On 2/3/24 19:48, Manu Bretelle wrote:
> On Fri, Feb 02, 2024 at 06:38:18PM -0700, Daniel Xu wrote:
>> 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
>
> I ran some test and tried and validated Jiri's patch in
> https://lore.kernel.org/bpf/Zb6Jt30bNcNhM6zR@surya/
Right, I'll include that patch. Thanks for testing!
Viktor
>
> Manu
>
