Re: Signedness of char in BTF

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> 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.

> 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?

> 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