Re: c6x linker issue on linux-next-20160808 + some linker table work

On Tue, 2016-08-09 at 19:09 -0700, Luis R. Rodriguez wrote:

On Aug 9, 2016 6:50 PM, "Mark Salter" [off-list ref] wrote:

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On Tue, 2016-08-09 at 20:40 +0200, Luis R. Rodriguez wrote:

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On Tue, Aug 09, 2016 at 01:04:00PM -0400, Mark Salter wrote:

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On Tue, 2016-08-09 at 06:37 -0700, Guenter Roeck wrote:

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On 08/09/2016 01:11 AM, Luis R. Rodriguez wrote:

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Mark, Aurelien,

I've run into a linker (ld) issue caused by the linker table work I've
been working on [0]. I looked into this and for the life of me, I
cannot comprehend what the problem is, so was hoping you folks might
be able to chime in.

For reference, the error is

c6x-elf-ld: drivers/built-in.o: SB-relative relocation but __c6xabi_DSBT_BASE not defined
c6x-elf-ld: drivers/built-in.o: SB-relative relocation but __c6xabi_DSBT_BASE not defined

DSBT is a reference to the no-MMU userspace ABI used by c6x. The kernel shouldn't
be referencing DSBT base. The -mno-dsbt gcc flag should prevent it.

I see -mno-dsbt on arch/c6x/Makefile already -- however at link time this is
an issue if linker tables are used it seems. Do you have any other recommendation?

I will note that it would seem that even i386 and x86-64 compiler/binutils seem
to have relocation issues on older compiler/binutils, for instance:

I see the problem with gcc 6 as well.

So there appears to be some toolchain issues at play here. We build the kernel with two
c6x-specific options: -mno-dsbt and -msdata=none. I already mentioned dsbt. The sdata
option may be one of:

-msdata=default
     Put small global and static data in the .neardata section, which is pointed to by
     register B14. Put small uninitialized global and static data in the .bss section,
     which is adjacent to the .neardata section. Put small read-only data into the 
     .rodata section. The corresponding sections used for large pieces of data are
     .fardata, .far and .const.

-msdata=all
    Put all data, not just small objects, into the sections reserved for small data,
    and use addressing relative to the B14 register to access them.

-msdata=none
    Make no use of the sections reserved for small data, and use absolute addresses
    to access all data. Put all initialized global and static data in the .fardata
    section, and all uninitialized data in the .far section. Put all constant data
    into the .const section.


Both small data and DSBT make use of base register + 15-bit offset to access data
and thus the SB-relative reloc in the above error message.

I think that gcc sees the .rodata section from DEFINE_LINKTABLE_RO() for builtin_fw
and thinks it needs an SB-relative reloc. When the linker sees that reloc, it thinks
it needs the dsbt base register and thus the error. Interestingly, weak data is
never put in the small data section so if gcc sees that data is weak, it doesn't
check the section name to see if it is a small data section. So SB-relative only
gets used for builtin_fw__end, but not the weak builtin_fw even though they both
are in the .rodata section.

I suspect gcc should avoid being fooled by .rodata if -msdata=none is used.
Regardless, I think this could all be avoided if the RO tables used .const
instead of .rodata for c6x.

Thanks for the thorough analysis, would you be OK for c6x to use .const for all read only linker tables or section ranges ?
I had not added #ifndef around the core-sections.h main ELF definitons but could add one as its needed. In this case perhals that is needed and fine by you
for SECTION_RODATA.
We can also override any of the core section setter helpers for archs but in this case based on what you say it seems this is needed. Unless of course just
-msdata=none is fine and that's not yet used and you prefer that.
  Luis

We're already using -msdata=none for kernel builds. From the gcc docs, one would think
all const data goes into .const with -msdata=none, but the kernel forces a lot of weak
const kallsyms data ,rodata so c6x vmlinux.lds still needs to have a .rodata section. I
think we need to use .const for the c6x read-only linker tables and keep .rodata for
RO_DATA_SECTION in vmlinux.lds.h.