Re: ERROR: INT DW_ATE_unsigned_1 Error emitting BTF type

[Yonghong Song <yhs@fb.com>]

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On 2/6/21 10:10 AM, Sedat Dilek wrote:
> On Sat, Feb 6, 2021 at 6:53 PM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> On 2/6/21 8:24 AM, Mark Wieelard wrote:
>>> Hi,
>>>
>>> On Sat, Feb 06, 2021 at 12:26:44AM -0800, Yonghong Song wrote:
>>>> With the above vmlinux, the issue appears to be handling
>>>> DW_ATE_signed_1, DW_ATE_unsigned_{1,24,40}.
>>>>
>>>> The following patch should fix the issue:
>>>
>>> That doesn't really make sense to me. Why is the compiler emitting a
>>> DW_TAG_base_type that needs to be interpreted according to the
>>> DW_AT_name attribute?
>>>
>>> If the issue is that the size of the base type cannot be expressed in
>>> bytes then the DWARF spec provides the following option:
>>>
>>>       If the value of an object of the given type does not fully occupy
>>>       the storage described by a byte size attribute, the base type
>>>       entry may also have a DW_AT_bit_size and a DW_AT_data_bit_offset
>>>       attribute, both of whose values are integer constant values (see
>>>       Section 2.19 on page 55). The bit size attribute describes the
>>>       actual size in bits used to represent values of the given
>>>       type. The data bit offset attribute is the offset in bits from the
>>>       beginning of the containing storage to the beginning of the
>>>       value. Bits that are part of the offset are padding.  If this
>>>       attribute is omitted a default data bit offset of zero is assumed.
>>>
>>> Would it be possible to use that encoding of those special types?  If
>>
>> I agree with you. I do not like comparing me as well. Unfortunately,
>> there is no enough information in dwarf to find out actual information.
>> The following is the dwarf dump with vmlinux (Sedat provided) for
>> DW_ATE_unsigned_1.
>>
>> 0x000e97e9:   DW_TAG_base_type
>>                   DW_AT_name      ("DW_ATE_unsigned_1")
>>                   DW_AT_encoding  (DW_ATE_unsigned)
>>                   DW_AT_byte_size (0x00)
>>
>> There is no DW_AT_bit_size and DW_AT_bit_offset for base type.
>> AFAIK, these two attributes typically appear in struct/union members
>> together with DW_AT_byte_size.
>>
>> Maybe compilers (clang in this case) can emit DW_AT_bit_size = 1
>> and DW_AT_bit_offset = 0/7 (depending on big/little endian) and
>> this case, we just test and get DW_AT_bit_size and it should work.
>>
>> But I think BTF does not need this (DW_ATE_unsigned_1) for now.
>> I checked dwarf dump and it is mostly used for some arith operation
>> encoded in dump (in this case, e.g., shift by 1 bit)
>>
>> 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")
>>
>> Look at clang frontend, only the following types are encoded with
>> unsigned dwarf type.
>>
>>     case BuiltinType::UShort:
>>     case BuiltinType::UInt:
>>     case BuiltinType::UInt128:
>>     case BuiltinType::ULong:
>>     case BuiltinType::WChar_U:
>>     case BuiltinType::ULongLong:
>>       Encoding = llvm::dwarf::DW_ATE_unsigned;
>>       break;
>>
>>
>>> not, can we try to come up with some extension that doesn't require
>>> consumers to match magic names?
>>>
> 
> You want me to upload mlx5_core.ko?

I just sent out a patch. You are cc'ed. I also attached in this email.
Yes, it would be great if you can upload mlx5_core.ko so I can
double check with this DW_ATE_unsigned_160 which is really usual.

> 
> When looking with llvm-dwarf for DW_ATE_unsigned_160:
> 
> 0x00d65616:   DW_TAG_base_type
>                 DW_AT_name      ("DW_ATE_unsigned_160")
>                 DW_AT_encoding  (DW_ATE_unsigned)
>                 DW_AT_byte_size (0x14)
> 
> If you need further information, please let me know.
> 
> Thanks.
> 
> - Sedat -
> 

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From 239c797090abbdc5253d0ff1e9e657c5006fbbee Mon Sep 17 00:00:00 2001
From: Yonghong Song <yhs@fb.com>
Date: Sat, 6 Feb 2021 10:21:45 -0800
Subject: [PATCH dwarves] btf_encoder: sanitize non-regular int base type

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>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 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";
+		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