241 lines
11 KiB
ReStructuredText
241 lines
11 KiB
ReStructuredText
|
.. 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
|