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#### 6.62.12 BPF Built-in Functions

The following built-in functions are available for eBPF targets.

Built-in Function: unsigned long long __builtin_bpf_load_byte (unsigned long long offset)

  Load a byte from the `struct sk_buff` packet data pointed to by the register `%r6`, and return it.

Built-in Function: unsigned long long __builtin_bpf_load_half (unsigned long long offset)

  Load 16 bits from the `struct sk_buff` packet data pointed to by the register `%r6`, and return it.

Built-in Function: unsigned long long __builtin_bpf_load_word (unsigned long long offset)

  Load 32 bits from the `struct sk_buff` packet data pointed to by the register `%r6`, and return it.

Built-in Function: type __builtin_preserve_access_index (type expr)

  BPF Compile Once-Run Everywhere (CO-RE) support. Instruct GCC to generate CO-RE relocation records for any accesses to aggregate data structures (struct, union, array types) in expr. This builtin is otherwise transparent; expr may have any type and its value is returned. This builtin has no effect if `-mco-re` is not in effect (either specified or implied).

Built-in Function: unsigned int __builtin_preserve_field_info (expr, unsigned int kind)

  BPF Compile Once-Run Everywhere (CO-RE) support. This builtin is used to extract information to aid in struct/union relocations. expr is an access to a field of a struct or union. Depending on kind, different information is returned to the program. A CO-RE relocation for the access in expr with kind kind is recorded if `-mco-re` is in effect.

  The following values are supported for kind:

  `FIELD_BYTE_OFFSET = 0`

    The returned value is the offset, in bytes, of the field from the beginning of the containing structure. For bit-fields, this is the byte offset of the containing word.

  `FIELD_BYTE_SIZE = 1`

    The returned value is the size, in bytes, of the field. For bit-fields, this is the size in bytes of the containing word.

  `FIELD_EXISTENCE = 2`

    The returned value is 1 if the field exists, 0 otherwise. Always 1 at compile time.

  `FIELD_SIGNEDNESS = 3`

    The returned value is 1 if the field is signed, 0 otherwise.

  `FIELD_LSHIFT_U64 = 4`
  `FIELD_RSHIFT_U64 = 5`

    The returned value is the number of bits of left- or right-shifting (respectively) needed in order to recover the original value of the field, after it has been loaded by a read of `FIELD_BYTE_SIZE` bytes into an unsigned 64-bit value. Primarily useful for reading bit-field values from structures that may change between kernel versions.

  Note that the return value is a constant which is known at compile time. If the field has a variable offset then `FIELD_BYTE_OFFSET`, `FIELD_LSHIFT_U64`, and `FIELD_RSHIFT_U64` are not supported. Similarly, if the field has a variable size then `FIELD_BYTE_SIZE`, `FIELD_LSHIFT_U64`, and `FIELD_RSHIFT_U64` are not supported.

  For example, `__builtin_preserve_field_info` can be used to reliably extract bit-field values from a structure that may change between kernel versions:

      struct S
      {
        short a;
        int x:7;
        int y:5;
      };

      int
      read_y (struct S *arg)
      {
        unsigned long long val;
        unsigned int offset
          = __builtin_preserve_field_info (arg->y, FIELD_BYTE_OFFSET);
        unsigned int size
          = __builtin_preserve_field_info (arg->y, FIELD_BYTE_SIZE);

        /* Read size bytes from arg + offset into val.  */
        bpf_probe_read (&val, size, arg + offset);

        val <<= __builtin_preserve_field_info (arg->y, FIELD_LSHIFT_U64);

        if (__builtin_preserve_field_info (arg->y, FIELD_SIGNEDNESS))
          val = ((long long) val
                 >> __builtin_preserve_field_info (arg->y, FIELD_RSHIFT_U64));
        else
          val >>= __builtin_preserve_field_info (arg->y, FIELD_RSHIFT_U64);

        return val;
      }

Built-in Function: unsigned int __builtin_preserve_enum_value (type, enum, unsigned int kind)

  BPF Compile Once-Run Everywhere (CO-RE) support. This builtin collects enum information and creates a CO-RE relocation relative to enum that should be of type. The kind specifies the action performed.

  The following values are supported for kind:

  `ENUM_VALUE_EXISTS = 0`

    The return value is either 0 or 1 depending if the enum value exists in the target.

  `ENUM_VALUE = 1`

    The return value is the enum value in the target kernel.

Built-in Function: unsigned int __builtin_btf_type_id (type, unsigned int kind)

  BPF Compile Once-Run Everywhere (CO-RE) support. This builtin is used to get the BTF type ID of a specified type. Depending on the kind argument, it either returns the ID of the local BTF information, or the BTF type ID in the target kernel.

  The following values are supported for kind:

  `BTF_TYPE_ID_LOCAL = 0`

    Return the local BTF type ID. Always succeeds.

  `BTF_TYPE_ID_TARGET = 1`

    Return the target BTF type ID. If type does not exist in the target, returns 0.

Built-in Function: unsigned int __builtin_preserve_type_info (type, unsigned int kind)

  BPF Compile Once-Run Everywhere (CO-RE) support. This builtin performs named type (struct/union/enum/typedef) verifications. The type of verification depends on the kind argument provided. This builtin always returns 0 if type does not exist in the target kernel.

  The following values are supported for kind:

  `BTF_TYPE_EXISTS = 0`

    Checks if type exists in the target.

  `BTF_TYPE_MATCHES = 1`

    Checks if type matches the local definition in the target kernel.

  `BTF_TYPE_SIZE = 2`

    Returns the size of the type within the target.

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