On 2/1/24 17:36, Daniel Borkmann wrote:
> On 1/31/24 5:24 PM, 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);
>> + }
>> + }
>> }
>>
>
> Could we improve that somewhat, e.g. gathering endianness could be moved into
> elf_collect() and the test could also be simplified (if I'm not missing sth) ?
>
> Like the below (not even compile-tested ...) :
Thanks for the suggestion, that looks nicer than my version. I'll use
the below, it should work pretty much out of the box.
Viktor
>
> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index 7badf1557e5c..7b5f592fe79c 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c
> @@ -90,6 +90,14 @@
>
> #define ADDR_CNT 100
>
> +#if __BYTE_ORDER == __LITTLE_ENDIAN
> +# define ELFDATANATIVE ELFDATA2LSB
> +#elif __BYTE_ORDER == __BIG_ENDIAN
> +# define ELFDATANATIVE ELFDATA2MSB
> +#else
> +# error "Unknown machine endianness!"
> +#endif
> +
> struct btf_id {
> struct rb_node rb_node;
> char *name;
> @@ -117,6 +125,7 @@ struct object {
> int idlist_shndx;
> size_t strtabidx;
> unsigned long idlist_addr;
> + int encoding;
> } efile;
>
> struct rb_root sets;
> @@ -320,6 +329,7 @@ static int elf_collect(struct object *obj)
> {
> Elf_Scn *scn = NULL;
> size_t shdrstrndx;
> + GElf_Ehdr ehdr;
> int idx = 0;
> Elf *elf;
> int fd;
> @@ -351,6 +361,13 @@ static int elf_collect(struct object *obj)
> return -1;
> }
>
> + if (gelf_getehdr(obj->efile.elf, &ehdr) == NULL) {
> + pr_err("FAILED cannot get ELF header: %s\n", elf_errmsg(-1));
> + return -1;
> + }
> +
> + obj->efile.encoding = ehdr.e_ident[EI_DATA];
> +
> /*
> * Scan all the elf sections and look for save data
> * from .BTF_ids section and symbols.
> @@ -649,6 +666,7 @@ static int cmp_id(const void *pa, const void *pb)
>
> static int sets_patch(struct object *obj)
> {
> + bool need_bswap = obj->efile.encoding != ELFDATANATIVE;
> Elf_Data *data = obj->efile.idlist;
> int *ptr = data->d_buf;
> struct rb_node *next;
> @@ -658,7 +676,7 @@ static int sets_patch(struct object *obj)
> 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 +704,21 @@ 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);
> + }
> + }
> }
>
> pr_debug("sorting addr %5lu: cnt %6d [%s]\n",
