On 8/25/23 3:45 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-strings from:
- include/uapi/linux/bpf.h:struct bpf_core_relo
- tools/lib/bpf/relo_core.c:__bpf_core_types_match()
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
LGTM with a couple minor nits below.
Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
Documentation/bpf/btf.rst | 31 +++-
Documentation/bpf/llvm_reloc.rst | 304 +++++++++++++++++++++++++++++++
2 files changed, 329 insertions(+), 6 deletions(-)
[...]
+.. 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 */
+ };
+
+Notes:
+
+* ``BPF_CORE_FIELD_LSHIFT_U64`` and ``BPF_CORE_FIELD_RSHIFT_U64`` are
+ supposed to be used to read bitfield values using the following
+ algorithm:
+
+ .. code-block:: c
+
+ // 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 = *((void *)s) + off)
parenthesis not matching in the above.
How about below to a little bit more precise?
v = *({s|u}<sz> *)((void *)s + off)
+ v <<= l
+ v >>= r
+
[...]
+
+CO-RE Relocation Examples
+=========================
+
+For the following C code:
+
+.. code-block:: c
+
+ struct foo {
+ int a;
+ int b;
+ unsigned c:15;
+ } __attribute__((preserve_access_index));
+
+ enum bar { U, V };
+
+With the following BTF definitions:
+
+.. code-block::
+
+ ...
+ [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
Misalignment in the above.
+ [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:
+
+.. code-block:: c
+
+ 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:
+
+.. code-block:: c
+
+ 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
+
+
[...]
