system: is anyone
> working on getting rid of this for other unixes?
>
There has been some work on this on Darwin (see #13624). Unfortunately
no work has been put into ridding other platforms of W|X mappings (e.g.
see #12657).

It would be great if you felt you could contribute some help!

Cheers,

- Ben

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