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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>,
        dwarves@vger.kernel.org, Arnaldo Carvalho de Melo <acme@redhat.com>
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Message-ID: <cbaf50c3-c85d-9239-0b37-c88e8cbed8c8@kernel.org>
Date:   Mon, 3 May 2021 10:59:44 +0200
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CCing pahole people.

On 03. 05. 21, 9:59, Jiri Slaby wrote:
> 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.

Attaching a patch for pahole which fixes the issue, but I have no idea 
whether it is the right fix at all.

> regards,-- 
js
suse labs

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From: Jiri Slaby <jslaby@suse.cz>
Subject: ppc64: .opd section fix
Patch-mainline: submitted 2021/05/03

Functions in the .opd section should be considered valid too. Otherwise,
pahole cannot produce a .BTF section from vmlinux and kernel build
fails on ppc64.
---
 btf_encoder.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -31,6 +31,8 @@ struct funcs_layout {
 	unsigned long mcount_start;
 	unsigned long mcount_stop;
 	unsigned long mcount_sec_idx;
+	unsigned long opd_start;
+	unsigned long opd_stop;
 };
 
 struct elf_function {
@@ -271,11 +273,24 @@ static int is_ftrace_func(struct elf_fun
 	return start <= addrs[r] && addrs[r] < end;
 }
 
+static int is_opd_func(struct elf_function *func, struct funcs_layout *fl)
+{
+	return fl->opd_start <= func->addr && func->addr < fl->opd_stop;
+}
+
 static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
 {
 	__u64 *addrs, count, i;
 	int functions_valid = 0;
 	bool kmod = false;
+	GElf_Shdr shdr;
+	Elf_Scn *sec;
+
+	sec = elf_section_by_name(btfe->elf, &btfe->ehdr, &shdr, ".opd", NULL);
+	if (sec) {
+		fl->opd_start = shdr.sh_addr;
+		fl->opd_stop = shdr.sh_addr + shdr.sh_size;
+	}
 
 	/*
 	 * Check if we are processing vmlinux image and
@@ -322,7 +337,8 @@ static int setup_functions(struct btf_el
 			func->addr += func->sh_addr;
 
 		/* Make sure function is within ftrace addresses. */
-		if (is_ftrace_func(func, addrs, count)) {
+		if (is_opd_func(func, fl) ||
+				is_ftrace_func(func, addrs, count)) {
 			/*
 			 * We iterate over sorted array, so we can easily skip
 			 * not valid item and move following valid field into

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