Re: Binutils 2.28 on PowerPC: dangerous relocation: generic linker can't handle R_PPC_SECTOFF_HA

[Sebastian Huber <sebastian dot huber at embedded-brains dot de>]

On 08/05/2019 10:41, Alan Modra wrote:
> On Tue, May 07, 2019 at 10:30:40AM +0200, Sebastian Huber wrote:
> > powerpc-rtems4.9-ld --version
> > GNU ld (GNU Binutils) 2.19.1
> >
> > With this LD version I can boot the application. The LD output is a binary
> > file. I had to remove the OUTPUT_ARCH(powerpc) from ppcboot.lds.
> >
> > Since at least Binutils 2.28, it no longer works:
> >
> > powerpc-rtems5-ld -o ticker.ralf
> > /build/git-build/b-any/powerpc-rtems5/mvme2100/lib/bootloader.o
> > --just-symbols=ticker.exe -b binary rtems.gz -T
> > ~/git-rtems-5/./bsps/powerpc/motorola_powerpc/bootloader/ppcboot.lds
> > /build/git-build/b-any/powerpc-rtems5/mvme2100/lib/bootloader.o: in function
> > `start':
> > /home/EB/sebastian_h/git-rtems-5/c/src/lib/libbsp/powerpc/motorola_powerpc/bootloader/../../../../../../../bsps/powerpc/motorola_powerpc/bootloader/head.S:140:(.text+0x8a):
> > dangerous relocation: generic linker can't handle R_PPC_SECTOFF_HA
> > /home/EB/sebastian_h/git-rtems-5/c/src/lib/libbsp/powerpc/motorola_powerpc/bootloader/../../../../../../../bsps/powerpc/motorola_powerpc/bootloader/head.S:141:(.text+0x8e):
> The truth is that linking to binary with 2.19 did not handle any of
> the R_PPC_SECTOFF relocations correctly.  They were handled just like
> the corresponding R_PPC_ADDR16 reloc, a fact that you can verify by
> comparing the reloc howtos.  (Reloc howtos are used by the generic
> linker to apply relocations.  When linking to powerpc ELF you'll be
> using the code in ppc_elf_relocate_section instead.)  So the values
> applied by 2.19 were symbol values rather than symbol values relative
> to their output section.
>
> I think that means your assembly code has compensated for the 2.19
> linker behaviour by some means or other, and therefore crashes with a
> fixed linker.
>
> You'll probably find that using 2.19 to link to ELF then objcopy to
> binary will also give you a "bad" output.

Lets look at the following assembler code in the .text input section:

https://git.rtems.org/rtems/tree/bsps/powerpc/motorola_powerpc/bootloader/head.S

/* Call the routine to fill boot_data structure from residual data.
 * And to find where the code has to be moved.
 */
    lis    r3,__size@sectoff@ha
    addi    r3,r3,__size@sectoff@l

This symbol is defined by the linker script:

https://git.rtems.org/rtems/tree/bsps/powerpc/motorola_powerpc/bootloader/ppcboot.lds

|.bss : { *(.sbss) *(.bss) . = ALIGN(4); } __bss_words = 
SIZEOF(.bss)>>2; __size = . ; /DISCARD/ : { *(.comment) } }|


I built the latest Binutils and invoked ld in GDB. I set a break point 
to _bfd_final_link_relocate(). Here I get this:

Breakpoint 1 at 0x44c590: file 
/home/EB/sebastian_h/archive/binutils-git/bfd/reloc.c, line 1340.
(gdb) r
Starting program: /opt/rtems/5/bin/powerpc-rtems5-ld -o ticker.ralf 
bootloader.o --just-symbols=ticker.exe -b binary rtems.gz -T ppcboot.lds 
-no-warn-mismatch

