Re: [PATCH dwarves] btf_encoder: sanitize non-regular int base type

[Yonghong Song <yhs@fb.com>]

On 2/6/21 10:36 PM, Andrii Nakryiko wrote:
> On Sat, Feb 6, 2021 at 11:21 AM Yonghong Song <yhs@fb.com> wrote:
>>
>> clang with dwarf5 may generate non-regular int base type,
>> i.e., not a signed/unsigned char/short/int/longlong/__int128.
>> Such base types are often used to describe
>> how an actual parameter or variable is generated. For example,
>>
>> 0x000015cf:   DW_TAG_base_type
>>                  DW_AT_name      ("DW_ATE_unsigned_1")
>>                  DW_AT_encoding  (DW_ATE_unsigned)
>>                  DW_AT_byte_size (0x00)
>>
>> 0x00010ed9:         DW_TAG_formal_parameter
>>                        DW_AT_location    (DW_OP_lit0,
>>                                           DW_OP_not,
>>                                           DW_OP_convert (0x000015cf) "DW_ATE_unsigned_1",
>>                                           DW_OP_convert (0x000015d4) "DW_ATE_unsigned_8",
>>                                           DW_OP_stack_value)
>>                        DW_AT_abstract_origin     (0x00013984 "branch")
>>
>> What it does is with a literal "0", did a "not" operation, and the converted to
>> one-bit unsigned int and then 8-bit unsigned int.
>>
>> Another example,
>>
>> 0x000e97e4:   DW_TAG_base_type
>>                  DW_AT_name      ("DW_ATE_unsigned_24")
>>                  DW_AT_encoding  (DW_ATE_unsigned)
>>                  DW_AT_byte_size (0x03)
>>
>> 0x000f88f8:     DW_TAG_variable
>>                    DW_AT_location        (indexed (0x3c) loclist = 0x00008fb0:
>>                       [0xffffffff82808812, 0xffffffff82808817):
>>                           DW_OP_breg0 RAX+0,
>>                           DW_OP_convert (0x000e97d5) "DW_ATE_unsigned_64",
>>                           DW_OP_convert (0x000e97df) "DW_ATE_unsigned_8",
>>                           DW_OP_stack_value,
>>                           DW_OP_piece 0x1,
>>                           DW_OP_breg0 RAX+0,
>>                           DW_OP_convert (0x000e97d5) "DW_ATE_unsigned_64",
>>                           DW_OP_convert (0x000e97da) "DW_ATE_unsigned_32",
>>                           DW_OP_lit8,
>>                           DW_OP_shr,
>>                           DW_OP_convert (0x000e97da) "DW_ATE_unsigned_32",
>>                           DW_OP_convert (0x000e97e4) "DW_ATE_unsigned_24",
>>                           DW_OP_stack_value,
>>                           DW_OP_piece 0x3
>>                       ......
>>
>> At one point, a right shift by 8 happens and the result is converted to
>> 32-bit unsigned int and then to 24-bit unsigned int.
>>
>> BTF does not need any of these DW_OP_* information and such non-regular int
>> types will cause libbpf to emit errors.
>> Let us sanitize them to generate BTF acceptable to libbpf and kernel.
>>
>> Cc: Sedat Dilek <sedat.dilek@gmail.com>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>>   libbtf.c | 39 ++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 38 insertions(+), 1 deletion(-)
>>
>> diff --git a/libbtf.c b/libbtf.c
>> index 9f76283..93fe185 100644
>> --- a/libbtf.c
>> +++ b/libbtf.c
>> @@ -373,6 +373,7 @@ int32_t btf_elf__add_base_type(struct btf_elf *btfe, const struct base_type *bt,
>>          struct btf *btf = btfe->btf;
>>          const struct btf_type *t;
>>          uint8_t encoding = 0;
>> +       uint16_t byte_sz;
>>          int32_t id;
>>
>>          if (bt->is_signed) {
>> @@ -384,7 +385,43 @@ int32_t btf_elf__add_base_type(struct btf_elf *btfe, const struct base_type *bt,
>>                  return -1;
>>          }
>>
>> -       id = btf__add_int(btf, name, BITS_ROUNDUP_BYTES(bt->bit_size), encoding);
>> +       /* dwarf5 may emit DW_ATE_[un]signed_{num} base types where
>> +        * {num} is not power of 2 and may exceed 128. Such attributes
>> +        * are mostly used to record operation for an actual parameter
>> +        * or variable.
>> +        * For example,
>> +        *     DW_AT_location        (indexed (0x3c) loclist = 0x00008fb0:
>> +        *         [0xffffffff82808812, 0xffffffff82808817):
>> +        *             DW_OP_breg0 RAX+0,
>> +        *             DW_OP_convert (0x000e97d5) "DW_ATE_unsigned_64",
>> +        *             DW_OP_convert (0x000e97df) "DW_ATE_unsigned_8",
>> +        *             DW_OP_stack_value,
>> +        *             DW_OP_piece 0x1,
>> +        *             DW_OP_breg0 RAX+0,
>> +        *             DW_OP_convert (0x000e97d5) "DW_ATE_unsigned_64",
>> +        *             DW_OP_convert (0x000e97da) "DW_ATE_unsigned_32",
>> +        *             DW_OP_lit8,
>> +        *             DW_OP_shr,
>> +        *             DW_OP_convert (0x000e97da) "DW_ATE_unsigned_32",
>> +        *             DW_OP_convert (0x000e97e4) "DW_ATE_unsigned_24",
>> +        *             DW_OP_stack_value, DW_OP_piece 0x3
>> +        *     DW_AT_name    ("ebx")
>> +        *     DW_AT_decl_file       ("/linux/arch/x86/events/intel/core.c")
>> +        *
>> +        * In the above example, at some point, one unsigned_32 value
>> +        * is right shifted by 8 and the result is converted to unsigned_32
>> +        * and then unsigned_24.
>> +        *
>> +        * BTF does not need such DW_OP_* information so let us sanitize
>> +        * these non-regular int types to avoid libbpf/kernel complaints.
>> +        */
>> +       byte_sz = BITS_ROUNDUP_BYTES(bt->bit_size);
>> +       if (!byte_sz || (byte_sz & (byte_sz - 1))) {
>> +               name = "sanitized_int";
> 
> DWARF never stops causing issues :( How about making this name stand
> out a bit more: __SANITIZED_FAKE_INT__ ? Similar in style to
> __ARRAY_INDEX_TYPE__?

Good idea. __SANITIZED_FAKE_INT__ can make it easy to understand
this is some kind of workaround.

Will send v2 soon.

> 
> Otherwise looks good to me, even though it's a bit sketchy to just
> "fix up" any integer that doesn't conform to our idea of "normal
> integer". But as I said, DWARF is DWARF...
> 
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> 
>> +               byte_sz = 4;
>> +       }
>> +
>> +       id = btf__add_int(btf, name, byte_sz, encoding);
>>          if (id < 0) {
>>                  btf_elf__log_err(btfe, BTF_KIND_INT, name, true, "Error emitting BTF type");
>>          } else {
>> --
>> 2.24.1
>>