Hi Viktor,
On Wed, Jan 31, 2024 at 05:24:09PM +0100, Viktor Malik 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 | 27 ++++++++++++++++++++++++++-
> 1 file changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index 7badf1557e5c..d01603ef6283 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c
> @@ -652,13 +652,23 @@ 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;
> + int need_bswap;
> +
> + if (gelf_getehdr(obj->efile.elf, &ehdr) == NULL) {
> + pr_err("FAILED cannot get ELF header: %s\n",
> + elf_errmsg(-1));
> + return -1;
> + }
> + need_bswap = (__BYTE_ORDER == __LITTLE_ENDIAN) !=
> + (ehdr.e_ident[EI_DATA] == ELFDATA2LSB);
>
> next = rb_first(&obj->sets);
> while (next) {
> unsigned long addr, idx;
> struct btf_id *id;
> void *base;
> - int cnt, size;
> + int cnt, size, i;
>
> id = rb_entry(next, struct btf_id, rb_node);
> addr = id->addr[0];
> @@ -686,6 +696,21 @@ static int sets_patch(struct object *obj)
> base = set8->pairs;
> cnt = set8->cnt;
> size = sizeof(set8->pairs[0]);
> +
> + /*
> + * 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 (need_bswap) {
> + for (i = 0; i < cnt; i++) {
> + set8->pairs[i].flags =
> + bswap_32(set8->pairs[i].flags);
> + }
Do we need this for btf_id_set8:flags as well? Didn't get a chance to
look too deeply yet.
Thanks,
Daniel
