Re: [PATCH dwarves] dwarf_loader: encode char type as signed

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On 8/8/22 3:52 PM, Andrii Nakryiko wrote:
On Sun, Aug 7, 2022 at 10:53 AM Yonghong Song <yhs@xxxxxx> wrote:

Currently, the pahole treats 'char' or 'signed char' type
as unsigned in BTF generation. The following is an example,
   $ cat t.c
   signed char a;
   char b;
   $ clang -O2 -g -c t.c
   $ pahole -JV t.o
   ...
   [1] INT signed char size=1 nr_bits=8 encoding=(none)
   [2] INT char size=1 nr_bits=8 encoding=(none)
In the above encoding '(none)' implies unsigned type.

But if the same program is compiled with bpf target,
   $ clang -target bpf -O2 -g -c t.c
   $ bpftool btf dump file t.o
   [1] INT 'signed char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
   [2] VAR 'a' type_id=1, linkage=global
   [3] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
   [4] VAR 'b' type_id=3, linkage=global
   [5] DATASEC '.bss' size=0 vlen=2
           type_id=2 offset=0 size=1 (VAR 'a')
           type_id=4 offset=0 size=1 (VAR 'b')
the 'char' and 'signed char' are encoded as SIGNED integers.

Encode 'char' and 'signed char' as SIGNED should be a right to
do and it will be consistent with bpf implementation.

With this patch,
   $ pahole -JV t.o
   ...
   [1] INT signed char size=1 nr_bits=8 encoding=SIGNED
   [2] INT char size=1 nr_bits=8 encoding=SIGNED

Signed-off-by: Yonghong Song <yhs@xxxxxx>
---

LGTM.

Is there a plan to also add CHAR encoding bit?

Not yet... Need more thinking about use cases and backward comparability issues.



  dwarf_loader.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/dwarf_loader.c b/dwarf_loader.c
index d892bc3..c2ad2a0 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -560,7 +560,7 @@ static struct base_type *base_type__new(Dwarf_Die *die, struct cu *cu, struct co
                 bt->bit_size = attr_numeric(die, DW_AT_byte_size) * 8;
                 uint64_t encoding = attr_numeric(die, DW_AT_encoding);
                 bt->is_bool = encoding == DW_ATE_boolean;
-               bt->is_signed = encoding == DW_ATE_signed;
+               bt->is_signed = (encoding == DW_ATE_signed) || (encoding == DW_ATE_signed_char);
                 bt->is_varargs = false;
                 bt->name_has_encoding = true;
                 bt->float_type = encoding_to_float_type(encoding);
--
2.30.2




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