Re: [PATCH] backends,bpf: add proper relocation support

[Yonghong Song <yhs at fb dot com>]

On 6/16/18 7:04 AM, Mark Wielaard wrote:
> Hi,
>
> I added Richard to the CC, who added the original BPF support.
> Who might remember where the R_BPF_MAP_FD comes from (see at the end).
>
> On Fri, 2018-06-15 at 15:40 -0700, Yonghong Song wrote:
> > Due to libdw does not have proper BPF relocation support,
> > the pahole cannot display filenames correctly for objects
> > with default llvm options. So we have to invent
> > a special option "llc -march=bpf -mattr=dwarfris" to
> > prevent llvm from generating cross-section dwarf relocation
> > records (https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_rL326505&d=DwIFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=reaovSjSH40LhrR6vohT7wIrLEVKTNdKYsXxw2wN0hM&s=_1Zapq0e2D1JVvkHQ6roxhWYP307V4tveppbAo2yTpc&e=).
> > The pahole related discussion is in linux netdev
> > mailing list (https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.openwall.net_netdev_2018_06_15_38&d=DwIFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=reaovSjSH40LhrR6vohT7wIrLEVKTNdKYsXxw2wN0hM&s=OJqgsG_v099f9jVBOq7h0SpxOSySbSy3Pcf7_XgIRG0&e=, etc.)
> >
> > We would like to add proper BPF relocation support
> > to libdw so eventually we could retire the special llc bpf
> > flag "-mattr=dwarfris".
>
> Yes. elfutils/libdwfl only does "simple relocations", but that is all
> you need anyway.

That is correct.

> Do you have a test file (binary for something simple/trivial generated
> by llc -march=bpf that contains at least one reloc). I looked at your
> implementation and I am sure it works correctly. But having a small
> testfile is always a plus.

Okay let me create a test case and submit the patch again.

> > The bpf relocations are defined in
> > llvm_repo:include/llvm/BinaryFormat/ELFRelocs/BPF.def:
> >    ELF_RELOC(R_BPF_NONE,        0)
> >    ELF_RELOC(R_BPF_64_64,       1)
> >    ELF_RELOC(R_BPF_64_32,      10)
> >
> > Removed the relocation type R_BPF_MAP_FD whoes name does not
> > confirm to llvm definition and replaced it with R_BPF_64_64.
> > The BPF object is just a relocatible object, not an executable or
> > a shared library, so assign ELF type to REL only in bpf_reloc.def.
> >
> > Tested locally with building pahole with this patch and
> > pahole is able to display structures in bpf object file properly.
>
> Patch looks good. Thanks. I'll add a ChangeLog entry because that is
> what we still do.

Sure.

> > Signed-off-by: Yonghong Song <yhs@fb.com>
> > ---
> >   backends/Makefile.am   |  2 +-
> >   backends/bpf_init.c    |  1 +
> >   backends/bpf_reloc.def |  3 ++-
> >   backends/bpf_symbol.c  | 54
> > ++++++++++++++++++++++++++++++++++++++++++++++++++
> >   libelf/elf.h           |  3 ++-
> >   5 files changed, 60 insertions(+), 3 deletions(-)
> >   create mode 100644 backends/bpf_symbol.c
> > [...]
> > diff --git a/libelf/elf.h b/libelf/elf.h
> > index f7748983..940e88dd 100644
> > --- a/libelf/elf.h
> > +++ b/libelf/elf.h
> > @@ -3848,7 +3848,8 @@ 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
>
> We should sync this with glibc. This file really is a copy of elf/elf.h
> in glibc, which we periodically sync. It would be good if all projects
> agree on the constants.
>
> I would like to understand where the R_BPF_MAP_FD comes from. But I
> assume it was a typo for BPF_PSEUDO_MAP_FD from bpf.h (which has the
> same constant number 1).
>
> I'll sent a patch to libc-alpha@sourceware.org unless you beat me to
> it.

Please go ahead to submit the necessary elf.h changes to libc. Thanks!

> Thanks,
>
> Mark