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@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
>
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
> >
