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
CO-RE Relocations¶
From object file point of view CO-RE mechanism is implemented as a set of CO-RE specific relocation records. These relocation records are not related to ELF relocations and are encoded in .BTF.ext section. See Documentation/bpf/btf.rst for more information on .BTF.ext structure.
CO-RE relocations are applied to BPF instructions to update immediate or offset fields of the instruction at load time with information relevant for target kernel.
Field to patch is selected basing on the instruction class:
For BPF_ALU, BPF_ALU64, BPF_LD immediate field is patched;
For BPF_LDX, BPF_STX, BPF_ST offset field is patched;
BPF_JMP, BPF_JMP32 instructions should not be patched.
Relocation kinds¶
There are several kinds of CO-RE relocations that could be split in three groups:
Field-based - patch instruction with field related information, e.g. change offset field of the BPF_LDX instruction to reflect offset of a specific structure field in the target kernel.
Type-based - patch instruction with type related information, e.g. change immediate field of the BPF_ALU move instruction to 0 or 1 to reflect if specific type is present in the target kernel.
Enum-based - patch instruction with enum related information, e.g. change immediate field of the BPF_LD_IMM64 instruction to reflect value of a specific enum literal in the target kernel.
The complete list of relocation kinds is represented by the following enum:
enum bpf_core_relo_kind {
BPF_CORE_FIELD_BYTE_OFFSET = 0, /* field byte offset */
BPF_CORE_FIELD_BYTE_SIZE = 1, /* field size in bytes */
BPF_CORE_FIELD_EXISTS = 2, /* field existence in target kernel */
BPF_CORE_FIELD_SIGNED = 3, /* field signedness (0 - unsigned, 1 - signed) */
BPF_CORE_FIELD_LSHIFT_U64 = 4, /* bitfield-specific left bitshift */
BPF_CORE_FIELD_RSHIFT_U64 = 5, /* bitfield-specific right bitshift */
BPF_CORE_TYPE_ID_LOCAL = 6, /* type ID in local BPF object */
BPF_CORE_TYPE_ID_TARGET = 7, /* type ID in target kernel */
BPF_CORE_TYPE_EXISTS = 8, /* type existence in target kernel */
BPF_CORE_TYPE_SIZE = 9, /* type size in bytes */
BPF_CORE_ENUMVAL_EXISTS = 10, /* enum value existence in target kernel */
BPF_CORE_ENUMVAL_VALUE = 11, /* enum value integer value */
BPF_CORE_TYPE_MATCHES = 12, /* type match in target kernel */
};
Notes:
BPF_CORE_FIELD_LSHIFT_U64andBPF_CORE_FIELD_RSHIFT_U64are supposed to be used to read bitfield values using the following algorithm:// To read bitfield ``f`` from ``struct s`` is_signed = relo(s->f, BPF_CORE_FIELD_SIGNED) off = relo(s->f, BPF_CORE_FIELD_BYTE_OFFSET) sz = relo(s->f, BPF_CORE_FIELD_BYTE_SIZE) l = relo(s->f, BPF_CORE_FIELD_LSHIFT_U64) r = relo(s->f, BPF_CORE_FIELD_RSHIFT_U64) // define ``v`` as signed or unsigned integer of size ``sz`` v = *({s|u}<sz> *)((void *)s + off) v <<= l v >>= r
The
BPF_CORE_TYPE_MATCHESqueries matching relation, defined as follows:for integers: types match if size and signedness match;
for arrays & pointers: target types are recursively matched;
for structs & unions:
local members need to exist in target with the same name;
for each member we recursively check match unless it is already behind a pointer, in which case we only check matching names and compatible kind;
for enums:
local variants have to have a match in target by symbolic name (but not numeric value);
size has to match (but enum may match enum64 and vice versa);
for function pointers:
number and position of arguments in local type has to match target;
for each argument and the return value we recursively check match.
CO-RE Relocation Record¶
Relocation record is encoded as the following structure:
struct bpf_core_relo {
__u32 insn_off;
__u32 type_id;
__u32 access_str_off;
enum bpf_core_relo_kind kind;
};
insn_off- instruction offset (in bytes) within a code section associated with this relocation;type_id- BTF type ID of the "root" (containing) entity of a relocatable type or field;access_str_off- offset into corresponding .BTF string section. String interpretation depends on specific relocation kind:for field-based relocations, string encodes an accessed field using a sequence of field and array indices, separated by colon (:). It's conceptually very close to LLVM's getelementptr instruction's arguments for identifying offset to a field. For example, consider the following C code:
struct sample { int a; int b; struct { int c[10]; }; } __attribute__((preserve_access_index)); struct sample *s;
Access to
s[0].awould be encoded as0:0:0: first element ofs(as ifsis an array);0: index of fieldainstruct sample.
Access to
s->awould be encoded as0:0as well.Access to
s->bwould be encoded as0:1:0: first element ofs;1: index of fieldbinstruct sample.
