On 6/5/21 2:03 PM, Fāng-ruì Sòng wrote:
On Tue, May 25, 2021 at 11:52 AM Yonghong Song <yhs@xxxxxx> wrote:
On 5/25/21 11:29 AM, Fangrui Song wrote:
I have a review queue with a huge pile of LLVM patches and have only
skimmed through this.
First, if the size benefit of REL over RELA isn't deem that necessary,
I will highly recommend RELA for simplicity and robustness.
REL is error-prone.
The worry is backward compatibility. Because of BPF ELF format
is so intervened with bpf eco system (loading, bpf map, etc.),
a lot of tools in the wild already implemented to parse REL...
It will be difficult to change...
It seems that the design did not get enough initial scrutiny...
(On https://reviews.llvm.org/D101336 , a reviewer who has apparently
never contributed to lld/ELF clicked LGTM without actual reviewing the
patch and that was why I have to click "Request Changes").
I worry that keeping the current state as-is can cause much
maintenance burden in the LLVM MC layer, linker, and other binary
utilities.
Some things can be improved without breaking backward compatibility.
On 2021-05-24, Yonghong Song 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 | 240 +++++++++++++++++++++++++
tools/testing/selftests/bpf/README.rst | 19 ++
3 files changed, 260 insertions(+)
create mode 100644 Documentation/bpf/llvm_reloc.rst
Changelogs:
v1 -> v2:
- add an example to illustrate how relocations related to base
section and symbol table and what is "Implicit Addend"
- clarify why we use 32bit read/write for R_BPF_64_64 (ld_imm64)
relocations.
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..5ade0244958f
--- /dev/null
+++ b/Documentation/bpf/llvm_reloc.rst
@@ -0,0 +1,240 @@
+.. 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 ``SECTION`` type
STT_SECTION. `SECTION` is just an abbreviated form used by some binary
tools.
This is just to match llvm-readelf output. I can add a reference
to STT_SECTION for the right macro name.
+and represents a section. So for static variable or function,
+the section offset is written to the original insn
+buffer, which is called ``IA`` (implicit 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``.
The other REL based psABIs all use `A` for addend.
I can use `A` as well. The reason I am using `IA` since it is not
shown in the relocation record and lld used API 'getImplicitAddend()`
get this value. But I can certainly use `A`.
An ABI document should stick with standard terms.
The variable names used in an implementation are just informative
(plus I don't see any `IA` in lld's source code).
+In general, the ``IA`` 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
+ IA
+ 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
Shifting the offset by 4 looks weird. R_386_32 applies at r_offset.
The call instruction R_BPF_64_32 is strange. Such special calculation
should not be named R_BPF_64_32.
Again, we have a backward compatibility issue here. I would like to
provide an alias for it in llvm relocation header file, but I don't
know how to do it.
It is very confusing that R_BPF_64_64 has a 32-bit value.
If you like, we can make it as 64bit value.
R_BPF_64_64 is for ld_imm64 insn which is a 16byte insn.
The bytes 4-7 and 12-15 forms a 64bit value for the instruction.
We can do
write32/read32 for bytes 4-7
write32/read32 for bytes 12-15
for the relocation. Currently, we limit to bytes 4-7 since
in BPF it is really unlikely we have section offset > 4G.
But we could extend to full 64bit section offset.
Since its computation is the same as R_BPF_64_ABS32, can R_BPF_64_64
be deprecated in favor of R_BPF_64_ABS32?
Its computation is the same but R_BPF_64_ABS32 starts from offset
and R_BPF_64_64 starts from offset + 4.
For R_BPF_64_64, the relocation offset is the *start* of the instruction
hence +4 is needed to actually read/write addend.
To deprecate R_BPF_64_64 to be the same as R_BPF_64_ABS32, we will
need to change relocation offset. This will break existing libbpf
and cilium and likely many other tools, so I would prefer not
to do this.
There is nothing preventing a relocation type from being used as data
in some cases while code in other cases.
R_BPF_64_64 can be renamed to indicate that it is deprecated.
R_BPF_64_32 can be confused with R_BPF_64_ABS32. You may rename
R_BPF_64_32 to say, R_BPF_64_CALL32.
For compatibility, only the values matter, not the names.
E.g. on x86, some unused GNU_PROPERTY values were renamed to
GNU_PROPERTY_X86_COMPAT_ISA_1_USED ("COMPAT" for compatibility) while
their values were kept.
Two aarch64 relocation types have been renamed.
Renaming sounds a more attractive choice. But a lot of other tools
have already used the name and it will be odd and not user friendly
to display a different name from llvm.
For example elfutils, we have
backends/bpf_symbol.c: case R_BPF_64_64:
libelf/elf.h:#define R_BPF_64_64
My /usr/include/elf.h (from glibc-headers-2.28-149.el8.x86_64) has:
/* BPF specific declarations. */
#define R_BPF_NONE 0 /* No reloc */
#define R_BPF_64_64 1
#define R_BPF_64_32 10
I agree the name is a little misleading, but renaming may cause
some confusion in bpf ecosystem. So we prefer to stay as is, but
with documentation to explain what each relocation intends to do.
