netdev - Re: [PATCH v2 bpf-next 05/17] libbpf: allow gaps in BPF program sections to support overriden weak functions

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Date:   Wed, 21 Apr 2021 23:25:50 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii@...nel.org>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <ast@...com>, <daniel@...earbox.net>
CC:     <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 05/17] libbpf: allow gaps in BPF program
 sections to support overriden weak functions



On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
> Currently libbpf is very strict about parsing BPF program isnstruction

isnstruction => instruction

> sections. No gaps are allowed between sequential BPF programs within a given
> ELF section. Libbpf enforced that by keeping track of the next section offset
> that should start a new BPF (sub)program and cross-checks that by searching for
> a corresponding STT_FUNC ELF symbol.
> 
> But this is too restrictive once we allow to have weak BPF programs and link
> together two or more BPF object files. In such case, some weak BPF programs
> might be "overriden" by either non-weak BPF program with the same name and

overriden => overridden

> signature, or even by another weak BPF program that just happened to be linked
> first. That, in turn, leaves BPF instructions of the "lost" BPF (sub)program
> intact, but there is no corresponding ELF symbol, because no one is going to
> be referencing it.
> 
> Libbpf already correctly handles such cases in the sense that it won't append
> such dead code to actual BPF programs loaded into kernel. So the only change
> that needs to be done is to relax the logic of parsing BPF instruction
> sections. Instead of assuming next BPF (sub)program section offset, iterate
> available STT_FUNC ELF symbols to discover all available BPF subprograms and
> programs.
> 
> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>

Ack with a minor suggestion below.
Acked-by: Yonghong Song <yhs@...com>

> ---
>   tools/lib/bpf/libbpf.c | 56 ++++++++++++++++--------------------------
>   1 file changed, 21 insertions(+), 35 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index ce5558d0a61b..a0e6d6bc47f3 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -502,8 +502,6 @@ static Elf_Scn *elf_sec_by_name(const struct bpf_object *obj, const char *name);
>   static int elf_sec_hdr(const struct bpf_object *obj, Elf_Scn *scn, GElf_Shdr *hdr);
>   static const char *elf_sec_name(const struct bpf_object *obj, Elf_Scn *scn);
>   static Elf_Data *elf_sec_data(const struct bpf_object *obj, Elf_Scn *scn);
> -static int elf_sym_by_sec_off(const struct bpf_object *obj, size_t sec_idx,
> -			      size_t off, __u32 sym_type, GElf_Sym *sym);
>   
>   void bpf_program__unload(struct bpf_program *prog)
>   {
> @@ -644,10 +642,12 @@ static int
>   bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
>   			 const char *sec_name, int sec_idx)
>   {
> +	Elf_Data *symbols = obj->efile.symbols;
>   	struct bpf_program *prog, *progs;
>   	void *data = sec_data->d_buf;
>   	size_t sec_sz = sec_data->d_size, sec_off, prog_sz;
> -	int nr_progs, err;
> +	size_t n = symbols->d_size / sizeof(GElf_Sym);

Maybe use "nr_syms" instead of "n" to be more descriptive?

> +	int nr_progs, err, i;
>   	const char *name;
>   	GElf_Sym sym;
>   
> @@ -655,14 +655,16 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
>   	nr_progs = obj->nr_programs;
>   	sec_off = 0;
>   
> -	while (sec_off < sec_sz) {
> -		if (elf_sym_by_sec_off(obj, sec_idx, sec_off, STT_FUNC, &sym)) {
> -			pr_warn("sec '%s': failed to find program symbol at offset %zu\n",
> -				sec_name, sec_off);
> -			return -LIBBPF_ERRNO__FORMAT;
> -		}
> +	for (i = 0; i < n; i++) {
> +		if (!gelf_getsym(symbols, i, &sym))
> +			continue;
> +		if (sym.st_shndx != sec_idx)
> +			continue;
> +		if (GELF_ST_TYPE(sym.st_info) != STT_FUNC)
> +			continue;
>   
>   		prog_sz = sym.st_size;
> +		sec_off = sym.st_value;
>   
>   		name = elf_sym_str(obj, sym.st_name);
>   		if (!name) {
> @@ -711,8 +713,6 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
>   
>   		nr_progs++;
>   		obj->nr_programs = nr_progs;
> -
> -		sec_off += prog_sz;
>   	}
>   
>   	return 0;
> @@ -2825,26 +2825,6 @@ static Elf_Data *elf_sec_data(const struct bpf_object *obj, Elf_Scn *scn)
>   	return data;
>   }
>   
[...]