Re: [PATCH -next 1/2] bpf: Unify data extension operation of jited_ksyms and jited_linfo

[Pu Lehui <pulehui@huawei.com>]

Hi Andrii,

On 2022/4/28 6:33, Andrii Nakryiko wrote:
> On Tue, Apr 26, 2022 at 6:40 AM Pu Lehui <pulehui@huawei.com> wrote:
>>
>> We found that 32-bit environment can not print bpf line info due
>> to data inconsistency between jited_ksyms[0] and jited_linfo[0].
>>
>> For example:
>> jited_kyms[0] = 0xb800067c, jited_linfo[0] = 0xffffffffb800067c
>>
>> We know that both of them store bpf func address, but due to the
>> different data extension operations when extended to u64, they may
>> not be the same. We need to unify the data extension operations of
>> them.
>>
>> Signed-off-by: Pu Lehui <pulehui@huawei.com>
>> ---
>>   kernel/bpf/syscall.c                         |  5 ++++-
>>   tools/lib/bpf/bpf_prog_linfo.c               |  8 ++++----
>>   tools/testing/selftests/bpf/prog_tests/btf.c | 18 +++++++++---------
> 
> please split kernel changes, libbpf changes, and selftests/bpf changes
> into separate patches
Thanks for your review. Alright, I will split it next time.

> 
>>   3 files changed, 17 insertions(+), 14 deletions(-)
>>
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index e9621cfa09f2..4c417c806d92 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -3868,13 +3868,16 @@ static int bpf_prog_get_info_by_fd(struct file *file,
>>                  info.nr_jited_line_info = 0;
>>          if (info.nr_jited_line_info && ulen) {
>>                  if (bpf_dump_raw_ok(file->f_cred)) {
>> +                       unsigned long jited_linfo_addr;
>>                          __u64 __user *user_linfo;
>>                          u32 i;
>>
>>                          user_linfo = u64_to_user_ptr(info.jited_line_info);
>>                          ulen = min_t(u32, info.nr_jited_line_info, ulen);
>>                          for (i = 0; i < ulen; i++) {
>> -                               if (put_user((__u64)(long)prog->aux->jited_linfo[i],
>> +                               jited_linfo_addr = (unsigned long)
>> +                                       prog->aux->jited_linfo[i];
>> +                               if (put_user((__u64) jited_linfo_addr,
>>                                               &user_linfo[i]))
>>                                          return -EFAULT;
>>                          }
Please let me to explain more detail, sorry if I'm wordy.
The main reason that 32-bit env does not print bpf line info is here:

kernel/bpf/syscall.c:
bpf_prog_get_info_by_fd {
	...
	user_ksyms = u64_to_user_ptr(info.jited_ksyms);
	ksym_addr = (unsigned long)prog->aux->func[i]->bpf_func;
	if (put_user((u64) ksym_addr, &user_ksyms[i]))
	...

	user_linfo = u64_to_user_ptr(info.jited_line_info);
	if (put_user((__u64)(long)prog->aux->jited_linfo[i],
		     &user_linfo[i]))
	...
}

In 32-bit env, ksym_addr and prog->aux->jited_linfo[0] both store the 
32-bit address of bpf_func, but the first one is zero-extension to u64, 
while the other is sign-extension to u64.
For example:
	prog->aux->func[0]->bpf_func = 0xb800067c
	user_ksyms[0] = 0xb800067c, user_linfo[0] = 0xffffffffb800067c

Both zero-extension and sign-extension are fine, but if operating 
directly between them without casting in 32-bit env, there will have 
some potential problems. Such as:

tools/lib/bpf/bpf_prog_linfo.c:
dissect_jited_func {
	...
	if (ksym_func[0] != *jited_linfo) //always missmatch in 32 env
		goto errout;
	...
	if (ksym_func[f] == *jited_linfo) {
	...
	last_jited_linfo = *jited_linfo;
	if (last_jited_linfo - ksym_func[f - 1] + 1 >
	    ksym_len[f - 1])
	...
}

We could cast them to 32-bit data type, but I think unify data extension 
operation will be better.

