Re: Signedness of char in BTF

[Yonghong Song <yhs@fb.com>]

On 7/21/22 7:54 AM, Jose E. Marchesi wrote:
> 
>> Hi Yonghong and Andrii,
>>
>> I have some questions re: signedness of chars in BTF. According to [1]
>> BTF_INT_ENCODING() may be one of SIGNED, CHAR or BOOL.
> 
> I have always assumed that the bits in `encoding' are non-exclusive
> i.e. it is a bitmap, not an enumerated.

Based on current BTF design, it is enumerated. So signed char
is 'signed 1-byte int', unsigned char is 'unsigned 1-byte int'
and 'char' could be BTF_INT_CHAR but since in debuginfo
any 'char' has a signedness bit, so it is folded into
'signed 1-byte int' or 'unsigned 1-byte int'.

> 
>> If I read [2] correctly the signedness of char is implementation
>> defined. Does this mean that I need to know which implementation
>> generated the BTF to interpret CHAR correctly?
>>
>> Somewhat related, how to I make clang emit BTF_INT_CHAR in the first
>> place? I've tried with clang-14, but only ever get
>>
>>      [6] INT 'unsigned char' size=1 bits_offset=0 nr_bits=8 encoding=(none)
>>      [6] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
> 
> Hm, in GCC we currently generate:
> 
> [1] int 'unsigned char'(0x00000001U#B) size=0x00000001U#B offset=0x00UB#b bits=0x08UB#b CHAR
> [2] int 'char'(0x00000001U#B) size=0x00000001U#B offset=0x00UB#b bits=0x08UB#b SIGNED CHAR
> 
> Which turns out is not correct?
> 
> We used a signed type for `char' because that was what the LLVM BPF
> toolchain uses, but then we assumed we had to emit the CHAR bit as
> well... wrong assumption apparently (I just tried with clang 15 and it
> doesn't set the CHAR bits for neither `char' nor `unsigned char').
> 
> But then what is the CHAR bit for?

This is not generated by llvm or pahole but apparently it may still
have some meaning when printing the value, a 'char c' may have
a dump like 'c' instead of '0x63'. In kernel/bpf/btf.c, we have

                 /*
                  * BTF_INT_CHAR encoding never seems to be set for
                  * char arrays, so if size is 1 and element is
                  * printable as a char, we'll do that.
                  */
                 if (elem_size == 1)
                         encoding = BTF_INT_CHAR;

> 
>> The kernel seems to agree that CHAR isn't a thing [3].
>>
>> Thanks!
>> Lorenz
>>
>> 1: https://www.kernel.org/doc/html/latest/bpf/btf.html#btf-kind-int
>> 2: https://stackoverflow.com/a/2054941/19544965
>> 3:
>> https://sourcegraph.com/github.com/torvalds/linux@353f7988dd8413c47718f7ca79c030b6fb62cfe5/-/blob/kernel/bpf/btf.c?L2928-2934