On Fri, Aug 23, 2024 at 12:47:52PM -0700, Andrii Nakryiko wrote:
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)
Yes, it's awkward. Fixed.
+#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;
Done.
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
Looking more closely, I see we already have this size check from: 32fa058398 ("libbpf: Add pr_warn() for EINVAL cases in linker_sanity_check_elf")
So can just drop the error-handling.
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
OK, merged as it's not complicated.
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?
Right, original code used extra "linker:" detail only for "debug" messages and not "warn" level. Changed to do the same. Thanks!
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