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

[Tony Ambardar <tony.ambardar@gmail.com>]

On Fri, Aug 30, 2024 at 02:25:07PM -0700, Andrii Nakryiko wrote:
> On Fri, Aug 30, 2024 at 12:30 AM Tony Ambardar <tony.ambardar@gmail.com> 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@gmail.com>
> > ---
> >  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
> 

I did a pass earlier to reorganize sanity checks and remove redundant
ones, and realized linker_sanity_check_elf() already does what we want
but overlooked dropping this from exec_sec_bswap(). Will do so now. Thanks
for catching!

> > +               /* 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

Right, makes sense.

> 
> > +       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
> >