Re: [PATCH bpf-next v2 2/2] tools/resolve_btfids: fix cross-compilation to non-host endianness

[Jiri Olsa <olsajiri@gmail.com>]

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@redhat.com>
> > ---
> >  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.

ah did not this, right, looks like we need that

jirka