[PATCH] bpf, docs: Document BPF insn encoding in term of stored bytes

Jose E. Marchesi
2023-02-24T20:06:14Z

This patch modifies instruction-set.rst so it documents the encoding
of BPF instructions in terms of how the bytes are stored (be it in an
ELF file or as bytes in a memory buffer to be loaded into the kernel
or some other BPF consumer) as opposed to how the instruction looks
like once loaded.

This is hopefully easier to understand by implementors looking to
generate and/or consume bytes conforming BPF instructions.

The patch also clarifies that the unused bytes in a pseudo-instruction
shall be cleared with zeros.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
---
Documentation/bpf/instruction-set.rst | 43 +++++++++++++--------------
1 file changed, 21 insertions(+), 22 deletions(-)

diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index 01802ed9b29b..9b28c0e15bb6 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -38,15 +38,13 @@ eBPF has two instruction encodings:
* the wide instruction encoding, which appends a second 64-bit immediate (i.e.,
constant) value after the basic instruction for a total of 128 bits.

-The basic instruction encoding looks as follows for a little-endian processor,
-where MSB and LSB mean the most significant bits and least significant bits,
-respectively:
+The fields conforming an encoded basic instruction are stored in the
+following order:

-============= ======= ======= ======= ============
-32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB)
-============= ======= ======= ======= ============
-imm offset src_reg dst_reg opcode
-============= ======= ======= ======= ============
+ opcode:8 src:4 dst:4 offset:16 imm:32 // In little-endian BPF.
+ opcode:8 dst:4 src:4 offset:16 imm:32 // In big-endian BPF.
+
+Where,

**imm**
signed integer immediate value
@@ -64,16 +62,17 @@ imm offset src_reg dst_reg opcode
**opcode**
operation to perform

-and as follows for a big-endian processor:
+Note that the contents of multi-byte fields ('imm' and 'offset') are
+stored using big-endian byte ordering in big-endian BPF and
+little-endian byte ordering in little-endian BPF.

-============= ======= ======= ======= ============
-32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB)
-============= ======= ======= ======= ============
-imm offset dst_reg src_reg opcode
-============= ======= ======= ======= ============
+For example:

-Multi-byte fields ('imm' and 'offset') are similarly stored in
-the byte order of the processor.
+ opcode offset imm assembly
+ src dst
+ 07 0 1 00 00 44 33 22 11 r1 += 0x11223344 // little
+ dst src
+ 07 1 0 00 00 11 22 33 44 r1 += 0x11223344 // big

Note that most instructions do not use all of the fields.
Unused fields shall be cleared to zero.
@@ -84,18 +83,18 @@ The 64 bits following the basic instruction contain a pseudo instruction
using the same format but with opcode, dst_reg, src_reg, and offset all set to zero,
and imm containing the high 32 bits of the immediate value.

-================= ==================
-64 bits (MSB) 64 bits (LSB)
-================= ==================
-basic instruction pseudo instruction
-================= ==================
+This is depicted in the following figure:
+
+ basic_instruction pseudo_instruction
+ code:8 regs:16 offset:16 imm:32 | unused:32 imm:32

Thus the 64-bit immediate value is constructed as follows:

imm64 = (next_imm << 32) | imm

where 'next_imm' refers to the imm value of the pseudo instruction
-following the basic instruction.
+following the basic instruction. The unused bytes in the pseudo
+instruction shall be cleared to zero.

Instruction classes
-------------------
--
2.30.2