[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

Wangnan (F)
Thu, 06 Aug 2015 04:31:26 -0000

On 2015/8/6 11:41, Alexei Starovoitov wrote:
> 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.
>

What about hacking ELF binary in memory?

1. load the object into memory;
2. twist the machine code to EM_X86_64;
3. load it using elf_begin;
4. return the twested elf memory image using libdwfl's find_elf callback.

Then libdw will recognise BPF's object file as a X86_64 object file. If
required,
relocation sections can also be twisted in this way. Should not very
hard since
we can only consider one relocation type.

Then let's start thinking how to introduce EM_BPF. We can rely on the
hacking
until EM_BPF symbol reaches elfutils in perf.

What do you think?

Thank you.