linux-kernel - Re: [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

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Message-ID: <20150806034125.GB52057@Alexeis-MBP.westell.com>
Date:	Wed, 5 Aug 2015 20:41:26 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	He Kuang <hekuang@...wei.com>
Cc:	Alexei Starovoitov <ast@...mgrid.com>,
	"Wangnan (F)" <wangnan0@...wei.com>, pi3orama <pi3orama@....com>,
	llvm-dev@...ts.llvm.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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

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.

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