On 8/25/23 4:40 AM, Eduard Zingerman wrote:
On Thu, 2023-08-24 at 23:05 -0700, Yonghong Song wrote:
On 8/24/23 4:01 PM, Eduard Zingerman wrote:
Add a section on CO-RE relocations to llvm_relo.rst.
Describe relevant .BTF.ext structure, `enum bpf_core_relo_kind`
and `struct bpf_core_relo` in some detail.
Description is based on doc-string from include/uapi/linux/bpf.h.
Thanks Eduard. This is very helpful to give bpf deverlopers
some insight about how different of core relocations are
supported in llvm and libbpf.
Hi Yonghong,
thank you for taking a look.
Some comments below.
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
Documentation/bpf/btf.rst | 27 ++++-
Documentation/bpf/llvm_reloc.rst | 178 +++++++++++++++++++++++++++++++
2 files changed, 201 insertions(+), 4 deletions(-)
diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
index f32db1f44ae9..c0530211c3c1 100644
--- a/Documentation/bpf/btf.rst
+++ b/Documentation/bpf/btf.rst
@@ -726,8 +726,8 @@ same as the one describe in :ref:`BTF_Type_String`.
4.2 .BTF.ext section
--------------------
[...]
+
+The complete list of relocation kinds is represented by the following enum:
+
+.. code-block:: c
+
+ 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 */
+ };
+
[...]
+
+CO-RE Relocation Examples
+=========================
+
+For the following C code:
+
+.. code-block:: c
+
+ struct foo {
+ int a;
+ int b;
+ } __attribute__((preserve_access_index));
+
+ enum bar { U, V };
+
+ void buz(struct foo *s, volatile unsigned long *g) {
+ s->a = 1;
+ *g = __builtin_preserve_field_info(s->b, 1);
+ *g = __builtin_preserve_type_info(*s, 1);
+ *g = __builtin_preserve_enum_value(*(enum bar *)V, 1);
Maybe __builtin_btf_type_id() can be added as well?
So far, clang only supports the above 4 builtin's for core
relocations.
Will add __builtin_btf_type_id() as well.
+ }
+
+With the following BTF definititions:
+
+.. code-block::
+
+ ...
+ [2] STRUCT 'foo' size=8 vlen=2
+ 'a' type_id=3 bits_offset=0
+ 'b' type_id=3 bits_offset=32
+ [3] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
+ ...
+ [9] ENUM 'bar' encoding=UNSIGNED size=4 vlen=2
+ 'U' val=0
+ 'V' val=1
+
+The following relocation entries would be generated:
+
+.. code-block:: c
+
+ <buz>:
+ 0: *(u32 *)(r1 + 0x0) = 0x1
+ 00: CO-RE <byte_off> [2] struct foo::a (0:0)
+ 1: r1 = 0x4
+ 08: CO-RE <byte_sz> [2] struct foo::b (0:1)
+ 2: *(u64 *)(r2 + 0x0) = r1
+ 3: r1 = 0x8
+ 18: CO-RE <type_size> [2] struct foo
+ 4: *(u64 *)(r2 + 0x0) = r1
+ 5: r1 = 0x1 ll
+ 28: CO-RE <enumval_value> [9] enum bar::V = 1
+ 7: *(u64 *)(r2 + 0x0) = r1
+ 8: exit
+
It would be great if we can have an example for each of above
core relocation kinds.
You mean all 13 kinds, right?
Yes, it would be great if we have at least one example for each kind
to illustrate what this relo kind intends to do.
+Note: modifications for llvm-objdump to show these relocation entries
+are currently work in progress.
