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

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On 2/6/21 10:36 PM, Andrii Nakryiko wrote:
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__?

Good idea. __SANITIZED_FAKE_INT__ can make it easy to understand
this is some kind of workaround.

Will send v2 soon.


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




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