Re: SH FDPIC ABI spec/binutils and kernel conflict on flag definitions

[Mike Frysinger <vapier@gentoo.org>]

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On 14 Sep 2015 12:09, Rich Felker wrote:
> On Thu, Sep 10, 2015 at 04:53:35PM +0100, David Howells wrote:
> > Rich Felker <dalias@libc.org> wrote:
> > > On the other hand, the only existing way to produce a binary that both
> > > (1) needs constant displacement, and (2) actually gets constant
> > > displacement from the kernel at load time, is to manually edit the ELF
> > > headers to flip the bit. So I really doubt any such binaries exist. Do
> > > you have a reason to believe they do?
> > 
> > Well, Fujitsu asked for it for FRV - I've no idea whether they have such
> > binaries still.
> 
> OK, I've solved part of the mystery: on FRV and Blackfin, binutils
> matches the kernel behavior and conflicts with the (effectively wrong)
> ABI documents. As can be seen at the following locations in the
> source, EF_$ARCH_PIC is cleared by default and set when there is a
> cross-segment relocation that forces constant displacement:
> 
> Blackfin:
> https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf32-bfin.c;h=152134ee7b9b445d96818fcab2350ffd11795897;hb=HEAD#l3140
> https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf32-bfin.c;h=152134ee7b9b445d96818fcab2350ffd11795897;hb=HEAD#l4978

it's a bit more complicated than that.  only one place does Blackfin
permit setting of EF_BFIN_PIC:
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf32-bfin.c;h=152134ee7b9b445d96818fcab2350ffd11795897;hb=HEAD#l3140

and if you look before that, it has:
  if (!silence_segment_error && bfd_link_pic (info))
    return FALSE;

and those are defined as:
  int silence_segment_error = !bfd_link_pic (info);
  #define bfd_link_pic(info)  (bfd_link_dll (info) || bfd_link_pie (info))
  #define bfd_link_pie(info)  ((info)->type == type_pie)

all FDPIC ELFs must be PIE which means it's not possible to create a
Blackfin FDPIC ELF with EF_BFIN_PIC set.

off the top of my head, i don't recall seeing EF_BFIN_PIC being set
in Blackfin FDPIC ELFs.  it would show up when generating some FLAT
code w/-fpic, but that doesn't matter here.

> FRV:
> https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf32-frv.c;h=fa12528b3d11ab9ed7a4d5d894f8b9c1a5e783a9;hb=HEAD#l3919
> https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf32-frv.c;h=fa12528b3d11ab9ed7a4d5d894f8b9c1a5e783a9;hb=HEAD#l6367

the Blackfin code was copied from FRV, and the codepath i noted above
is the same for FRV.
-mike

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