On Mon, Feb 8, 2021 at 8:55 PM Arnaldo Carvalho de Melo
<arnaldo.melo@xxxxxxxxx> wrote:
>
> Em Sun, Feb 07, 2021 at 11:32:00AM +0100, Sedat Dilek escreveu:
> > On Sun, Feb 7, 2021 at 8:17 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>
> >
> > Thanks for v2.
> >
> > For both v1 and v2:
> >
> > Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> > Reported-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
>
> Thanks, applied.
>
Great.
I cannot see it yet in [1] or [2].
More important to me is is this worth a pahole v1.20.1 release?
This patch is required to successfully build with BTF and DWARF-5 and Clang-12.
I have Nathan's "bpf: Hoise pahole version checks into Kconfig" patch
in my custom patchset (together with Nick's DWARF-v5 patchset) which
makes it easier to do (depends on PAHOLE_VERSION >= 1201) via Kconfig
for example.
- Sedat -
[1] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/
[2] https://github.com/acmel/dwarves/
> - Arnaldo
>
> > My development and testing environment:
> >
> > 1. Debian/testing AMD64
> > 2. Linux v5.11-rc6+ with custom mostly Clang fixes
> > 3. Debug-Info: BTF + DWARF-v5 enabled
> > 4. Compiler and tools (incl. Clang's Integrated ASsembler IAS):
> > LLVM/Clang v12.0.0-rc1 (make LLVM=1 LLVM_IAS=1)
> >
> > Build and boot on bare metal.
> >
> > - Sedat -
> >
> > > Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > > 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..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
> > >
>
> --
>
> - Arnaldo
