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linux-next/Documentation/bpf/llvm_reloc.rst
Yonghong Song fc8c262e0e bpf, docs: Add llvm_reloc.rst to explain llvm bpf relocations 
LLVM upstream commit https://reviews.llvm.org/D102712 made some changes
to bpf relocations to make them llvm linker lld friendly. The scope of
existing relocations R_BPF_64_{64,32} is narrowed and new relocations
R_BPF_64_{ABS32,ABS64,NODYLD32} are introduced.

Let us add some documentation about llvm bpf relocations so people can
understand how to resolve them properly in their respective tools.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210526152457.335210-1-yhs@fb.com
2021-05-28 22:12:04 +02:00

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.. 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 relations 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,
for global variable ``g2`` which has a symbol value 4, the offset
from the start of ``.data`` section.
The third and fourth relocations refers to static variables ``l1``
and ``l2``. From ``.rel.text`` section above, it is not clear
which symbols they really refers to as they both refers to
symbol table entry 4, symbol ``sec``, which has ``STT_SECTION`` type
and represents a section. So for 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
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