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

Hi, Alexei

Hi, Alexei

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?

Otherwise, for not touching libdw, we should reimplement the
relocation codes already in libdw for bpf elf file with "unknown"
machine info specially in perf. I wonder whether it is worth doing
this and what's your opinion?

Thank you.

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.

sounds crazy, but may work. let's try it :slight_smile: