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Subject: Re: linux-next failing build due to missing cubictcp_state symbol
From:   Jiri Slaby <jirislaby@kernel.org>
To:     =?UTF-8?Q?Michal_Such=c3=a1nek?= <msuchanek@suse.de>,
        Jiri Olsa <jolsa@redhat.com>
Cc:     Yonghong Song <yhs@fb.com>, linux-kernel@vger.kernel.org,
        Martin KaFai Lau <kafai@fb.com>,
        "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Song Liu <songliubraving@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, netdev@vger.kernel.org,
        bpf@vger.kernel.org, Jiri Olsa <jolsa@kernel.org>,
        Jesper Dangaard Brouer <brouer@redhat.com>
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Date:   Mon, 3 May 2021 09:59:37 +0200
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On 03. 05. 21, 8:11, Jiri Slaby wrote:
>>>>>> looks like vfs_truncate did not get into BTF data,
>>>>>> I'll try to reproduce
>>
>> _None_ of the functions are generated by pahole -J from debuginfo on 
>> ppc64. debuginfo appears to be correct. Neither pahole -J fs/open.o 
>> works correctly. collect_functions in dwarves seems to be defunct on 
>> ppc64... "functions" array is bogus (so find_function -- the bsearch 
>> -- fails).
> 
> It's not that bogus. I forgot an asterisk:
>> #0? find_function (btfe=0x100269f80, name=0x10024631c "stream_open") 
>> at /usr/src/debug/dwarves-1.21-1.1.ppc64/btf_encoder.c:350
>> (gdb) p (*functions)@84
>> $5 = {{name = 0x7ffff68e0922 ".__se_compat_sys_ftruncate", addr = 
>> 75232, size = 72, sh_addr = 65536, generated = false}, {
>> ??? name = 0x7ffff68e019e ".__se_compat_sys_open", addr = 80592, size 
>> = 216, sh_addr = 65536, generated = false}, {
>> ??? name = 0x7ffff68e0076 ".__se_compat_sys_openat", addr = 80816, 
>> size = 232, sh_addr = 65536, generated = false}, {
>> ??? name = 0x7ffff68e0908 ".__se_compat_sys_truncate", addr = 74304, 
>> size = 100, sh_addr = 65536, generated = false}, {
> ...
>> ??? name = 0x7ffff68e0808 ".stream_open", addr = 65824, size = 72, 
>> sh_addr = 65536, generated = false}, {
> ...
>> ??? name = 0x7ffff68e0751 ".vfs_truncate", addr = 73392, size = 544, 
>> sh_addr = 65536, generated = false}}
> 
> The dot makes the difference, of course. The question is why is it 
> there? I keep looking into it. Only if someone has an immediate idea...

Well, .vfs_truncate is in .text (and contains an ._mcount call). And 
vfs_truncate is in .opd (w/o an ._mcount call). Since setup_functions 
excludes all functions without the ._mcount call, is_ftrace_func later 
returns false for such functions and they are filtered before the BTF 
processing.

Technically, get_vmlinux_addrs looks at a list of functions between 
__start_mcount_loc and __stop_mcount_loc and considers only the listed.

I don't know what the correct fix is (exclude .opd functions from the 
filter?). Neither why cross compiler doesn't fail, nor why ebi v2 avoids 
this too.

regards,
-- 
js