Breakpoint 1, _bfd_final_link_relocate (howto=howto@entry=0x647340 
<ppc_elf_howto_raw+1440>, input_bfd=input_bfd@entry=0x8a9c50, 
input_section=input_section@entry=0x8aae30, 
contents=contents@entry=0x99b5e0 "H", address=138, 
value=value@entry=155668, addend=18446744073709433152) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/reloc.c:1340
1340                              bfd_vma value,
(gdb) bt
#0  _bfd_final_link_relocate (howto=howto@entry=0x647340 
<ppc_elf_howto_raw+1440>, input_bfd=input_bfd@entry=0x8a9c50, 
input_section=input_section@entry=0x8aae30, 
contents=contents@entry=0x99b5e0 "H", address=138, 
value=value@entry=155668, addend=18446744073709433152) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/reloc.c:1340
#1  0x000000000045e54d in ppc_elf_relocate_section 
(output_bfd=output_bfd@entry=0x89d220, info=0x88c820 <link_info>, 
input_bfd=input_bfd@entry=0x8a9c50, 
input_section=input_section@entry=0x8aae30, contents=0x99b5e0 "H", 
relocs=relocs@entry=0x8d11d8, local_syms=0x9c5890, 
local_sections=0x9cdec0) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/elf32-ppc.c:9039
#2  0x0000000000487029 in elf_link_input_bfd 
(flinfo=flinfo@entry=0x7fffffffd540, input_bfd=input_bfd@entry=0x8a9c50) 
at /home/EB/sebastian_h/archive/binutils-git/bfd/elflink.c:10887
#3  0x0000000000488897 in bfd_elf_final_link (abfd=<optimized out>, 
info=<optimized out>) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/elflink.c:12214
#4  0x000000000041874d in ldwrite () at 
/home/EB/sebastian_h/archive/binutils-git/ld/ldwrite.c:581
#5  0x0000000000402f20 in main (argc=<optimized out>, argv=<optimized 
out>) at /home/EB/sebastian_h/archive/binutils-git/ld/ldmain.c:457
(gdb) frame 1
#1  0x000000000045e54d in ppc_elf_relocate_section 
(output_bfd=output_bfd@entry=0x89d220, info=0x88c820 <link_info>, 
input_bfd=input_bfd@entry=0x8a9c50, 
input_section=input_section@entry=0x8aae30, contents=0x99b5e0 "H", 
relocs=relocs@entry=0x8d11d8, local_syms=0x9c5890, 
local_sections=0x9cdec0) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/elf32-ppc.c:9039
9039            r = _bfd_final_link_relocate (howto, input_bfd, 
input_section, contents,
(gdb) p sym_name
$1 = 0x8d4231 "__size"
(gdb) p sec->output_section ->name
$2 = 0x89cc60 ".bss"
(gdb) p input_section->name
$3 = 0x8b2c50 ".text"

Why is the output section .bss for __size and not *ABS*?

If I do this with the latest Binutils 2.19 branch. I get:

(gdb) b _bfd_final_link_relocate
Breakpoint 1 at 0x427b40: file 
/home/EB/sebastian_h/archive/binutils-git/bfd/reloc.c, line 1346.
(gdb) r
Starting program: /opt/rtems/4.9/bin/powerpc-rtems4.9-ld -o ticker.ralf 
bootloader.o --just-symbols=ticker.exe -b binary rtems.gz -T ppcboot.lds 
-no-warn-mismatch

Breakpoint 1, _bfd_final_link_relocate (howto=howto@entry=0x4dcfe0 
<ppc_elf_howto_raw+2880>, input_bfd=input_bfd@entry=0x7298e0, 
input_section=input_section@entry=0x72cdc8, 
contents=contents@entry=0x822e90 "H", address=138, 
value=value@entry=155668, addend=32768) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/reloc.c:1346
1346      if (address > bfd_get_section_limit (input_bfd, input_section))
(gdb) bt
#0  _bfd_final_link_relocate (howto=howto@entry=0x4dcfe0 
<ppc_elf_howto_raw+2880>, input_bfd=input_bfd@entry=0x7298e0, 
input_section=input_section@entry=0x72cdc8, 
contents=contents@entry=0x822e90 "H", address=138, 
value=value@entry=155668, addend=32768) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/reloc.c:1346
#1  0x000000000043a1ba in ppc_elf_relocate_section 
(output_bfd=output_bfd@entry=0x71d2c0, info=0x70f900 <link_info>, 
input_bfd=input_bfd@entry=0x7298e0, 
input_section=input_section@entry=0x72cdc8, 
contents=contents@entry=0x822e90 "H", relocs=relocs@entry=0x757d88, 
local_syms=0x84d140, local_sections=0x855770) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/elf32-ppc.c:7318
#2  0x000000000045c359 in elf_link_input_bfd 
(finfo=finfo@entry=0x7fffffffd520, input_bfd=input_bfd@entry=0x7298e0) 
at /home/EB/sebastian_h/archive/binutils-git/bfd/elflink.c:9308
#3  0x000000000045df3e in bfd_elf_final_link (abfd=0x71d2c0, 
info=0x70f900 <link_info>) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/elflink.c:10448
#4  0x000000000041534d in ldwrite () at 
/home/EB/sebastian_h/archive/binutils-git/ld/ldwrite.c:567
#5  0x000000000040283e in main (argc=<optimized out>, argv=<optimized 
out>) at /home/EB/sebastian_h/archive/binutils-git/ld/ldmain.c:462
(gdb) frame 1
#1  0x000000000043a1ba in ppc_elf_relocate_section 
(output_bfd=output_bfd@entry=0x71d2c0, info=0x70f900 <link_info>, 
input_bfd=input_bfd@entry=0x7298e0, 
input_section=input_section@entry=0x72cdc8, 
contents=contents@entry=0x822e90 "H", relocs=relocs@entry=0x757d88, 
local_syms=0x84d140, local_sections=0x855770) at 
/home/EB/sebastian_h/archive/binutils-git/bfd/elf32-ppc.c:7318
7318          r = _bfd_final_link_relocate (howto,
(gdb) p sym_name
$1 = 0x75aaa1 "__size"
(gdb) p sec->output_section ->name
$2 = 0x4acd54 "*ABS*"
(gdb) p input_section ->name
$3 = 0x73b770 ".text"

--
Sebastian Huber, embedded brains GmbH

Address : Dornierstr. 4, D-82178 Puchheim, Germany
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