On Sat, Feb 6, 2021 at 11:21 AM Yonghong Song <yhs@xxxxxx> wrote:
>
> clang with dwarf5 may generate non-regular int base type,
> i.e., not a signed/unsigned char/short/int/longlong/__int128.
> Such base types are often used to describe
> how an actual parameter or variable is generated. For example,
>
> 0x000015cf: DW_TAG_base_type
> DW_AT_name ("DW_ATE_unsigned_1")
> DW_AT_encoding (DW_ATE_unsigned)
> DW_AT_byte_size (0x00)
>
> 0x00010ed9: DW_TAG_formal_parameter
> DW_AT_location (DW_OP_lit0,
> DW_OP_not,
> DW_OP_convert (0x000015cf) "DW_ATE_unsigned_1",
> DW_OP_convert (0x000015d4) "DW_ATE_unsigned_8",
> DW_OP_stack_value)
> DW_AT_abstract_origin (0x00013984 "branch")
>
> What it does is with a literal "0", did a "not" operation, and the converted to
> one-bit unsigned int and then 8-bit unsigned int.
>
> Another example,
>
> 0x000e97e4: DW_TAG_base_type
> DW_AT_name ("DW_ATE_unsigned_24")
> DW_AT_encoding (DW_ATE_unsigned)
> DW_AT_byte_size (0x03)
>
> 0x000f88f8: DW_TAG_variable
> DW_AT_location (indexed (0x3c) loclist = 0x00008fb0:
> [0xffffffff82808812, 0xffffffff82808817):
> DW_OP_breg0 RAX+0,
> DW_OP_convert (0x000e97d5) "DW_ATE_unsigned_64",
> DW_OP_convert (0x000e97df) "DW_ATE_unsigned_8",
> DW_OP_stack_value,
> DW_OP_piece 0x1,
> DW_OP_breg0 RAX+0,
> DW_OP_convert (0x000e97d5) "DW_ATE_unsigned_64",
> DW_OP_convert (0x000e97da) "DW_ATE_unsigned_32",
> DW_OP_lit8,
> DW_OP_shr,
> DW_OP_convert (0x000e97da) "DW_ATE_unsigned_32",
> DW_OP_convert (0x000e97e4) "DW_ATE_unsigned_24",
> DW_OP_stack_value,
> DW_OP_piece 0x3
> ......
>
> At one point, a right shift by 8 happens and the result is converted to
> 32-bit unsigned int and then to 24-bit unsigned int.
>
> BTF does not need any of these DW_OP_* information and such non-regular int
> types will cause libbpf to emit errors.
> Let us sanitize them to generate BTF acceptable to libbpf and kernel.
>
> Cc: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
> libbtf.c | 39 ++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 38 insertions(+), 1 deletion(-)
>
> diff --git a/libbtf.c b/libbtf.c
> index 9f76283..93fe185 100644
> --- a/libbtf.c
> +++ b/libbtf.c
> @@ -373,6 +373,7 @@ int32_t btf_elf__add_base_type(struct btf_elf *btfe, const struct base_type *bt,
> struct btf *btf = btfe->btf;
> const struct btf_type *t;
> uint8_t encoding = 0;
> + uint16_t byte_sz;
> int32_t id;
>
> if (bt->is_signed) {
> @@ -384,7 +385,43 @@ int32_t btf_elf__add_base_type(struct btf_elf *btfe, const struct base_type *bt,
> return -1;
> }
>
> - id = btf__add_int(btf, name, BITS_ROUNDUP_BYTES(bt->bit_size), encoding);
> + /* dwarf5 may emit DW_ATE_[un]signed_{num} base types where
> + * {num} is not power of 2 and may exceed 128. Such attributes
> + * are mostly used to record operation for an actual parameter
> + * or variable.
> + * For example,
> + * DW_AT_location (indexed (0x3c) loclist = 0x00008fb0:
> + * [0xffffffff82808812, 0xffffffff82808817):
> + * DW_OP_breg0 RAX+0,
> + * DW_OP_convert (0x000e97d5) "DW_ATE_unsigned_64",
> + * DW_OP_convert (0x000e97df) "DW_ATE_unsigned_8",
> + * DW_OP_stack_value,
> + * DW_OP_piece 0x1,
> + * DW_OP_breg0 RAX+0,
> + * DW_OP_convert (0x000e97d5) "DW_ATE_unsigned_64",
> + * DW_OP_convert (0x000e97da) "DW_ATE_unsigned_32",
> + * DW_OP_lit8,
> + * DW_OP_shr,
> + * DW_OP_convert (0x000e97da) "DW_ATE_unsigned_32",
> + * DW_OP_convert (0x000e97e4) "DW_ATE_unsigned_24",
> + * DW_OP_stack_value, DW_OP_piece 0x3
> + * DW_AT_name ("ebx")
> + * DW_AT_decl_file ("/linux/arch/x86/events/intel/core.c")
> + *
> + * In the above example, at some point, one unsigned_32 value
> + * is right shifted by 8 and the result is converted to unsigned_32
> + * and then unsigned_24.
> + *
> + * BTF does not need such DW_OP_* information so let us sanitize
> + * these non-regular int types to avoid libbpf/kernel complaints.
> + */
> + byte_sz = BITS_ROUNDUP_BYTES(bt->bit_size);
> + if (!byte_sz || (byte_sz & (byte_sz - 1))) {
> + name = "sanitized_int";
DWARF never stops causing issues :( How about making this name stand
out a bit more: __SANITIZED_FAKE_INT__ ? Similar in style to
__ARRAY_INDEX_TYPE__?
Otherwise looks good to me, even though it's a bit sketchy to just
"fix up" any integer that doesn't conform to our idea of "normal
integer". But as I said, DWARF is DWARF...
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> + byte_sz = 4;
> + }
> +
> + id = btf__add_int(btf, name, byte_sz, encoding);
> if (id < 0) {
> btf_elf__log_err(btfe, BTF_KIND_INT, name, true, "Error emitting BTF type");
> } else {
> --
> 2.24.1
>
