Re: [PATCH] elf.h: Add BPF relocation types.

[Carlos O'Donell <carlos@redhat.com>]

On 06/16/2018 05:45 PM, Mark Wielaard wrote:
> The BPF ELF format has new relocation types R_BPF_64_64 and R_BPF_64_32.
> The existing R_BPF_MAP_FD was an extension that never got implemented.
> Remove it, because its constant conflicts with the official R_BPF_64_64.
> ---
>  ChangeLog | 5 +++++
>  elf/elf.h | 3 ++-
>  2 files changed, 7 insertions(+), 1 deletion(-)

I trust you that R_BPF_MAP_FD was never implemented, but I consider it
bad engineering to publish and then reuse a constant, even if you might
argue the entire pseudo-architecture is "unstable."

Having said that we should align the values here with the values being used
by llvm and Facebook. They represent the most recent values being used for
relocations in this pseudo-machine. Without an officially published ABI for
these relocations we have no real way to review what is canonical and what
is not.

I don't see any objections from Florian Weimer (the other reviewer), just
the same frowning face I have :/ at the lack of a proper engineering
specification.

OK to checkin.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> diff --git a/ChangeLog b/ChangeLog
> index 9dd87eebb8..40175d254a 100644
> --- a/ChangeLog
> +++ b/ChangeLog
> @@ -1,3 +1,8 @@
> +2018-06-16  Mark Wielaard  <mark@klomp.org>
> +
> +	* elf/elf.h (R_BPF_MAP_FD): Removed.
> +	(R_BPF_64_64, R_BPF_64_32): New.
> +
>  2018-06-15  Samuel Thibault  <samuel.thibault@ens-lyon.org>
>  
>  	* include/sys/sendfile.h (__sendfile64): Declare hidden prototype.
> diff --git a/elf/elf.h b/elf/elf.h
> index a5b2811ce0..75043bcbf9 100644
> --- a/elf/elf.h
> +++ b/elf/elf.h
> @@ -3850,7 +3850,8 @@ enum
>  /* BPF specific declarations.  */
>  
>  #define R_BPF_NONE		0	/* No reloc */
> -#define R_BPF_MAP_FD		1	/* Map fd to pointer */
> +#define R_BPF_64_64		1
> +#define R_BPF_64_32		10

This matches what is in LLVM upstream (not the git mirror).

Changed by this:

commit db476dbf2ee03f8ebdacc439bdde1cf4f0ee629a
Author: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Mon Nov 21 06:21:23 2016 +0000

    [bpf] fix dwarf elf relocs and line numbers
    
    - teach RelocVisitor to recognize bpf relocations
    - fix AsmInfo->PointerSize to make sure dwarf is emitted correctly
    - add a test for the above
    
    Signed-off-by: Alexei Starovoitov <ast@kernel.org>
    
    git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287521 91177308-0d34-0410-b5e6-96231b3b80d8

>  
>  /* Imagination Meta specific relocations. */
>  
>