[llvm-dev] [LLVMdev] Cc llvmdev: Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

Alexei Starovoitov
Thu, 06 Aug 2015 03:41:26 -0000

On Wed, Aug 05, 2015 at 04:59:01PM +0800, He Kuang
wrote:
> Hi, Alexei
>
> On 2015/7/30 1:13, Alexei Starovoitov wrote:
> >On 7/29/15 2:38 AM, He Kuang wrote:
> >>Hi, Alexei
> >>
> >>On 2015/7/28 10:18, Alexei Starovoitov wrote:
> >>>On 7/25/15 3:04 AM, He Kuang wrote:
> >>>>I noticed that for 64-bit elf format, the reloc sections
have
> >>>>'Addend' in the entry, but there's no
'Addend' info in bpf elf
> >>>>file(64bit). I think there must be something wrong in the
process
> >>>>of .s -> .o, which related to 64bit/32bit. Anyway, we
can parse out the
> >>>>AT_name now, DW_AT_LOCATION still missed and need your
help.
> >>>
>
> Another thing about DW_AT_name, we've already found that the name
> string is stored indirectly and needs relocation which is
> architecture specific, while the e_machine info in bpf obj file
> is "unknown", both objdump and libdw cannot parse DW_AT_name
> correctly.
>
> Should we just use a known architeture for bpf object file
> instead of "unknown"? If so, we can use the existing relocation
> codes in libdw and get DIE name by simply invoking
> dwarf_diename(). The drawback of this method is that, e.g. we
> use "x86-64" instead, is hard to distinguish bpf obj file with
> x86-64 elf file. Do you think this is ok?

The only clean way would be to register bpf as an architecture
with elf standards committee. I have no idea who is doing that and
how much such new e_machine registration may cost.
So far using EM_NONE is a hack to avoid bureaucracy.
Are dwarf relocation processor specific?
Then simple hack to elfutils/libdw to treat EM_NONE as X64
should do the trick, right?
If that indeed works, we can tweak bpf backend to use EM_X86_64,
but then the danger that such .o file will be wrongly
recognized by elf utils. imo it's safer to keep it as EM_NONE
until real number is assigned, but even after it's assigned it
will take time to propagate that value. So for now I would try
to find a solution keeping EM_NONE hack.