Re: linux-next failing build due to missing cubictcp_state symbol

[Jiri Olsa <jolsa@redhat.com>]

On Mon, May 03, 2021 at 10:59:44AM +0200, Jiri Slaby wrote:
> 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.

hi,
we're considering to disable ftrace filter completely,
I guess that would solve this issue for ppc as well

  https://lore.kernel.org/bpf/20210501001653.x3b4rk4vk4iqv3n7@kafai-mbp.dhcp.thefacebook.com/

jirka

> 
> > regards,--
> js
> suse labs

> 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