linux-zen-desktop/Documentation/bpf/llvm_reloc.rst

.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)

====================
BPF LLVM Relocations
====================

This document describes LLVM BPF backend relocation types.

Relocation Record
=================

LLVM BPF backend records each relocation with the following 16-byte
ELF structure::

  typedef struct
  {
    Elf64_Addr    r_offset;  // Offset from the beginning of section.
    Elf64_Xword   r_info;    // Relocation type and symbol index.
  } Elf64_Rel;

For example, for the following code::

  int g1 __attribute__((section("sec")));
  int g2 __attribute__((section("sec")));
  static volatile int l1 __attribute__((section("sec")));
  static volatile int l2 __attribute__((section("sec")));
  int test() {
    return g1 + g2 + l1 + l2;
  }

Compiled with ``clang -target bpf -O2 -c test.c``, the following is
the code with ``llvm-objdump -dr test.o``::

       0:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
                0000000000000000:  R_BPF_64_64  g1
       2:       61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
       3:       18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll
                0000000000000018:  R_BPF_64_64  g2
       5:       61 20 00 00 00 00 00 00 r0 = *(u32 *)(r2 + 0)
       6:       0f 10 00 00 00 00 00 00 r0 += r1
       7:       18 01 00 00 08 00 00 00 00 00 00 00 00 00 00 00 r1 = 8 ll
                0000000000000038:  R_BPF_64_64  sec
       9:       61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
      10:       0f 10 00 00 00 00 00 00 r0 += r1
      11:       18 01 00 00 0c 00 00 00 00 00 00 00 00 00 00 00 r1 = 12 ll
                0000000000000058:  R_BPF_64_64  sec
      13:       61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
      14:       0f 10 00 00 00 00 00 00 r0 += r1
      15:       95 00 00 00 00 00 00 00 exit

There are four relocations in the above for four ``LD_imm64`` instructions.
The following ``llvm-readelf -r test.o`` shows the binary values of the four
relocations::

  Relocation section '.rel.text' at offset 0x190 contains 4 entries:
      Offset             Info             Type               Symbol's Value  Symbol's Name
  0000000000000000  0000000600000001 R_BPF_64_64            0000000000000000 g1
  0000000000000018  0000000700000001 R_BPF_64_64            0000000000000004 g2
  0000000000000038  0000000400000001 R_BPF_64_64            0000000000000000 sec
  0000000000000058  0000000400000001 R_BPF_64_64            0000000000000000 sec

Each relocation is represented by ``Offset`` (8 bytes) and ``Info`` (8 bytes).
For example, the first relocation corresponds to the first instruction
(Offset 0x0) and the corresponding ``Info`` indicates the relocation type
of ``R_BPF_64_64`` (type 1) and the entry in the symbol table (entry 6).
The following is the symbol table with ``llvm-readelf -s test.o``::

  Symbol table '.symtab' contains 8 entries:
     Num:    Value          Size Type    Bind   Vis       Ndx Name
       0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT   UND
       1: 0000000000000000     0 FILE    LOCAL  DEFAULT   ABS test.c
       2: 0000000000000008     4 OBJECT  LOCAL  DEFAULT     4 l1
       3: 000000000000000c     4 OBJECT  LOCAL  DEFAULT     4 l2
       4: 0000000000000000     0 SECTION LOCAL  DEFAULT     4 sec
       5: 0000000000000000   128 FUNC    GLOBAL DEFAULT     2 test
       6: 0000000000000000     4 OBJECT  GLOBAL DEFAULT     4 g1
       7: 0000000000000004     4 OBJECT  GLOBAL DEFAULT     4 g2

The 6th entry is global variable ``g1`` with value 0.

Similarly, the second relocation is at ``.text`` offset ``0x18``, instruction 3,
has a type of ``R_BPF_64_64`` and refers to entry 7 in the symbol table.
The second relocation resolves to global variable ``g2`` which has a symbol
value 4. The symbol value represents the offset from the start of ``.data``
section where the initial value of the global variable ``g2`` is stored.

