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Commit f063c889 authored by Daniel Borkmann's avatar Daniel Borkmann Committed by Alexei Starovoitov
Browse files

bpf: add specification for BTF Var and DataSec kinds 



This adds the BTF specification and UAPI bits for supporting BTF Var
and DataSec kinds. This is following LLVM upstream commit ac4082b77e07
("[BPF] Add BTF Var and DataSec Support") which has been merged recently.
Var itself is for describing a global variable and DataSec to describe
ELF sections e.g. data/bss/rodata sections that hold one or multiple
global variables.

Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarMartin KaFai Lau <kafai@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 3e0ddc4f

Documentation/bpf/btf.rst

+57 −0
Original line number Diff line number Diff line
@@ -82,6 +82,8 @@ sequentially and type id is assigned to each recognized type starting from id 
    #define BTF_KIND_RESTRICT       11      /* Restrict     */

    #define BTF_KIND_FUNC           12      /* Function     */

    #define BTF_KIND_FUNC_PROTO     13      /* Function Proto       */

    #define BTF_KIND_VAR            14      /* Variable     */

    #define BTF_KIND_DATASEC        15      /* Section      */



Note that the type section encodes debug info, not just pure types.

``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
@@ -393,6 +395,61 @@ refers to parameter type. 
If the function has variable arguments, the last parameter is encoded with

``name_off = 0`` and ``type = 0``.



2.2.14 BTF_KIND_VAR

~~~~~~~~~~~~~~~~~~~



``struct btf_type`` encoding requirement:

  * ``name_off``: offset to a valid C identifier

  * ``info.kind_flag``: 0

  * ``info.kind``: BTF_KIND_VAR

  * ``info.vlen``: 0

  * ``type``: the type of the variable



``btf_type`` is followed by a single ``struct btf_variable`` with the

following data::



    struct btf_var {

        __u32   linkage;

    };



``struct btf_var`` encoding:

  * ``linkage``: currently only static variable 0, or globally allocated

                 variable in ELF sections 1



Not all type of global variables are supported by LLVM at this point.

The following is currently available:



  * static variables with or without section attributes

  * global variables with section attributes



The latter is for future extraction of map key/value type id's from a

map definition.



2.2.15 BTF_KIND_DATASEC

~~~~~~~~~~~~~~~~~~~~~~~



``struct btf_type`` encoding requirement:

  * ``name_off``: offset to a valid name associated with a variable or

                  one of .data/.bss/.rodata

  * ``info.kind_flag``: 0

  * ``info.kind``: BTF_KIND_DATASEC

  * ``info.vlen``: # of variables

  * ``size``: total section size in bytes (0 at compilation time, patched

              to actual size by BPF loaders such as libbpf)



``btf_type`` is followed by ``info.vlen`` number of ``struct btf_var_secinfo``.::



    struct btf_var_secinfo {

        __u32   type;

        __u32   offset;

        __u32   size;

    };



``struct btf_var_secinfo`` encoding:

  * ``type``: the type of the BTF_KIND_VAR variable

  * ``offset``: the in-section offset of the variable

  * ``size``: the size of the variable in bytes



3. BTF Kernel API

*****************

include/uapi/linux/btf.h

+28 −4
Original line number Diff line number Diff line
@@ -39,11 +39,11 @@ struct btf_type { 
	 *             struct, union and fwd

	 */

	__u32 info;

	/* "size" is used by INT, ENUM, STRUCT and UNION.

	/* "size" is used by INT, ENUM, STRUCT, UNION and DATASEC.

	 * "size" tells the size of the type it is describing.

	 *

	 * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,

	 * FUNC and FUNC_PROTO.

	 * FUNC, FUNC_PROTO and VAR.

	 * "type" is a type_id referring to another type.

	 */

	union {
@@ -70,8 +70,10 @@ struct btf_type { 
#define BTF_KIND_RESTRICT	11	/* Restrict	*/

#define BTF_KIND_FUNC		12	/* Function	*/

#define BTF_KIND_FUNC_PROTO	13	/* Function Proto	*/

#define BTF_KIND_MAX		13

#define NR_BTF_KINDS		14

#define BTF_KIND_VAR		14	/* Variable	*/

#define BTF_KIND_DATASEC	15	/* Section	*/

#define BTF_KIND_MAX		BTF_KIND_DATASEC

#define NR_BTF_KINDS		(BTF_KIND_MAX + 1)



/* For some specific BTF_KIND, "struct btf_type" is immediately

 * followed by extra data.
@@ -138,4 +140,26 @@ struct btf_param { 
	__u32	type;

};



enum {

	BTF_VAR_STATIC = 0,

	BTF_VAR_GLOBAL_ALLOCATED,

};



/* BTF_KIND_VAR is followed by a single "struct btf_var" to describe

 * additional information related to the variable such as its linkage.

 */

struct btf_var {

	__u32	linkage;

};



/* BTF_KIND_DATASEC is followed by multiple "struct btf_var_secinfo"

 * to describe all BTF_KIND_VAR types it contains along with it's

 * in-section offset as well as size.

 */

struct btf_var_secinfo {

	__u32	type;

	__u32	offset;

	__u32	size;

};



#endif /* _UAPI__LINUX_BTF_H__ */