>> diff --git a/tools/lib/bpf/bpf_prog_linfo.c b/tools/lib/bpf/bpf_prog_linfo.c
>> index 5c503096ef43..5cf41a563ef5 100644
>> --- a/tools/lib/bpf/bpf_prog_linfo.c
>> +++ b/tools/lib/bpf/bpf_prog_linfo.c
>> @@ -127,7 +127,7 @@ struct bpf_prog_linfo *bpf_prog_linfo__new(const struct bpf_prog_info *info)
>>          prog_linfo->raw_linfo = malloc(data_sz);
>>          if (!prog_linfo->raw_linfo)
>>                  goto err_free;
>> -       memcpy(prog_linfo->raw_linfo, (void *)(long)info->line_info, data_sz);
>> +       memcpy(prog_linfo->raw_linfo, (void *)(unsigned long)info->line_info, data_sz);
>>
>>          nr_jited_func = info->nr_jited_ksyms;
>>          if (!nr_jited_func ||
>> @@ -148,7 +148,7 @@ struct bpf_prog_linfo *bpf_prog_linfo__new(const struct bpf_prog_info *info)
>>          if (!prog_linfo->raw_jited_linfo)
>>                  goto err_free;
>>          memcpy(prog_linfo->raw_jited_linfo,
>> -              (void *)(long)info->jited_line_info, data_sz);
>> +              (void *)(unsigned long)info->jited_line_info, data_sz);
>>
>>          /* Number of jited_line_info per jited func */
>>          prog_linfo->nr_jited_linfo_per_func = malloc(nr_jited_func *
>> @@ -166,8 +166,8 @@ struct bpf_prog_linfo *bpf_prog_linfo__new(const struct bpf_prog_info *info)
>>                  goto err_free;
>>
>>          if (dissect_jited_func(prog_linfo,
>> -                              (__u64 *)(long)info->jited_ksyms,
>> -                              (__u32 *)(long)info->jited_func_lens))
>> +                              (__u64 *)(unsigned long)info->jited_ksyms,
>> +                              (__u32 *)(unsigned long)info->jited_func_lens))
> 
> so I'm trying to understand how this is changing anything for 32-bit
> architecture and I must be missing something, sorry if I'm being
> dense. The example you used below
> 
> jited_kyms[0] = 0xb800067c, jited_linfo[0] = 0xffffffffb800067c
> 
> Wouldn't (unsigned long)0xffffffffb800067c == (long)0xffffffffb800067c
> == 0xb800067c ?
If I understand correctly, info->jited_ksyms or info->jited_func_lens is 
just a u64 address that point to the corresponding space. The bpf_func 
address is stored in the item of info->jited_ksyms but not 
info->jited_ksyms.

And here, I may have misled you. Both (__u64 *)(long)info->jited_ksyms 
and (__u64 *)(unsigned long)info->jited_ksyms are the same, I just want 
to unify the style. I will remove them in v2.

Please let me know if there is any problem with my understanding.

Thanks,
Lehui
> 
> isn't sizeof(long) == sizeof(void*) == 4?
> 
> It would be nice if you could elaborate a bit more on what problems
> did you see in practice?
> 
>>                  goto err_free;
>>
>>          return prog_linfo;
>> diff --git a/tools/testing/selftests/bpf/prog_tests/btf.c b/tools/testing/selftests/bpf/prog_tests/btf.c
>> index 84aae639ddb5..d9ba1ec1d5b3 100644
>> --- a/tools/testing/selftests/bpf/prog_tests/btf.c
>> +++ b/tools/testing/selftests/bpf/prog_tests/btf.c
>> @@ -6451,8 +6451,8 @@ static int test_get_linfo(const struct prog_info_raw_test *test,
>>                    info.nr_jited_line_info, jited_cnt,
>>                    info.line_info_rec_size, rec_size,
>>                    info.jited_line_info_rec_size, jited_rec_size,
>> -                 (void *)(long)info.line_info,
>> -                 (void *)(long)info.jited_line_info)) {
>> +                 (void *)(unsigned long)info.line_info,
>> +                 (void *)(unsigned long)info.jited_line_info)) {
>>                  err = -1;
>>                  goto done;
>>          }
>> @@ -6500,8 +6500,8 @@ static int test_get_linfo(const struct prog_info_raw_test *test,
>>          }
>>
>>          if (CHECK(jited_linfo[0] != jited_ksyms[0],
>> -                 "jited_linfo[0]:%lx != jited_ksyms[0]:%lx",
>> -                 (long)(jited_linfo[0]), (long)(jited_ksyms[0]))) {
>> +                 "jited_linfo[0]:%llx != jited_ksyms[0]:%llx",
>> +                 jited_linfo[0], jited_ksyms[0])) {
>>                  err = -1;
>>                  goto done;
>>          }
>> @@ -6519,16 +6519,16 @@ static int test_get_linfo(const struct prog_info_raw_test *test,
>>                  }
>>
>>                  if (CHECK(jited_linfo[i] <= jited_linfo[i - 1],
>> -                         "jited_linfo[%u]:%lx <= jited_linfo[%u]:%lx",
>> -                         i, (long)jited_linfo[i],
>> -                         i - 1, (long)(jited_linfo[i - 1]))) {
>> +                         "jited_linfo[%u]:%llx <= jited_linfo[%u]:%llx",
>> +                         i, jited_linfo[i],
>> +                         i - 1, (jited_linfo[i - 1]))) {
>>                          err = -1;
>>                          goto done;
>>                  }
>>
>>                  if (CHECK(jited_linfo[i] - cur_func_ksyms > cur_func_len,
>> -                         "jited_linfo[%u]:%lx - %lx > %u",
>> -                         i, (long)jited_linfo[i], (long)cur_func_ksyms,
>> +                         "jited_linfo[%u]:%llx - %llx > %u",
>> +                         i, jited_linfo[i], cur_func_ksyms,
>>                            cur_func_len)) {
>>                          err = -1;
>>                          goto done;
>> --
>> 2.25.1
>>
> .
> 

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