The third and fourth relocations refer to static variables ``l1``
and ``l2``. From the ``.rel.text`` section above, it is not clear
to which symbols they really refer as they both refer to
symbol table entry 4, symbol ``sec``, which has ``STT_SECTION`` type
and represents a section. So for a static variable or function,
the section offset is written to the original insn
buffer, which is called ``A`` (addend). Looking at
above insn ``7`` and ``11``, they have section offset ``8`` and ``12``.
From symbol table, we can find that they correspond to entries ``2``
and ``3`` for ``l1`` and ``l2``.

In general, the ``A`` is 0 for global variables and functions,
and is the section offset or some computation result based on
section offset for static variables/functions. The non-section-offset
case refers to function calls. See below for more details.

Different Relocation Types
==========================

Six relocation types are supported. The following is an overview and
``S`` represents the value of the symbol in the symbol table::

  Enum  ELF Reloc Type     Description      BitSize  Offset        Calculation
  0     R_BPF_NONE         None
  1     R_BPF_64_64        ld_imm64 insn    32       r_offset + 4  S + A
  2     R_BPF_64_ABS64     normal data      64       r_offset      S + A
  3     R_BPF_64_ABS32     normal data      32       r_offset      S + A
  4     R_BPF_64_NODYLD32  .BTF[.ext] data  32       r_offset      S + A
  10    R_BPF_64_32        call insn        32       r_offset + 4  (S + A) / 8 - 1

For example, ``R_BPF_64_64`` relocation type is used for ``ld_imm64`` instruction.
The actual to-be-relocated data (0 or section offset)
is stored at ``r_offset + 4`` and the read/write
data bitsize is 32 (4 bytes). The relocation can be resolved with
the symbol value plus implicit addend. Note that the ``BitSize`` is 32 which
means the section offset must be less than or equal to ``UINT32_MAX`` and this
is enforced by LLVM BPF backend.

In another case, ``R_BPF_64_ABS64`` relocation type is used for normal 64-bit data.
The actual to-be-relocated data is stored at ``r_offset`` and the read/write data
bitsize is 64 (8 bytes). The relocation can be resolved with
the symbol value plus implicit addend.

Both ``R_BPF_64_ABS32`` and ``R_BPF_64_NODYLD32`` types are for 32-bit data.
But ``R_BPF_64_NODYLD32`` specifically refers to relocations in ``.BTF`` and
``.BTF.ext`` sections. For cases like bcc where llvm ``ExecutionEngine RuntimeDyld``
is involved, ``R_BPF_64_NODYLD32`` types of relocations should not be resolved
to actual function/variable address. Otherwise, ``.BTF`` and ``.BTF.ext``
become unusable by bcc and kernel.

Type ``R_BPF_64_32`` is used for call instruction. The call target section
offset is stored at ``r_offset + 4`` (32bit) and calculated as
``(S + A) / 8 - 1``.

Examples
========

Types ``R_BPF_64_64`` and ``R_BPF_64_32`` are used to resolve ``ld_imm64``
and ``call`` instructions. For example::

  __attribute__((noinline)) __attribute__((section("sec1")))
  int gfunc(int a, int b) {
    return a * b;
  }
  static __attribute__((noinline)) __attribute__((section("sec1")))
  int lfunc(int a, int b) {
    return a + b;
  }
  int global __attribute__((section("sec2")));
  int test(int a, int b) {
    return gfunc(a, b) +  lfunc(a, b) + global;
  }

Compiled with ``clang -target bpf -O2 -c test.c``, we will have
following code with `llvm-objdump -dr test.o``::

  Disassembly of section .text:

  0000000000000000 <test>:
         0:       bf 26 00 00 00 00 00 00 r6 = r2
         1:       bf 17 00 00 00 00 00 00 r7 = r1
         2:       85 10 00 00 ff ff ff ff call -1
                  0000000000000010:  R_BPF_64_32  gfunc
         3:       bf 08 00 00 00 00 00 00 r8 = r0
         4:       bf 71 00 00 00 00 00 00 r1 = r7
         5:       bf 62 00 00 00 00 00 00 r2 = r6
         6:       85 10 00 00 02 00 00 00 call 2
                  0000000000000030:  R_BPF_64_32  sec1
         7:       0f 80 00 00 00 00 00 00 r0 += r8
         8:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
                  0000000000000040:  R_BPF_64_64  global
        10:       61 11 00 00 00 00 00 00 r1 = *(u32 *)(r1 + 0)
        11:       0f 10 00 00 00 00 00 00 r0 += r1
        12:       95 00 00 00 00 00 00 00 exit

