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Move legacy packet instructions to linux historical notes
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Also specify that div-by-zero results in zero
and mod-zero results in the source value

Signed-off-by: Dave Thaler <[email protected]>
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@dthaler
dthaler committed Sep 13, 2022
1 parent 9db8d5b commit 9246c66
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86 changes: 28 additions & 58 deletions isa/kernel.org/instruction-set.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -182,25 +182,27 @@ code value description
BPF_ADD 0x00 dst += src
BPF_SUB 0x10 dst -= src
BPF_MUL 0x20 dst \*= src
BPF_DIV 0x30 dst /= src
BPF_DIV 0x30 dst = (src != 0) ? (dst / src) : 0
BPF_OR 0x40 dst \|= src
BPF_AND 0x50 dst &= src
BPF_LSH 0x60 dst <<= src
BPF_RSH 0x70 dst >>= src
BPF_NEG 0x80 dst = ~src
BPF_MOD 0x90 dst %= src
BPF_MOD 0x90 dst = (src != 0) ? (dst % src) : src
BPF_XOR 0xa0 dst ^= src
BPF_MOV 0xb0 dst = src
BPF_ARSH 0xc0 sign extending shift right
BPF_END 0xd0 byte swap operations (see `Byte swap instructions`_ below)
======== ===== =================================================

Underflow and overflow are allowed during arithmetic operations,
meaning the 64-bit or 32-bit value will wrap.
where 'src' is the source operand value.

``BPF_DIV`` has an implicit program exit condition as well. If
Underflow and overflow are allowed during arithmetic operations,
meaning the 64-bit or 32-bit value will wrap. If
eBPF program execution would result in division by zero,
program execution must be gracefully aborted.
the destination register is instead set to zero.
If execution would result in module by zero,
the destination register is instead set to the source value.

Examples:

Expand Down Expand Up @@ -480,45 +482,13 @@ There is currently only one such instruction.
Legacy BPF Packet access instructions
-------------------------------------

eBPF has special instructions for access to packet data that have been
carried over from classic BPF to retain the performance of legacy socket
filters running in an eBPF interpreter.

The instructions come in two forms: ``BPF_ABS | <size> | BPF_LD`` and
``BPF_IND | <size> | BPF_LD``.

These instructions are used to access packet data and can only be used when
the program context contains a pointer to a networking packet. ``BPF_ABS``
accesses packet data at an absolute offset specified by the immediate data
and ``BPF_IND`` access packet data at an offset that includes the value of
a register in addition to the immediate data.

These instructions have seven implicit operands:

* Register R6 is an implicit input that must contain a pointer to a
context structure with a packet data pointer.
* Register R0 is an implicit output which contains the data fetched from
the packet.
* Registers R1-R5 are scratch registers that are clobbered by the
instruction.
Linux introduced special instructions for access to packet data that were
carried over from classic BPF. However, these instructions are
deprecated and should no longer be used in any version of the ISA.

**Note**

*Linux implementation*: In Linux, R6 references a struct sk_buff.

These instructions have an implicit program exit condition as well. If an
eBPF program attempts access data beyond the packet boundary, the
program execution must be gracefully aborted.

``BPF_ABS | BPF_W | BPF_LD`` (0x20) means::

R0 = ntohl(*(uint32_t *) (R6->data + imm))

where ``ntohl()`` converts a 32-bit value from network byte order to host byte order.

``BPF_IND | BPF_W | BPF_LD`` (0x40) means::

R0 = ntohl(*(uint32_t *) (R6->data + src + imm))
*Linux implementation*: Details can be found in the `Linux historical notes <https://github.com/dthaler/ebpf-docs/blob/update/isa/kernel.org/linux-historical-notes.rst#legacy-bpf-packet-access-instructions>`_.

