Re: static musl-based gdb and -fPIC

["writeonce@midipix.org" <writeonce@midipix.org>]

On 04/20/2014 04:31 PM, Rich Felker wrote:
> On Sun, Apr 20, 2014 at 01:03:12PM -0400, writeonce@midipix.org wrote:
>> Greetings,
>>
>> While building a statically linked musl-based gdb, ld asked that
>> libc.a be recompiled with -fPIC.
> This is a bug in the gdb build process. Despite your request for a
> static gdb, it's trying to build a shared library for something.
> There's a way to disable it (IIRC --disable-gdbserver is a big hammer
> that can do it, and there might be a more fine-grained approach) but
> the real issue is that the build process is broken and doing something
> that can't work.

Thanks!  The offending part was indeed in gdbserver.  With 
--disable-gdbserver, -fPIC is no longer necessary.  Looking into this, I 
thought the problem lied in gdb/gdbserver/Makefile.in

     LDFLAGS = @LDFLAGS@
     INTERNAL_LDFLAGS = $(LDFLAGS) @RDYNAMIC@

however removing @RDYNAMIC@ did not solve the issue, so for the time 
being gdbserver will have to be disabled.

For the record: python's Modules/posixmodule.c has a static 
implementation of posix_close that is incompatible with musl's.  My 
first take on that was to make python use musl's posix_close, which 
resulted in a very subtle bug leading to a segmentation fault (not to 
mention all of those lost hours...)  Renaming the module's posix_close 
to __posix_close solved the problem.  The code that triggered the bug was

     import subprocess
     subprocess.Popen(['ls'])

With the newer approach (__posix_close) everything seems to work fine.

Thanks again,
zg


>
>> After recompiling musl with the
>> above flag, gdb built successfully.  The reason I wanted to have a
>> static gdb (other than the trivial ones) was to be able to debug a
>> musl-based python.  The distribution's gdb has a dynamic dependency
>> on a glibc-based libpython, and the two friends don't play well
>> together.
>>
>> Now that the static gdb is up and running, my questions are:
>>
>> 1) is there any reason not to "always" compile musl with -fPIC, at
>> least on x86_64?
> Compare the .lo and .o files. I think you'll find the .lo files are a
> considerably more bloated and slower -- not as bad as on 32-bit x86,
> but still undesirable.
>
> Some users will want to use -fPIC even for static linking to be able
> to produce static PIE binaries, but this is not a mainstream usage
> (there's not even any official toolchain support for it, just a local
> hack I posted to the list a year or two back) and not something we
> would want to impose on everyone.
>
>> 2) is there any reason to revert to the old build of libc.so?
>> Although I rebuilt musl because of libc.a, it turns out that the
>> -fPIC flag also helped libc.so become much smaller: 699299 bytes,
>> instead of 2767910 bytes (musl v1.0.0, binutils v2.24).  Any other
>> factors to consider?
> You must have done something else like disabling debugging info at the
> same time.
>
> Rich
>
>