Access to
s[1].c[5]would be encoded as1:2:0:5:1: second element ofs;2: index of anonymous structure field instruct sample;0: index of fieldcin anonymous structure;5: access to array element #5.
for type-based relocations, string is expected to be just "0";
- for enum value-based relocations, string contains an index of enum
value within its enum type;
kind- one ofenum bpf_core_relo_kind.
CO-RE Relocation Examples¶
For the following C code:
struct foo {
int a;
int b;
unsigned c:15;
} __attribute__((preserve_access_index));
enum bar { U, V };
With the following BTF definitions:
...
[2] STRUCT 'foo' size=8 vlen=2
'a' type_id=3 bits_offset=0
'b' type_id=3 bits_offset=32
'c' type_id=4 bits_offset=64 bitfield_size=15
[3] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[4] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
...
[16] ENUM 'bar' encoding=UNSIGNED size=4 vlen=2
'U' val=0
'V' val=1
Field offset relocations are generated automatically when
__attribute__((preserve_access_index)) is used, for example:
void alpha(struct foo *s, volatile unsigned long *g) {
*g = s->a;
s->a = 1;
}
00 <alpha>:
0: r3 = *(s32 *)(r1 + 0x0)
00: CO-RE <byte_off> [2] struct foo::a (0:0)
1: *(u64 *)(r2 + 0x0) = r3
2: *(u32 *)(r1 + 0x0) = 0x1
10: CO-RE <byte_off> [2] struct foo::a (0:0)
3: exit
All relocation kinds could be requested via built-in functions. E.g. field-based relocations:
void bravo(struct foo *s, volatile unsigned long *g) {
*g = __builtin_preserve_field_info(s->b, 0 /* field byte offset */);
*g = __builtin_preserve_field_info(s->b, 1 /* field byte size */);
*g = __builtin_preserve_field_info(s->b, 2 /* field existence */);
*g = __builtin_preserve_field_info(s->b, 3 /* field signedness */);
*g = __builtin_preserve_field_info(s->c, 4 /* bitfield left shift */);
*g = __builtin_preserve_field_info(s->c, 5 /* bitfield right shift */);
}
20 <bravo>:
4: r1 = 0x4
20: CO-RE <byte_off> [2] struct foo::b (0:1)
5: *(u64 *)(r2 + 0x0) = r1
6: r1 = 0x4
30: CO-RE <byte_sz> [2] struct foo::b (0:1)
7: *(u64 *)(r2 + 0x0) = r1
8: r1 = 0x1
40: CO-RE <field_exists> [2] struct foo::b (0:1)
9: *(u64 *)(r2 + 0x0) = r1
10: r1 = 0x1
50: CO-RE <signed> [2] struct foo::b (0:1)
11: *(u64 *)(r2 + 0x0) = r1
12: r1 = 0x31
60: CO-RE <lshift_u64> [2] struct foo::c (0:2)
13: *(u64 *)(r2 + 0x0) = r1
14: r1 = 0x31
70: CO-RE <rshift_u64> [2] struct foo::c (0:2)
15: *(u64 *)(r2 + 0x0) = r1
16: exit
Type-based relocations:
void charlie(struct foo *s, volatile unsigned long *g) {
*g = __builtin_preserve_type_info(*s, 0 /* type existence */);
*g = __builtin_preserve_type_info(*s, 1 /* type size */);
*g = __builtin_preserve_type_info(*s, 2 /* type matches */);
*g = __builtin_btf_type_id(*s, 0 /* type id in this object file */);
*g = __builtin_btf_type_id(*s, 1 /* type id in target kernel */);
}
88 <charlie>:
17: r1 = 0x1
88: CO-RE <type_exists> [2] struct foo
18: *(u64 *)(r2 + 0x0) = r1
19: r1 = 0xc
98: CO-RE <type_size> [2] struct foo
20: *(u64 *)(r2 + 0x0) = r1
21: r1 = 0x1
a8: CO-RE <type_matches> [2] struct foo
22: *(u64 *)(r2 + 0x0) = r1
23: r1 = 0x2 ll
b8: CO-RE <local_type_id> [2] struct foo
25: *(u64 *)(r2 + 0x0) = r1
26: r1 = 0x2 ll
d0: CO-RE <target_type_id> [2] struct foo
28: *(u64 *)(r2 + 0x0) = r1
29: exit
Enum-based relocations:
void delta(struct foo *s, volatile unsigned long *g) {
*g = __builtin_preserve_enum_value(*(enum bar *)U, 0 /* enum literal existence */);
*g = __builtin_preserve_enum_value(*(enum bar *)V, 1 /* enum literal value */);
}
f0 <delta>:
30: r1 = 0x1 ll
f0: CO-RE <enumval_exists> [16] enum bar::U = 0
32: *(u64 *)(r2 + 0x0) = r1
33: r1 = 0x1 ll
108: CO-RE <enumval_value> [16] enum bar::V = 1
35: *(u64 *)(r2 + 0x0) = r1
36: exit