Re: [PATCH bpf-next v4 6/8] libbpf: Support linking bpf objects of either endianness

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On Fri, Aug 30, 2024 at 12:30 AM Tony Ambardar <tony.ambardar@xxxxxxxxx> wrote:
>
> Allow static linking object files of either endianness, checking that input
> files have consistent byte-order, and setting output endianness from input.
>
> Linking requires in-memory processing of programs, relocations, sections,
> etc. in native endianness, and output conversion to target byte-order. This
> is enabled by built-in ELF translation and recent BTF/BTF.ext endianness
> functions. Further add local functions for swapping byte-order of sections
> containing BPF insns.
>
> Signed-off-by: Tony Ambardar <tony.ambardar@xxxxxxxxx>
> ---
>  tools/lib/bpf/linker.c | 90 ++++++++++++++++++++++++++++++++++--------
>  1 file changed, 74 insertions(+), 16 deletions(-)
>

[...]

>
> +static bool is_exec_sec(struct dst_sec *sec)
> +{
> +       if (!sec || sec->ephemeral)
> +               return false;
> +       return (sec->shdr->sh_type == SHT_PROGBITS) &&
> +              (sec->shdr->sh_flags & SHF_EXECINSTR);
> +}
> +
> +static int exec_sec_bswap(void *raw_data, int size)
> +{
> +       const int insn_cnt = size / sizeof(struct bpf_insn);
> +       struct bpf_insn *insn = raw_data;
> +       int i;
> +
> +       if (size % sizeof(struct bpf_insn))
> +               return -EINVAL;

this shouldn't be checked here, it should be assumed this is valid and
was ensured by the caller. And make exec_sec_bswap() a void function,
please.

> +       for (i = 0; i < insn_cnt; i++, insn++)
> +               bpf_insn_bswap(insn);
> +       return 0;
> +}
> +
>  static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src_sec *src)
>  {
>         void *tmp;
> @@ -1170,6 +1203,10 @@ static int extend_sec(struct bpf_linker *linker, struct dst_sec *dst, struct src
>                 memset(dst->raw_data + dst->sec_sz, 0, dst_align_sz - dst->sec_sz);
>                 /* now copy src data at a properly aligned offset */
>                 memcpy(dst->raw_data + dst_align_sz, src->data->d_buf, src->shdr->sh_size);
> +

the check for size % sizeof(struct bpf_insn) should be somewhere here
(if is_exec_sec()), right?

> +               /* convert added bpf insns to native byte-order */
> +               if (linker->swapped_endian && is_exec_sec(dst))
> +                       exec_sec_bswap(dst->raw_data + dst_align_sz, src->shdr->sh_size);
>         }
>
>         dst->sec_sz = dst_final_sz;
> @@ -2630,6 +2667,10 @@ int bpf_linker__finalize(struct bpf_linker *linker)
>                 if (!sec->scn)
>                         continue;
>

but no need to check here, we know it's correct, if we got all the way here

> +               /* restore sections with bpf insns to target byte-order */
> +               if (linker->swapped_endian && is_exec_sec(sec))
> +                       exec_sec_bswap(sec->raw_data, sec->sec_sz);
> +
>                 sec->data->d_buf = sec->raw_data;
>         }
>
> @@ -2698,6 +2739,7 @@ static int emit_elf_data_sec(struct bpf_linker *linker, const char *sec_name,
>
>  static int finalize_btf(struct bpf_linker *linker)
>  {
> +       enum btf_endianness link_endianness;
>         LIBBPF_OPTS(btf_dedup_opts, opts);
>         struct btf *btf = linker->btf;
>         const void *raw_data;
> @@ -2742,6 +2784,22 @@ static int finalize_btf(struct bpf_linker *linker)
>                 return err;
>         }
>
> +       /* Set .BTF and .BTF.ext output byte order */
> +       link_endianness = linker->elf_hdr->e_ident[EI_DATA] == ELFDATA2MSB ?
> +                         BTF_BIG_ENDIAN : BTF_LITTLE_ENDIAN;
> +       err = btf__set_endianness(linker->btf, link_endianness);
> +       if (err) {
> +               pr_warn("failed to set .BTF output endianness: %d\n", err);
> +               return err;
> +       }

link_endianness is always well-formed enum, there is no need to check
errors, here and for btf_ext__set_endianness, please drop both

> +       if (linker->btf_ext) {
> +               err = btf_ext__set_endianness(linker->btf_ext, link_endianness);
> +               if (err) {
> +                       pr_warn("failed to set .BTF.ext output endianness: %d\n", err);
> +                       return err;
> +               }
> +       }
> +
>         /* Emit .BTF section */
>         raw_data = btf__raw_data(linker->btf, &raw_sz);
>         if (!raw_data)
> --
> 2.34.1
>





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