  Disassembly of section sec1:

  0000000000000000 <gfunc>:
         0:       bf 20 00 00 00 00 00 00 r0 = r2
         1:       2f 10 00 00 00 00 00 00 r0 *= r1
         2:       95 00 00 00 00 00 00 00 exit

  0000000000000018 <lfunc>:
         3:       bf 20 00 00 00 00 00 00 r0 = r2
         4:       0f 10 00 00 00 00 00 00 r0 += r1
         5:       95 00 00 00 00 00 00 00 exit

The first relocation corresponds to ``gfunc(a, b)`` where ``gfunc`` has a value of 0,
so the ``call`` instruction offset is ``(0 + 0)/8 - 1 = -1``.
The second relocation corresponds to ``lfunc(a, b)`` where ``lfunc`` has a section
offset ``0x18``, so the ``call`` instruction offset is ``(0 + 0x18)/8 - 1 = 2``.
The third relocation corresponds to ld_imm64 of ``global``, which has a section
offset ``0``.

The following is an example to show how R_BPF_64_ABS64 could be generated::

  int global() { return 0; }
  struct t { void *g; } gbl = { global };

Compiled with ``clang -target bpf -O2 -g -c test.c``, we will see a
relocation below in ``.data`` section with command
``llvm-readelf -r test.o``::

  Relocation section '.rel.data' at offset 0x458 contains 1 entries:
      Offset             Info             Type               Symbol's Value  Symbol's Name
  0000000000000000  0000000700000002 R_BPF_64_ABS64         0000000000000000 global

The relocation says the first 8-byte of ``.data`` section should be
filled with address of ``global`` variable.

With ``llvm-readelf`` output, we can see that dwarf sections have a bunch of
``R_BPF_64_ABS32`` and ``R_BPF_64_ABS64`` relocations::

  Relocation section '.rel.debug_info' at offset 0x468 contains 13 entries:
      Offset             Info             Type               Symbol's Value  Symbol's Name
  0000000000000006  0000000300000003 R_BPF_64_ABS32         0000000000000000 .debug_abbrev
  000000000000000c  0000000400000003 R_BPF_64_ABS32         0000000000000000 .debug_str
  0000000000000012  0000000400000003 R_BPF_64_ABS32         0000000000000000 .debug_str
  0000000000000016  0000000600000003 R_BPF_64_ABS32         0000000000000000 .debug_line
  000000000000001a  0000000400000003 R_BPF_64_ABS32         0000000000000000 .debug_str
  000000000000001e  0000000200000002 R_BPF_64_ABS64         0000000000000000 .text
  000000000000002b  0000000400000003 R_BPF_64_ABS32         0000000000000000 .debug_str
  0000000000000037  0000000800000002 R_BPF_64_ABS64         0000000000000000 gbl
  0000000000000040  0000000400000003 R_BPF_64_ABS32         0000000000000000 .debug_str
  ......

The .BTF/.BTF.ext sections has R_BPF_64_NODYLD32 relocations::

  Relocation section '.rel.BTF' at offset 0x538 contains 1 entries:
      Offset             Info             Type               Symbol's Value  Symbol's Name
  0000000000000084  0000000800000004 R_BPF_64_NODYLD32      0000000000000000 gbl

  Relocation section '.rel.BTF.ext' at offset 0x548 contains 2 entries:
      Offset             Info             Type               Symbol's Value  Symbol's Name
  000000000000002c  0000000200000004 R_BPF_64_NODYLD32      0000000000000000 .text
  0000000000000040  0000000200000004 R_BPF_64_NODYLD32      0000000000000000 .text