netdev - Re: [PATCH v2 bpf-next 06/11] libbpf: add BPF static linker APIs

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Message-ID: <43d9bd1c-fba6-f653-7374-cc593f01bc72@fb.com>
Date:   Mon, 7 Jun 2021 23:47:08 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Tom Stellard <tstellar@...hat.com>
CC:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...com>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 06/11] libbpf: add BPF static linker APIs



On 6/7/21 9:08 PM, Andrii Nakryiko wrote:
> On Mon, Jun 7, 2021 at 7:41 PM Tom Stellard <tstellar@...hat.com> wrote:
>>
>> On 6/7/21 5:25 PM, Andrii Nakryiko wrote:
>>> On Mon, Jun 7, 2021 at 4:12 PM Tom Stellard <tstellar@...hat.com> wrote:
>>>>
>>>>
>>>> Hi,
>>>>
>>>>> +                               } else {
>>>>> +                                       pr_warn("relocation against STT_SECTION in non-exec section is not supported!\n");
>>>>> +                                       return -EINVAL;
>>>>> +                               }
>>>>
>>>> Kernel build of commit 324c92e5e0ee are failing for me with this error
>>>> message:
>>>>
>>>> /builddir/build/BUILD/kernel-5.13-rc4-61-g324c92e5e0ee/linux-5.13.0-0.rc4.20210603git324c92e5e0ee.35.fc35.x86_64/tools/bpf/bpftool/bpftool gen object /builddir/build/BUILD/kernel-5.13-rc4-61-g324c92e5e0ee/linux-5.13.0-0.rc4.20210603git324c92e5e0ee.35.fc35.x86_64/tools/testing/selftests/bpf/bind_perm.linked1.o /builddir/build/BUILD/kernel-5.13-rc4-61-g324c92e5e0ee/linux-5.13.0-0.rc4.20210603git324c92e5e0ee.35.fc35.x86_64/tools/testing/selftests/bpf/bind_perm.o
>>>> libbpf: relocation against STT_SECTION in non-exec section is not supported!
>>>>
>>>> What information can I provide to help debug this failure?
>>>
>>> Can you please send that bind_perm.o file? Also what's your `clang
>>> --version` output?
>>>
>>
>> clang version 12.0.0 (Fedora 12.0.0-2.fc35)
>>
>>>> I suspect this might be due to Clang commit 6a2ea84600ba ("BPF: Add
>>>> more relocation kinds"), but I get a different error on 324c92e5e0ee.
>>>> So meanwhile you might try applying 9f0c317f6aa1 ("libbpf: Add support
>>>> for new llvm bpf relocations") from bpf-next/master and check if that
>>>> helps. But please do share bind_perm.o, just to double-check what's
>>>> going on.
>>>>
>>
>> Here is bind_perm.o: https://fedorapeople.org/~tstellar/bind_perm.o
>>
> 
> So somehow you end up with .eh_frame section in BPF object file, which
> shouldn't ever happen. So there must be something that you are doing
> differently (compiler flags or something else) that makes Clang
> produce .eh_frame. So we need to figure out why .eh_frame gets
> generated. Not sure how, but maybe you have some ideas of what might
> be different about your build.

Tom,

I tried with latest release/12.x branch with top commit
   0826268d59c6 [PowerPC] Fix x86 vector intrinsics wrapper
                compilation under C++
and I cannot reproduce the issue.

$ clang --version
clang version 12.0.1 (https://github.com/llvm/llvm-project.git 
0826268d59c6e1bb3530dffd9dc5f6038774486d)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/yhs/work/llvm-project/llvm/build/install/bin

The following is my compilation flag, captured with
   make -C tools/testing/selftests/bpf -j60 V=1
$ cat log.sh
clang  -g -D__TARGET_ARCH_x86 -mlittle-endian 
-I/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include 
-I/home/yhs/work/bpf-next/tools/testing/selftests/bpf 
-I/home/yhs/work/bpf-next/tools/include/uapi 
-I/home/yhs/work/bpf-next/tools/testing/selftests/usr/include -idirafter 
/home/yhs/work/llvm-project/llvm/build.cur/install/lib/clang/13.0.0/include 
-idirafter /usr/local/include -idirafter /usr/include 
-Wno-compare-distinct-pointer-types -DENABLE_ATOMICS_TESTS -O2 -target 
bpf -c progs/bind_perm.c -o 
/home/yhs/work/bpf-next/tools/testing/selftests/bpf/bind_perm.o -mcpu=v3

Looking at progs/bind_perm.c, I can see
#include <linux/stddef.h>
#include <linux/bpf.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>

So it included some system header files. Maybe some of these
header files changed some function attributes which triggered
.cfi_startproc/.cfi_endproc generation which enabled .eh_frame section
generation.

Maybe you can produce an .i file in your environment
so we can do some further analysis.

> 
>> Thanks,
>> Tom
>>
>>>>
>>>>>
>>>>> Thanks,
>>>>> Tom
>>>>>
>>>>
>>>
>>