Re: [PATCH dwarves v2] btf_encoder: sanitize non-regular int base type

[Nick Desaulniers <ndesaulniers@google.com>]

On Sat, Feb 6, 2021 at 11:17 PM Yonghong Song <yhs@fb.com> 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@gmail.com>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Yonghong Song <yhs@fb.com>

Thanks for the patch!

Tested-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  libbtf.c | 39 ++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 38 insertions(+), 1 deletion(-)
>
> diff --git a/libbtf.c b/libbtf.c
> index 9f76283..5843200 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_FAKE_INT__";
> +               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
>


-- 
Thanks,
~Nick Desaulniers