netdev - Re: [PATCHv5 bpf] bpf: Move iterator functions into special init section

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Date:   Mon, 9 Nov 2020 23:04:34 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Yonghong Song <yhs@...com>, Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCHv5 bpf] bpf: Move iterator functions into special init
 section

On 11/9/20 7:57 PM, Jiri Olsa wrote:
> With upcoming changes to pahole, that change the way how and
> which kernel functions are stored in BTF data, we need a way
> to recognize iterator functions.
> 
> Iterator functions need to be in BTF data, but have no real
> body and are currently placed in .init.text section, so they
> are freed after kernel init and are filtered out of BTF data
> because of that.
> 
> The solution is to place these functions under new section:
>    .init.bpf.preserve_type
> 
> And add 2 new symbols to mark that area:
>    __init_bpf_preserve_type_begin
>    __init_bpf_preserve_type_end
> 
> The code in pahole responsible for picking up the functions will
> be able to recognize functions from this section and add them to
> the BTF data and filter out all other .init.text functions.
> 
> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
> Suggested-by: Yonghong Song <yhs@...com>
> Acked-by: Song Liu <songliubraving@...com>
> Acked-by: Andrii Nakryiko <andrii@...nel.org>
> Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> ---
> v5 changes:
>    - use "" in __section macro due to:
>      33def8498fdde180 ("treewide: Convert macro and uses of __section(foo) to __section("foo")")
>      [Arnaldo]
> 
> v4: https://lore.kernel.org/bpf/20201106222512.52454-1-jolsa@kernel.org/
> 
>   include/asm-generic/vmlinux.lds.h | 16 +++++++++++++++-
>   include/linux/bpf.h               |  8 +++++++-
>   include/linux/init.h              |  1 +
>   3 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index b2b3d81b1535..f91029b3443b 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -685,8 +685,21 @@
>   	.BTF_ids : AT(ADDR(.BTF_ids) - LOAD_OFFSET) {			\
>   		*(.BTF_ids)						\
>   	}
> +
> +/*
> + * .init.bpf.preserve_type
> + *
> + * This section store special BPF function and marks them
> + * with begin/end symbols pair for the sake of pahole tool.
> + */
> +#define INIT_BPF_PRESERVE_TYPE						\
> +	__init_bpf_preserve_type_begin = .;                             \
> +	*(.init.bpf.preserve_type)                                      \
> +	__init_bpf_preserve_type_end = .;				\
> +	MEM_DISCARD(init.bpf.preserve_type)
>   #else
>   #define BTF
> +#define INIT_BPF_PRESERVE_TYPE
>   #endif
>   
>   /*
> @@ -741,7 +754,8 @@
>   #define INIT_TEXT							\
>   	*(.init.text .init.text.*)					\
>   	*(.text.startup)						\
> -	MEM_DISCARD(init.text*)
> +	MEM_DISCARD(init.text*)						\
> +	INIT_BPF_PRESERVE_TYPE
>   
>   #define EXIT_DATA							\
>   	*(.exit.data .exit.data.*)					\
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 2b16bf48aab6..73e8ededde3e 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1276,10 +1276,16 @@ struct bpf_link *bpf_link_get_from_fd(u32 ufd);
>   int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
>   int bpf_obj_get_user(const char __user *pathname, int flags);
>   
> +#ifdef CONFIG_DEBUG_INFO_BTF
> +#define BPF_INIT __init_bpf_preserve_type
> +#else
> +#define BPF_INIT __init
> +#endif
> +
>   #define BPF_ITER_FUNC_PREFIX "bpf_iter_"
>   #define DEFINE_BPF_ITER_FUNC(target, args...)			\
>   	extern int bpf_iter_ ## target(args);			\
> -	int __init bpf_iter_ ## target(args) { return 0; }
> +	int BPF_INIT bpf_iter_ ## target(args) { return 0; }
>   
>   struct bpf_iter_aux_info {
>   	struct bpf_map *map;
> diff --git a/include/linux/init.h b/include/linux/init.h
> index 7b53cb3092ee..a7c71e3b5f9a 100644
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -52,6 +52,7 @@
>   #define __initconst	__section(".init.rodata")
>   #define __exitdata	__section(".exit.data")
>   #define __exit_call	__used __section(".exitcall.exit")
> +#define __init_bpf_preserve_type __section(".init.bpf.preserve_type")

Small nit, why this detour via BPF_INIT define? Couldn't we just:

#ifdef CONFIG_DEBUG_INFO_BTF
#define __init_bpf_preserve_type   __section(".init.bpf.preserve_type")
#else
#define __init_bpf_preserve_type   __init
#endif

Also, the comment above the existing defines says '/* These are for everybody (although
not all archs will actually discard it in modules) */' ... We should probably not add
the __init_bpf_preserve_type right under this listing as-is in your patch, but instead
'separate' it by adding a small comment on top of its definition by explaining its
purpose more clearly for others.

>   /*
>    * modpost check for section mismatches during the kernel build.
>