Re: [PATCH] libbpf: Fix determine_ptr_size() guessing

[Douglas Raillard <douglas.raillard@arm.com>]

Hi Yonghong,

On 5/20/22 20:19, Yonghong Song wrote:
>
>
> On 5/20/22 8:38 AM, Douglas RAILLARD wrote:
>> From: Douglas Raillard <douglas.raillard@arm.com>
>>
>> One strategy employed by libbpf to guess the pointer size is by finding
>> the size of "unsigned long" type. This is achieved by looking for a type
>> of with the expected name and checking its size.
>>
>> Unfortunately, the C syntax is friendlier to humans than to computers
>> as there is some variety in how such a type can be named. Specifically,
>> gcc and clang do not use the same name in debug info.
>
> Yes, this is indeed the case.
>
>>
>> Lookup all the names for such a type so that libbpf can hope to find the
>> information it wants.
>>
>> Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>
>
> LGTM with some comments and needed change below.
>
> Acked-by: Yonghong Song <yhs@fb.com>
>
>> ---
>>   tools/lib/bpf/btf.c | 14 ++++++++++++--
>>   1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
>> index 1383e26c5d1f..ce05e4b1febd 100644
>> --- a/tools/lib/bpf/btf.c
>> +++ b/tools/lib/bpf/btf.c
>> @@ -489,8 +489,18 @@ static int determine_ptr_size(const struct btf *btf)
>>           if (!name)
>>               continue;
>> -        if (strcmp(name, "long int") == 0 ||
>> -            strcmp(name, "long unsigned int") == 0) {
>> +        if (
>> +            strcmp(name, "long int") == 0 ||
>> +            strcmp(name, "int long") == 0 ||
>> +            strcmp(name, "unsigned long") == 0 ||
>> +            strcmp(name, "long unsigned") == 0 ||
>> +            strcmp(name, "unsigned long int") == 0 ||
>> +            strcmp(name, "unsigned int long") == 0 ||
>> +            strcmp(name, "long unsigned int") == 0 ||
>> +            strcmp(name, "long int unsigned") == 0 ||
>> +            strcmp(name, "int unsigned long") == 0 ||
>> +            strcmp(name, "int long unsigned") == 0
>
> Please add "long" as well. For "long t" declaration, clang generates
> the following dwarf:

Good catch, I missed this one thanks.

>
> 0x00000029:   DW_TAG_base_type
>                  DW_AT_name      ("long")
>                  DW_AT_encoding  (DW_ATE_signed)
>                  DW_AT_byte_size (0x08)
>
> If the type name can be sorted with words, we only need
> to compare the following 4 instead of 11
>    "long"
>    "int long"
>    "long unsigned"
>    "int long unsigned"
>
> But I don't know whether we have an existing function
> to do word sorting or not.

I could not find anything and was thinking it's probably more hassle than it's worth but if
we really want that I could add it somewhere.

>
>> +        ) {
>>               if (t->size != 4 && t->size != 8)
>>                   continue;
>>               return t->size;


Thanks,

Douglas
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