+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.
Why cannot R_BPF_64_ABS32 cover the use cases of R_BPF_64_NODYLD32?
I haven't seen any relocation type which hard coding knowledge on data
sections.
This is due to how .BTF relocation is done. Relocation is against
loadable symbols but it does not want dynamic linker to resolve them.
Instead it wants libbpf and kernel to resolve them in a different
way.
How is R_BPF_64_NODYLD32 different?
I don't see it is different on https://reviews.llvm.org/D101336 .
I cannot find R_BPF_64_NODYLD32 in the kernel code as well.
As I mentioned in the above this is to deal with a case related to
runtime dynamic linking relocation (DYLD), JIT style of compilation.
kernel won't use this paradigm so you won't find it in the kenrel.
There may be a misconception that different sections need different
relocation types,
even if the semantics are the same. Such understanding is incorrect.
We know this and we don't have this perception such .BTF/.BTF.ext
relocation types must be different due to it is a different relocation.
It needs a different relocation because its relocation resolution
is different from ABS32.
I guess I didn't give enough details on why we need this new relocation
kind and let me try again.
First, the use case is for bcc/bpftrace style LLVM JIT (ExecutionEngine)
based compilation. The related bcc code is here:
https://github.com/iovisor/bcc/blob/master/src/cc/bpf_module.cc#L453-L498
Basically, we will invoke clang CompilerInvocation to generate IR and
do a bpf target IR optimization and call
ExecutionEngine->finalizeObject()
to generate code.
In this particular setting, we set ExecutionEngine to process
all sections (including dwarf and BTF sections). And among others,
ExecutionEngine will try to *resolve* all relocations for dwarf and
BTF sections.
The core loop for relocation resolution,
void RuntimeDyldImpl::resolveLocalRelocations() {
// Iterate over all outstanding relocations
for (auto it = Relocations.begin(), e = Relocations.end(); it != e;
++it) {
// The Section here (Sections[i]) refers to the section in which the
// symbol for the relocation is located. The SectionID in the
relocation
// entry provides the section to which the relocation will be applied.
unsigned Idx = it->first;
uint64_t Addr = getSectionLoadAddress(Idx);
LLVM_DEBUG(dbgs() << "Resolving relocations Section #" << Idx << "\t"
<< format("%p", (uintptr_t)Addr) << "\n");
resolveRelocationList(it->second, Addr);
}
Relocations.clear();
}
For example, for the following code,
$ cat t.c
int g;
int test() { return g; }
$ clang -target bpf -O2 -g -c t.c
$ llvm-readelf -r t.o
...
Relocation section '.rel.debug_info' at offset 0x3e0 contains 11
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 g
0000000000000040 0000000400000003 R_BPF_64_ABS32
0000000000000000 .debug_str
0000000000000047 0000000200000002 R_BPF_64_ABS64
0000000000000000 .text
0000000000000055 0000000400000003 R_BPF_64_ABS32
0000000000000000 .debug_str
Relocation section '.rel.BTF' at offset 0x490 contains 1 entries:
Offset Info Type Symbol's
Value Symbol's Name
0000000000000060 0000000800000004 R_BPF_64_NODYLD32 0000000000000000 g
Relocation section '.rel.BTF.ext' at offset 0x4a0 contains 3 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
0000000000000050 0000000200000004 R_BPF_64_NODYLD32
0000000000000000 .text
During JIT relocation resolution, it is OKAY to resolve 'g' to
be actual address and actually it is a good thing since tools
like bcc actually go through dwarf interface to dump instructions
interleaved with source codes.
Note that dwarf won't be loaded into the kernel.
But we don't want relocations in .BTF/.BTF.ext to be resolved with
actually addresses. They will be processed by bpf libraries and the
kernel. The reason not to have relocation resolution
is not due to their names, but due
to their functionality. If we intend to load dwarf to kernel, we
will issue R_BPF_64_NODYLD32 to dwarf as well.
One can argue we should have fine control in RuntimeDyld so
that which section to have runtime relocation resolution
and which section does not. I don't know whether
ExecutionEngine/RuntimeDyld agree with that or not. But
BPF backend perspective, R_BPF_64_ABS32 and R_BPF_64_NODYLD32
are two different relocations, they may do something common
in some places like lld, but they may do different things
in other places like dyld.
Is the above reasonable or you have some further suggestions
on how to resolve this? I guess I do need to add some of above
discussion in the documentation so it will be clear why we added
this relocation.
+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``.
In other ABIs, names like 32/ABS32/ABS64 refer to absolute relocation types
without such complex operation.
Again, this is a historical artifact to handle call instruction. I am
aware that this might be different from other architectures. But we have
to keep backward compatibility...
+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;
+ }
+
[...]
