Date:   Thu, 24 Oct 2019 20:31:14 +0200
From:   Ilya Leoshkevich <iii@...ux.ibm.com>
To:     Yonghong Song <yhs@...com>
Cc:     Prabhakar Kushwaha <prabhakar.pkin@...il.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: Linux-5.4: bpf: test_core_reloc_arrays.o: Segmentation fault with
 llc -march=bpf

> Am 24.10.2019 um 19:49 schrieb Yonghong Song <yhs@...com>:
> 
> 
> 
> On 10/24/19 9:04 AM, Ilya Leoshkevich wrote:
>>> Am 23.10.2019 um 03:35 schrieb Prabhakar Kushwaha <prabhakar.pkin@...il.com>:
>>> 
>>> 
>>> Adding other mailing list, folks...
>>> 
>>> Hi All,
>>> 
>>> I am trying to build kselftest on Linux-5.4 on ubuntu 18.04. I installed
>>> LLVM-9.0.0 and Clang-9.0.0 from below links after following steps from
>>> [1] because of discussion [2]
>>> 
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__releases.llvm.org_9.0.0_llvm-2D9.0.0.src.tar.xz&d=DwIFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=se8pV6OlDAeF2g5iEAvSB2qhLBJGPaHADv3NQVNFx6U&s=IzBxNhAvcILfAD_XcSB7t0s6-B-wFY3TBoVGH6WhRK8&e=
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__releases.llvm.org_9.0.0_clang-2Dtools-2Dextra-2D9.0.0.src.tar.xz&d=DwIFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=se8pV6OlDAeF2g5iEAvSB2qhLBJGPaHADv3NQVNFx6U&s=KkjCjWm_q2iMfFh50rTKtFqQEMbRBVhT9Oh8KMfgwW4&e=
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__releases.llvm.org_9.0.0_cfe-2D9.0.0.src.tar.xz&d=DwIFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=se8pV6OlDAeF2g5iEAvSB2qhLBJGPaHADv3NQVNFx6U&s=TvkN9sb5rSB5BNxJP27UmCsfNHsRQdaVeAnBa1TkyjM&e=
>>> 
>>> Now, i am trying with llc -march=bpf, with this segmentation fault is
>>> coming as below:
>>> 
>>> gcc -g -Wall -O2 -I../../../include/uapi -I../../../lib
>>> -I../../../lib/bpf -I../../../../include/generated -DHAVE_GENHDR
>>> -I../../../include -Dbpf_prog_load=bpf_prog_test_load
>>> -Dbpf_load_program=bpf_test_load_program    test_flow_dissector.c
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/test_stub.o
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/libbpf.a -lcap -lelf
>>> -lrt -lpthread -o
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/test_flow_dissector
>>> gcc -g -Wall -O2 -I../../../include/uapi -I../../../lib
>>> -I../../../lib/bpf -I../../../../include/generated -DHAVE_GENHDR
>>> -I../../../include -Dbpf_prog_load=bpf_prog_test_load
>>> -Dbpf_load_program=bpf_test_load_program
>>> test_tcp_check_syncookie_user.c
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/test_stub.o
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/libbpf.a -lcap -lelf
>>> -lrt -lpthread -o
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/test_tcp_check_syncookie_user
>>> gcc -g -Wall -O2 -I../../../include/uapi -I../../../lib
>>> -I../../../lib/bpf -I../../../../include/generated -DHAVE_GENHDR
>>> -I../../../include -Dbpf_prog_load=bpf_prog_test_load
>>> -Dbpf_load_program=bpf_test_load_program    test_lirc_mode2_user.c
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/test_stub.o
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/libbpf.a -lcap -lelf
>>> -lrt -lpthread -o
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/test_lirc_mode2_user
>>> (clang -I. -I./include/uapi -I../../../include/uapi
>>> -I/usr/src/tovards/linux/tools/testing/selftests/bpf/../usr/include
>>> -D__TARGET_ARCH_arm64 -g -idirafter /usr/local/include -idirafter
>>> /usr/local/lib/clang/9.0.0/include -idirafter
>>> /usr/include/aarch64-linux-gnu -idirafter /usr/include
>>> -Wno-compare-distinct-pointer-types -O2 -target bpf -emit-llvm \
>>> -c progs/test_core_reloc_arrays.c -o - || echo "clang failed") | \
>>> llc -march=bpf -mcpu=probe  -filetype=obj -o
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/test_core_reloc_arrays.o
>>> Stack dump:
>>> 0. Program arguments: llc -march=bpf -mcpu=probe -filetype=obj -o
>>> /usr/src/tovards/linux/tools/testing/selftests/bpf/test_core_reloc_arrays.o
>>> 1. Running pass 'Function Pass Manager' on module '<stdin>'.
>>> 2. Running pass 'BPF Assembly Printer' on function '@...t_core_arrays'
>>> #0 0x0000aaaac618db08 llvm::sys::PrintStackTrace(llvm::raw_ostream&)
>>> (/usr/local/bin/llc+0x152eb08)
>>> Segmentation fault
>> 
>> Hi,
>> 
>> FWIW I can confirm that this is happening on s390 too with llvm-project
>> commit 950b800c451f.
>> 
>> Here is the reduced sample that triggers this (with -march=bpf
>> -mattr=+alu32):
>> 
>> struct b {
>>   int e;
>> } c;
>> int f() {
>>   return __builtin_preserve_field_info(c.e, 0);
>> }
>> 
>> This is compiled into:
>> 
>> 0B      bb.0 (%ir-block.0):
>> 16B       %0:gpr = LD_imm64 @"b:0:0$0:0"
>> 32B       $w0 = COPY %0:gpr, debug-location !17; 1-E.c:5:3
>> 48B       RET implicit killed $w0, debug-location !17; 1-E.c:5:3
>> 
>> and then BPFInstrInfo::copyPhysReg chokes on COPY, since $w0 and %0 are
>> in different register classes.
> 
> Ilya,
> 
> Thanks for reporting. I can reproduce the issue with latest trunk.
> I will investigate and fix the problem soon.
> 
> Yonghong

Thanks for taking care of this! Just FYI, bisect pointed to 05e46979d2f4
("[BPF] do compile-once run-everywhere relocation for bitfields").

Could you please add me to Phabricator review? I'm curious what the
proper solution is going to be, as I'm still not sure whether handling
asymmetric copies is the right approach, or whether they should rather
be prevented from occuring in the first place.

Best regards,
Ilya

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