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

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

•           /* 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