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/
Manu
