Re: [PATCH dwarves] btf: Remove ftrace filter

[Jiri Olsa <jolsa@redhat.com>]

On Wed, May 05, 2021 at 06:58:24PM -0700, Martin KaFai Lau wrote:
> BTF is currently generated for functions that are in ftrace list
> or extern.
> 
> A recent use case also needs BTF generated for functions included in
> allowlist.  In particular, the kernel
> commit e78aea8b2170 ("bpf: tcp: Put some tcp cong functions in allowlist for bpf-tcp-cc")
> allows bpf program to directly call a few tcp cc kernel functions. Those
> kernel functions are currently allowed only if CONFIG_DYNAMIC_FTRACE
> is set to ensure they are in the ftrace list but this kconfig dependency
> is unnecessary.
> 
> Those kernel functions are specified under an ELF section .BTF_ids.
> There was an earlier attempt [0] to add another filter for the functions in
> the .BTF_ids section.  That discussion concluded that the ftrace filter
> should be removed instead.
> 
> This patch is to remove the ftrace filter and its related functions.
> 
> Number of BTF FUNC with and without is_ftrace_func():
> My kconfig in x86: 40643 vs 46225
> Jiri reported on arm: 25022 vs 55812
> 
> [0]: https://lore.kernel.org/dwarves/20210423213728.3538141-1-kafai@fb.com/
> 
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> ---
>  btf_encoder.c | 272 +-------------------------------------------------
>  1 file changed, 5 insertions(+), 267 deletions(-)
> 
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 80e896961d4e..55c5f8e30cac 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -27,17 +27,8 @@
>   */
>  #define KSYM_NAME_LEN 128
>  
> -struct funcs_layout {
> -	unsigned long mcount_start;
> -	unsigned long mcount_stop;
> -	unsigned long mcount_sec_idx;
> -};
> -
>  struct elf_function {
>  	const char	*name;
> -	unsigned long	 addr;
> -	unsigned long	 size;
> -	unsigned long	 sh_addr;
>  	bool		 generated;
>  };
>  
> @@ -98,250 +89,11 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
>  	}
>  

we could also remove sym_sec_idx/last_idx right?
it's there for the sh.sh_addr, which got removed

jirka

>  	functions[functions_cnt].name = name;
> -	functions[functions_cnt].addr = elf_sym__value(sym);
> -	functions[functions_cnt].size = elf_sym__size(sym);
> -	functions[functions_cnt].sh_addr = sh.sh_addr;
>  	functions[functions_cnt].generated = false;
>  	functions_cnt++;
>  	return 0;
>  }
>  

SNIP