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Message-ID: <20151019151840.GM8645@brightrain.aerifal.cx>
Date: Mon, 19 Oct 2015 11:18:40 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: musl mips n64 dynamic build
On Mon, Oct 19, 2015 at 02:51:12PM +0530, Mahesh Bodapati wrote:
> Hi Rich,
>
> I have ported musl libraries for MIPS n64 ABI architecture and I have setup
> $gp as
>
> Crt1.s:
crt1.s is obsolete and should not be used for new archs. The benefit
to you is that crt_arch.h (the new way of doing it) is a lot easier to
write. All you need to do is jump to the C entry point _start_c with
the right calling convention for a C function and the first 2 argument
slots containing the original stack pointer and a pointer to _DYNAMIC,
respectively. _start_c can be assumed to be defined in the same source
file.
> *lui $3,%hi(%neg(%gp_rel(_start)))*
>
> * daddu
> $3,$3,$25 /* same in dlsym.s
> and pipe.s */*
>
> * daddiu $gp,$3,%lo(%neg(%gp_rel(_start)))*
>
> ld $4, %got_disp(main)($gp) # Get main() ..
Jumping to main from crt1 is not valid. You need to call
__libc_start_main. But if you use crt_arch.h this part happens
automatically for you.
> I didn’t setup $gp as in MIPS o32 ABI as we shouldn’t use _*gp*_disp for
> setting up $gp
>
> “The special symbol name _gp_disp, used for relocating the calculation of gp
> on entry to a DSO in 32-bit files, is
>
> not supported in ELF-64 or in the new 32-bit ABI. Instead, these relocations
> should be composed with
>
> R_MIPS_GPREL applied to an explicit symbol for the entry point of the
> subprogram
I'm not sure what document you're quoting, but sometimes these things
express assumptions about a particular usage case that don't apply to
what musl is doing. But you should be able to avoid writing this code
anyway.
> involves loading the difference between the entry
>
> address of a subprogram s and the runtime gp for establishing gp:
>
> A: lui rx, %hi(%neg(%gp_rel(s)))# load high part of diff
>
> B: daddiu rx, rx, %lo(%neg(%gp_rel(s)))# add low part
>
> C: dadd gp, t9, rx # add to entry address “
>
>
>
> Now I am able to build dynamic n64 musl libraries and n64 dynamic
> application but it’s not showing any relocation with it
>
> and execution of application is giving errors like “unsupported relocation
> type errors”
Did you make a reloc.h defining the relocation types needed for mips
n64 and mapping them to the generic-reloc names musl uses for them?
See the existing arch/*/reloc.h files. Without these being defined
directly the dynamic linker will not be able to process the
relocations it finds in itself or in the main program or other
libraries.
But without a working crt_arch.h I'm surprised the dynamic linker can
even start at all. Are you sure you're not using the mips32 dynamic
linker? Look at the program headers (using readelf -l) for INTERP.
> $ objdump -R main
>
>
>
> a.out: file format elf64-tradbigmips
>
>
>
> DYNAMIC RELOCATION RECORDS (none)
>
>
>
> $ ./main
>
> Error relocating libc.so: unsupported relocation type 482431
>
> Error relocating libc.so: unsupported relocation type 482687
>
> Error relocating libc.so: unsupported relocation type 482943
>
> Error relocating libc.so: unsupported relocation type 483199
>
> ;
>
> ;
>
> ;
>
> Segmentation fault
>
>
>
> So Can you suggest how can I resolve these errors? I am thinking like
> either doing changes in do_mips_reloc(),do_relocs() functions in musl
> sources or in usage of relocations.
OK, I see what's going wrong here. The invalid relocation types are
being produced by this line in do_mips_relocs:
rel[1] = sym-p->syms << 8 | R_MIPS_JUMP_SLOT;
That produces a relocation in the elf32 encoding, because I just
assumes mips is elf32 when I wrote it. I believe the 64-bit version
should be:
rel[1] = sym-p->syms << 32 | R_MIPS_JUMP_SLOT;
Try just making that change and see if it works. If so we can work on
making a clean conditional for 32- vs 64-bit here later.
Rich
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