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
