Hi Yonghong. > 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'. Ok, we will change GCC so it does the same thing. What about BOOL? I don't think we ever use that bit. Does LLVM generate it for any case? >>> 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
