Yonghong Song <yhs@xxxxxx> writes:
> On 12/3/20 9:55 AM, Toke Høiland-Jørgensen wrote:
>> Hi Andrii
>>
>> I noticed that recent libbpf versions fail to load BPF files compiled
>> with old versions of LLVM. E.g., if I compile xdp-tools with LLVM 7 I
>> get:
>>
>> $ sudo ./xdp-loader load testns ../lib/testing/xdp_drop.o -vv
>> Loading 1 files on interface 'testns'.
>> libbpf: loading ../lib/testing/xdp_drop.o
>> libbpf: elf: section(3) prog, size 16, link 0, flags 6, type=1
>> libbpf: sec 'prog': failed to find program symbol at offset 0
>> Couldn't open file '../lib/testing/xdp_drop.o': BPF object format invalid
>>
>> The 'failed to find program symbol' error seems to have been introduced
>> with commit c112239272c6 ("libbpf: Parse multi-function sections into
>> multiple BPF programs").
>>
>> Looking at the object file in question, indeed it seems to not have any
>> function symbols defined:
>>
>> $ llvm-objdump --syms ../lib/testing/xdp_drop.o
>>
>> ../lib/testing/xdp_drop.o: file format elf64-bpf
>>
>> SYMBOL TABLE:
>> 0000000000000000 l .debug_str 0000000000000000
>> 0000000000000037 l .debug_str 0000000000000000
>> 0000000000000042 l .debug_str 0000000000000000
>> 0000000000000068 l .debug_str 0000000000000000
>> 0000000000000071 l .debug_str 0000000000000000
>> 0000000000000076 l .debug_str 0000000000000000
>> 000000000000008a l .debug_str 0000000000000000
>> 0000000000000097 l .debug_str 0000000000000000
>> 00000000000000a3 l .debug_str 0000000000000000
>> 00000000000000ac l .debug_str 0000000000000000
>> 00000000000000b5 l .debug_str 0000000000000000
>> 00000000000000bc l .debug_str 0000000000000000
>> 00000000000000c9 l .debug_str 0000000000000000
>> 00000000000000d4 l .debug_str 0000000000000000
>> 00000000000000dd l .debug_str 0000000000000000
>> 00000000000000e1 l .debug_str 0000000000000000
>> 00000000000000e5 l .debug_str 0000000000000000
>> 00000000000000ea l .debug_str 0000000000000000
>> 00000000000000f0 l .debug_str 0000000000000000
>> 00000000000000f9 l .debug_str 0000000000000000
>> 0000000000000103 l .debug_str 0000000000000000
>> 0000000000000113 l .debug_str 0000000000000000
>> 0000000000000122 l .debug_str 0000000000000000
>> 0000000000000131 l .debug_str 0000000000000000
>> 0000000000000000 l d prog 0000000000000000 prog
>> 0000000000000000 l d .debug_abbrev 0000000000000000 .debug_abbrev
>> 0000000000000000 l d .debug_info 0000000000000000 .debug_info
>> 0000000000000000 l d .debug_frame 0000000000000000 .debug_frame
>> 0000000000000000 l d .debug_line 0000000000000000 .debug_line
>> 0000000000000000 g license 0000000000000000 _license
>> 0000000000000000 g prog 0000000000000000 xdp_drop
>>
>>
>> I assume this is because old LLVM versions simply don't emit that symbol
>> information?
>
> Could you share xdp_drop.c or other test which I can compile and check
> to understand the issue?
It's just an empty program returning XDP_DROP:
https://github.com/xdp-project/xdp-tools/blob/master/lib/testing/xdp_drop.c
I basically just did this on Debian buster:
$ sudo apt install gcc-multilib build-essential libpcap-dev libelf-dev git llc lld clang gcc-multilib pkt-config m4
$ git clone --recurse-submodules https://github.com/xdp-project/xdp-tools
$ cd xdp-tools
$ LLC=llc-7 ./configure
$ make -k
$ cd xdp-loader
$ sudo ip link add dev testns type veth
$ sudo ./xdp-loader load testns ../lib/testing/xdp_drop.o -vv
(xdpdump will fail to build with llvm7, but the rest should work)
-Toke
