[FFmpeg-devel] PIC and YASM

[Reimar Döffinger]

On Mon, Nov 09, 2009 at 10:16:33PM +0000, M?ns Rullg?rd wrote:
> Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:
> 
> > On Mon, Nov 09, 2009 at 08:26:05PM +0300, Michael Kostylev wrote:
> >> 
> >> On Mon Nov  9 18:02:54 2009
> >> Reimar D?ffinger wrote:
> >> 
> >> >>> @@ -1919,7 +1917,6 @@
> >> >>>          ;;
> >> >>>      openbsd)
> >> >>>          enable malloc_aligned
> >> >>> -        enable pic
> >> >> 
> >> >> OpenBSD builds fail without that currently.  Does your new __PIC__
> >> >> check fix that?
> >> > 
> >> > I expect
> >> 
> >> Unfounded.
> >
> > Interesting. I do not know 100% what OpenBSD does, but FFmpeg builds
> > just with with PIC disabled, the generated program just does not run.
> 
> I knew there was something fishy there.
> 
> > However, I'd like to suggest the change below.
> > Thing is, when PIC is enabled we already take the performance hit
> > anyway, so I suggest we at least try to also get the security benefit
> > by linking the executable programs as PIE.
> >
> > What do you think about that? Any known issues with -pie linker flag?
> >
> > Index: configure
> > ===================================================================
> > --- configure   (revision 20476)
> > +++ configure   (working copy)
> > @@ -2103,6 +2103,7 @@
> >      add_cppflags -DPIC
> >      add_cflags   -fPIC
> >      add_asflags  -fPIC
> > +    add_ldflags  -pie
> >  }
> >
> >  enabled pic && enable_pic
> 
> That is wrong.  Let's read the gcc manual for a bit:
> 
> `-fpic'
> `-fPIC'
>      Generate position-independent code (PIC) suitable for use in a
>      shared library, if supported for the target machine.  Such code
>      accesses all constant addresses through a global offset table
>      (GOT). [...]
> 
> `-fpie'
> `-fPIE'
>      These options are similar to `-fpic' and `-fPIC', but generated
>      position independent code can be only linked into executables.
>      Usually these options are used when `-pie' GCC option will be used
>      during linking.
> 
> `-pie'
>      Produce a position independent executable on targets which support
>      it.  For predictable results, you must also specify the same set
>      of options that were used to generate code (`-fpie', `-fPIE', or
>      model suboptions) when you specify this option.
> 
> Can -pie be used with -fPIC-compiled code?  After reading the above,
> I'm not entirely certain.

Yes, -fPIE is a subset of -fPIC and -fpie is a subset of -fpic, where
subset means that pie just allows a few optimizations more (like calling
static functions without going through the GOT).
Though I am quite surprised to hear that -pie will break things with -shared,
that would be an issue.
I thought it only wouldn't work with -static (on OpenBSD that is), but i
may have confused it.