On Tue, Jan 23, 2024 at 4:08 AM Viktor Malik <vmalik@xxxxxxxxxx> wrote:
>
> The .BTF_ids section is pre-filled with zeroed BTF ID entries during the
> build and afterwards patched by resolve_btfids with correct values.
> Since resolve_btfids always writes in host-native endianness, it relies
> on libelf to do the translation when the target ELF is cross-compiled to
> a different endianness (this was introduced in commit 61e8aeda9398
> ("bpf: Fix libelf endian handling in resolv_btfids")).
>
> Unfortunately, the translation will corrupt the flags fields of SET8
> entries because these were written during vmlinux compilation and are in
> the correct endianness already. This will lead to numerous selftests
> failures such as:
>
> $ sudo ./test_verifier 502 502
> #502/p sleepable fentry accept FAIL
> Failed to load prog 'Invalid argument'!
> bpf_fentry_test1 is not sleepable
> verification time 34 usec
> stack depth 0
> processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> Summary: 0 PASSED, 0 SKIPPED, 1 FAILED
>
> Since it's not possible to instruct libelf to translate just certain
> values, let's manually bswap the flags in resolve_btfids when needed, so
> that libelf then translates everything correctly.
>
> Fixes: ef2c6f370a63 ("tools/resolve_btfids: Add support for 8-byte BTF sets")
> Signed-off-by: Viktor Malik <vmalik@xxxxxxxxxx>
> ---
> tools/bpf/resolve_btfids/main.c | 35 +++++++++++++++++++++++++++++++--
> 1 file changed, 33 insertions(+), 2 deletions(-)
>
> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index 27a23196d58e..440d3d066ce4 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c
> @@ -646,18 +646,31 @@ static int cmp_id(const void *pa, const void *pb)
> return *a - *b;
> }
>
> +static int need_bswap(int elf_byte_order)
> +{
> + return __BYTE_ORDER == __LITTLE_ENDIAN && elf_byte_order != ELFDATA2LSB ||
> + __BYTE_ORDER == __BIG_ENDIAN && elf_byte_order != ELFDATA2MSB;
return (__BYTE_ORDER == __LITTLE_ENDIAN) != (elf_byte_order == ELFDATA2LSB);
?
> +}
> +
> static int sets_patch(struct object *obj)
> {
> Elf_Data *data = obj->efile.idlist;
> int *ptr = data->d_buf;
> struct rb_node *next;
> + GElf_Ehdr ehdr;
> +
> + if (gelf_getehdr(obj->efile.elf, &ehdr) == NULL) {
> + pr_err("FAILED cannot get ELF header: %s\n",
> + elf_errmsg(-1));
> + return -1;
> + }
calculate needs_bswap() once here?
>
> next = rb_first(&obj->sets);
> while (next) {
> - unsigned long addr, idx;
> + unsigned long addr, idx, flags;
> struct btf_id *id;
> int *base;
> - int cnt;
> + int cnt, i;
>
> id = rb_entry(next, struct btf_id, rb_node);
> addr = id->addr[0];
> @@ -679,6 +692,24 @@ static int sets_patch(struct object *obj)
>
> qsort(base, cnt, id->is_set8 ? sizeof(uint64_t) : sizeof(int), cmp_id);
>
> + /*
> + * When ELF endianness does not match endianness of the host,
> + * libelf will do the translation when updating the ELF. This,
> + * however, corrupts SET8 flags which are already in the target
> + * endianness. So, let's bswap them to the host endianness and
> + * libelf will then correctly translate everything.
> + */
> + if (id->is_set8 && need_bswap(ehdr.e_ident[EI_DATA])) {
> + for (i = 0; i < cnt; i++) {
> + /*
> + * header and entries are 8-byte, flags is the
> + * second half of an entry
> + */
> + flags = idx + (i + 1) * 2 + 1;
> + ptr[flags] = bswap_32(ptr[flags]);
we are dealing with struct btf_id_set8, right? Can't we #include
include/linux/btf_ids.h and use that type for all these offset
calculations?..
I have the same question for existing code, tbh, so maybe there was
some good reason, not sure...
> + }
> + }
> +
> next = rb_next(next);
> }
> return 0;
> --
> 2.43.0
>
>
