On 5/24/21 10:23 AM, Andrii Nakryiko wrote:
On Sat, May 22, 2021 at 9:39 AM Yonghong Song <yhs@xxxxxx> wrote:
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.
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
Documentation/bpf/index.rst | 1 +
Documentation/bpf/llvm_reloc.rst | 168 +++++++++++++++++++++++++++++++
2 files changed, 169 insertions(+)
create mode 100644 Documentation/bpf/llvm_reloc.rst
diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst
index a702f67dd45f..93e8cf12a6d4 100644
--- a/Documentation/bpf/index.rst
+++ b/Documentation/bpf/index.rst
@@ -84,6 +84,7 @@ Other
:maxdepth: 1
ringbuf
+ llvm_reloc
.. Links:
.. _networking-filter: ../networking/filter.rst
diff --git a/Documentation/bpf/llvm_reloc.rst b/Documentation/bpf/llvm_reloc.rst
new file mode 100644
index 000000000000..bc62bce591b1
--- /dev/null
+++ b/Documentation/bpf/llvm_reloc.rst
@@ -0,0 +1,168 @@
+.. 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 static function/variable references, the symbol often refers to
+the section itself which has a value of 0. To identify actual static
+function/variable, its section offset or some computation result
+based on section offset is written to the original insn/data buffer,
+which is called ``IA`` (implicit addend) below. For global
+function/variables, the symbol refers to actual global and the implicit
+addend is 0.
+
+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 + IA
There are cases where we set all 64-bits of ld_imm64 (e.g., extern
ksym, global variables). Or those will be a different relocation now
(R_BPF_64_ABS64?). If not, I think BitSize 64 is more correct here.
It is still R_BPF_64_64. In llvm, we have restriction that section
offset must be <= UINT32_MAX, and that is why only 32bit is used
to find the actual symbol in symbol table. 32bit permits 4GB section
which should enough in practice for a bpf program.
libbpf or tools can write to full 64bits of imm values of ld_imm64 insn.
The name is a little bit misleading, but it has become part of ABI
and lives in /usr/include/elf.h and we are not able to change it
any more.
Looking at LLVM diff I haven't found a test for global variables (at
least I didn't realize it was there), so double-checking here (and it
might be a good idea to have an explicit test for global variables?)
We have llvm/test/CodeGen/BPF/reloc.ll and
llvm/test/CodeGen/BPF/reloc-btf.ll covering R_BPF_64_ABS64. But I think
I can enhance
llvm/test/CodeGen/BPF/reloc-2.ll to cover an explicit global variable case.
+ 2 R_BPF_64_ABS64 normal data 64 r_offset S + IA
+ 3 R_BPF_64_ABS32 normal data 32 r_offset S + IA
+ 4 R_BPF_64_NODYLD32 .BTF[.ext] data 32 r_offset S + IA
+ 10 R_BPF_64_32 call insn 32 r_offset + 4 (S + IA) / 8 - 1
+
+For example, ``R_BPF_64_64`` relocation type is used for ``ld_imm64`` instruction.
+The actual to-be-relocated data 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.
+
+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 + IA) / 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 -d test.o``::
I recently learned about `llvm-objdump -dr test.o`, which shows
relocations inline, it would be nice to use that output here.
Yes, will do.
+
+ 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
+ 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
+ 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
+ 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
+
+Three relocations are generated with ``llvm-readelf -r test.o``::
+
+ Relocation section '.rel.text' at offset 0x188 contains 3 entries:
+ Offset Info Type Symbol's Value Symbol's Name
+ 0000000000000010 000000040000000a R_BPF_64_32 0000000000000000 gfunc
+ 0000000000000030 000000020000000a R_BPF_64_32 0000000000000000 sec1
+ 0000000000000040 0000000600000001 R_BPF_64_64 0000000000000000 global
+
+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 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
--
2.30.2
