Re: [PATCH bpf-next v2 6/8] libbpf: Support linking bpf objects of either endianness

On Thu, Aug 22, 2024 at 2:25 AM Tony Ambardar tony.ambardar@gmail.com wrote:

From: Tony Ambardar tony.ambardar@gmail.com

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 | 106 ++++++++++++++++++++++++++++++++++------- 1 file changed, 90 insertions(+), 16 deletions(-)

Mostly just stylistic and code organization nits, the change overall looks good.

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c index 7489306cd6f7..9bf218db443e 100644 --- a/tools/lib/bpf/linker.c +++ b/tools/lib/bpf/linker.c @@ -135,6 +135,7 @@ struct bpf_linker { int fd; Elf *elf; Elf64_Ehdr *elf_hdr;

•   bool swapped_endian;
  
    /* Output sections metadata */
    struct dst_sec *secs;
  

@@ -324,13 +325,8 @@ static int init_output_elf(struct bpf_linker *linker, const char *file)

    linker->elf_hdr->e_machine = EM_BPF;
    linker->elf_hdr->e_type = ET_REL;

-#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__

•   linker->elf_hdr->e_ident[EI_DATA] = ELFDATA2LSB;
  

-#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__

•   linker->elf_hdr->e_ident[EI_DATA] = ELFDATA2MSB;
  

-#else -#error "Unknown __BYTE_ORDER__" -#endif

•   /* Set unknown ELF endianness, assign later from input files */
  

•   linker->elf_hdr->e_ident[EI_DATA] = ELFDATANONE;
  
    /* STRTAB */
    /* initialize strset with an empty string to conform to ELF */
  

@@ -541,19 +537,21 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename, const struct bpf_linker_file_opts *opts, struct src_obj *obj) { -#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__

•   const int host_endianness = ELFDATA2LSB;
  

-#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__

•   const int host_endianness = ELFDATA2MSB;
  

-#else -#error "Unknown __BYTE_ORDER__" -#endif int err = 0; Elf_Scn *scn; Elf_Data *data; Elf64_Ehdr *ehdr; Elf64_Shdr *shdr; struct src_sec *sec;

•   unsigned char obj_byteorder;
  

•   unsigned char *link_byteorder = &linker->elf_hdr->e_ident[EI_DATA];
  

nit: not a fan of pointer into e_ident, just read local value of byte order, and then assign it directly below (it's only in one of the branches, no duplication, really)

+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__

•   const unsigned char host_byteorder = ELFDATA2LSB;
  

+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__

•   const unsigned char host_byteorder = ELFDATA2MSB;
  

+#else +#error "Unknown __BYTE_ORDER__" +#endif

    pr_debug("linker: adding object file '%s'...\n", filename);

@@ -579,11 +577,25 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename, pr_warn_elf("failed to get ELF header for %s", filename); return err; }

•   if (ehdr->e_ident[EI_DATA] != host_endianness) {
  

•   /* Linker output endianness set by first input object */
  

•   obj_byteorder = ehdr->e_ident[EI_DATA];
  

•   if (obj_byteorder != ELFDATA2LSB && obj_byteorder != ELFDATA2MSB) {
            err = -EOPNOTSUPP;
  

•           pr_warn_elf("unsupported byte order of ELF file %s", filename);
  

•           pr_warn("linker: unknown byte order of ELF file %s\n", filename);
            return err;
    }
  

•   if (*link_byteorder == ELFDATANONE) {
  

•           *link_byteorder = obj_byteorder;
  

see above, I'd prefer:

linker->elf_hdr->e_ident[EI_DATA] = obj_byteorder;

•           linker->swapped_endian = obj_byteorder != host_byteorder;
  

•           pr_debug("linker: set %s-endian output byte order\n",
  

•                    obj_byteorder == ELFDATA2MSB ? "big" : "little");
  

•   } else if (*link_byteorder != obj_byteorder) {
  

•           err = -EOPNOTSUPP;
  

•           pr_warn("linker: byte order mismatch with ELF file %s\n", filename);
  

•           return err;
  

•   }
  

•   if (ehdr->e_type != ET_REL
        || ehdr->e_machine != EM_BPF
        || ehdr->e_ident[EI_CLASS] != ELFCLASS64) {
  

@@ -1111,6 +1123,27 @@ static bool sec_content_is_same(struct dst_sec *dst_sec, struct src_sec *src_sec return true; }

+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;
  

•   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 +1203,16 @@ 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);

•           /* convert added bpf insns to native byte-order */
  

•           if (linker->swapped_endian && is_exec_sec(dst)) {
  

•                   err = exec_sec_bswap(dst->raw_data + dst_align_sz,
  

•                                        src->shdr->sh_size);
  

nit: I think exec_sec_bswap() shouldn't ever fail, so given we have is_exec_sec() now, let's do the size alignment check early on (and regardless of swapped_endian), and then just proceed with byte swap that can't fail

•                   if (err) {
  

•                           pr_warn("%s: error changing insns endianness\n", __func__);
  

•                           return err;
  

•                   }
  

•           }
    }
  
    dst->sec_sz = dst_final_sz;
  

@@ -2630,6 +2673,14 @@ int bpf_linker__finalize(struct bpf_linker *linker) if (!sec->scn) continue;

•           /* restore sections with bpf insns to target byte-order */
  

•           if (linker->swapped_endian && is_exec_sec(sec)) {
  

•                   err = exec_sec_bswap(sec->raw_data, sec->sec_sz);
  

and here we'll know that size is validly aligned anyways, so no checks required

•                   if (err) {
  

•                           pr_warn("error finalizing insns endianness\n");
  

•                           return libbpf_err(err);
  

•                   }
  

•           }
            sec->data->d_buf = sec->raw_data;
    }
  
  

@@ -2696,6 +2747,13 @@ static int emit_elf_data_sec(struct bpf_linker *linker, const char *sec_name, return 0; }

+static enum btf_endianness +linker_btf_endianness(const struct bpf_linker *linker) +{

•   unsigned char byteorder = linker->elf_hdr->e_ident[EI_DATA];
  

empty line between variable declaration and the rest of the code

•   return byteorder == ELFDATA2MSB ? BTF_BIG_ENDIAN : BTF_LITTLE_ENDIAN;
  

+}

but actually, this whole helper function seems unnecessary, just do everything inside finalize_btf, it's a pretty trivial piece of logic

static int finalize_btf(struct bpf_linker *linker) { LIBBPF_OPTS(btf_dedup_opts, opts); @@ -2742,6 +2800,22 @@ static int finalize_btf(struct bpf_linker *linker) return err; }

•   /* Set .BTF and .BTF.ext output byte order */
  

•   err = btf__set_endianness(linker->btf,
  

•                             linker_btf_endianness(linker));
  

•   if (err) {
  

•           pr_warn("failed to set .BTF output endianness: %d\n", err);
  

nit: you used "linker: " prefix for messages like this, stay consistent?

•           return err;
  

•   }
  

•   if (linker->btf_ext) {
  

•           err = btf_ext__set_endianness(linker->btf_ext,
  

•                                         linker_btf_endianness(linker));
  

•           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