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Sun, 28 Jan 2024 12:17:20 -0800 (PST) Received: from krava ([83.240.60.213]) by smtp.gmail.com with ESMTPSA id u11-20020a056000038b00b0033aed5feea4sm1043426wrf.54.2024.01.28.12.17.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 28 Jan 2024 12:17:20 -0800 (PST) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Sun, 28 Jan 2024 21:17:18 +0100 To: Andrii Nakryiko Cc: Viktor Malik , bpf@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Ian Rogers , Andrew Morton , Alexey Dobriyan , Kumar Kartikeya Dwivedi Subject: Re: [PATCH bpf-next] tools/resolve_btfids: fix cross-compilation to non-host endianness Message-ID: References: <20240123120759.1865189-1-vmalik@redhat.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Jan 26, 2024 at 03:40:11PM -0800, Andrii Nakryiko wrote: > On Tue, Jan 23, 2024 at 4:08 AM 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 hum, I'd think we should have hit such bug long time ago.. set8 is there for some time already.. nice ;-) > > > > 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 > > --- > > 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?.. we could, there's tools/include/linux/btf_ids.h, which we could include in here, we do that in selftests.. but it needs to be updated with latest kernel updates (at least with set8 struct) > > I have the same question for existing code, tbh, so maybe there was > some good reason, not sure... I think the test came later and I did not think of it for the resolve_btfids itself, I guess it might make the code more readable thanks, jirka > > > + } > > + } > > + > > next = rb_next(next); > > } > > return 0; > > -- > > 2.43.0 > > > >