Appendix
========
Expand All @@ -542,42 +512,42 @@ opcode imm description reference
0x1d 0x00 if dst == src goto +offset `Jump instructions`_
0x1e 0x00 if (uint32_t)dst == (uint32_t)src goto +offset `Jump instructions`_
0x1f 0x00 dst -= src `Arithmetic instructions`_
0x20 any dst = ntohl(\*(uint32_t \*)(R6->data + imm)) `Load and store instructions`_
0x20 any (deprecated, implementation-specific) `Legacy BPF Packet access instructions`_
0x24 any dst = (uint32_t)(dst \* imm) `Arithmetic instructions`_
0x25 any if dst > imm goto +offset `Jump instructions`_
0x26 any if (uint32_t)dst > imm goto +offset `Jump instructions`_
0x27 any dst \*= imm `Arithmetic instructions`_
0x28 any dst = ntohs(\*(uint16_t \*)(R6->data + imm)) `Load and store instructions`_
0x28 any (deprecated, implementation-specific) `Legacy BPF Packet access instructions`_
0x2c 0x00 dst = (uint32_t)(dst \* src) `Arithmetic instructions`_
0x2d 0x00 if dst > src goto +offset `Jump instructions`_
0x2e 0x00 if (uint32_t)dst > (uint32_t)src goto +offset `Jump instructions`_
0x2f 0x00 dst \*= src `Arithmetic instructions`_
0x30 any dst = (\*(uint8_t \*)(R6->data + imm)) `Load and store instructions`_
0x34 any dst = (uint32_t)(dst / imm) `Arithmetic instructions`_
0x30 any (deprecated, implementation-specific) `Legacy BPF Packet access instructions`_
0x34 any dst = (uint32_t)((imm != 0) ? (dst / imm) : 0) `Arithmetic instructions`_
0x35 any if dst >= imm goto +offset `Jump instructions`_
0x36 any if (uint32_t)dst >= imm goto +offset `Jump instructions`_
0x37 any dst /= imm `Arithmetic instructions`_
0x38 any dst = ntohll(\*(uint64_t \*)(R6->data + imm)) `Load and store instructions`_
0x3c 0x00 dst = (uint32_t)(dst / src) `Arithmetic instructions`_
0x37 any dst = (imm != 0) ? (dst / imm) : 0 `Arithmetic instructions`_
0x38 any (deprecated, implementation-specific) `Legacy BPF Packet access instructions`_
0x3c 0x00 dst = (uint32_t)((imm != 0) ? (dst / src) : 0) `Arithmetic instructions`_
0x3d 0x00 if dst >= src goto +offset `Jump instructions`_
0x3e 0x00 if (uint32_t)dst >= (uint32_t)src goto +offset `Jump instructions`_
0x3f 0x00 dst /= src `Arithmetic instructions`_
0x40 any dst = ntohl(\*(uint32_t \*)(R6->data + src + imm)) `Load and store instructions`_
0x3f 0x00 dst = (src !+ 0) ? (dst / src) : 0 `Arithmetic instructions`_
0x40 any (deprecated, implementation-specific) `Legacy BPF Packet access instructions`_
0x44 any dst = (uint32_t)(dst \| imm) `Arithmetic instructions`_
0x45 any if dst & imm goto +offset `Jump instructions`_
0x46 any if (uint32_t)dst & imm goto +offset `Jump instructions`_
0x47 any dst \|= imm `Arithmetic instructions`_
0x48 any dst = ntohs(\*(uint16_t \*)(R6->data + src + imm)) `Load and store instructions`_
0x48 any (deprecated, implementation-specific) `Legacy BPF Packet access instructions`_
0x4c 0x00 dst = (uint32_t)(dst \| src) `Arithmetic instructions`_
0x4d 0x00 if dst & src goto +offset `Jump instructions`_
0x4e 0x00 if (uint32_t)dst & (uint32_t)src goto +offset `Jump instructions`_
0x4f 0x00 dst \|= src `Arithmetic instructions`_
0x50 any dst = \*(uint8_t \*)(R6->data + src + imm)) `Load and store instructions`_
0x50 any (deprecated, implementation-specific) `Legacy BPF Packet access instructions`_
0x54 any dst = (uint32_t)(dst & imm) `Arithmetic instructions`_
0x55 any if dst != imm goto +offset `Jump instructions`_
0x56 any if (uint32_t)dst != imm goto +offset `Jump instructions`_
0x57 any dst &= imm `Arithmetic instructions`_
0x58 any dst = ntohll(\*(uint64_t \*)(R6->data + src + imm)) `Load and store instructions`_
0x58 any (deprecated, implementation-specific) `Legacy BPF Packet access instructions`_
0x5c 0x00 dst = (uint32_t)(dst & src) `Arithmetic instructions`_
0x5d 0x00 if dst != src goto +offset `Jump instructions`_
0x5e 0x00 if (uint32_t)dst != (uint32_t)src goto +offset `Jump instructions`_
Expand Down Expand Up @@ -613,11 +583,11 @@ opcode imm description reference
0x84 0x00 dst = (uint32_t)-dst `Arithmetic instructions`_
0x85 any call imm `Jump instructions`_
0x87 0x00 dst = -dst `Arithmetic instructions`_
0x94 any dst = (uint32_t)(dst % imm) `Arithmetic instructions`_
0x94 any dst = (uint32_t)((imm != 0) ? (dst % imm) : imm) `Arithmetic instructions`_
0x95 0x00 return `Jump instructions`_
0x97 any dst %= imm `Arithmetic instructions`_
0x9c 0x00 dst = (uint32_t)(dst % src) `Arithmetic instructions`_
0x9f 0x00 dst %= src `Arithmetic instructions`_
0x97 any dst = (imm != 0) ? (dst % imm) : imm `Arithmetic instructions`_
0x9c 0x00 dst = (uint32_t)((src != 0) ? (dst % src) : src) `Arithmetic instructions`_
0x9f 0x00 dst = (src != 0) ? (dst % src) : src `Arithmetic instructions`_
0xa4 any dst = (uint32_t)(dst ^ imm) `Arithmetic instructions`_
0xa5 any if dst < imm goto +offset `Jump instructions`_
0xa6 any if (uint32_t)dst < imm goto +offset `Jump instructions`_
Expand Down
63 changes: 63 additions & 0 deletions isa/kernel.org/linux-historical-notes.rst
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@@ -0,0 +1,63 @@
.. contents::
.. sectnum::

=====================
Linux historial notes
=====================

Legacy BPF Packet access instructions
=====================================

As mentioned in the `ISA standard documentation <https://github.com/dthaler/ebpf-docs/blob/update/isa/kernel.org/instruction-set.rst#legacy-bpf-packet-access-instructions>`_,
Linux has special eBPF instructions for access to packet data that have been
carried over from classic BPF to retain the performance of legacy socket
filters running in the eBPF interpreter.

The instructions come in two forms: ``BPF_ABS | <size> | BPF_LD`` and
``BPF_IND | <size> | BPF_LD``.

These instructions are used to access packet data and can only be used when
the program context is a pointer to a networking packet. ``BPF_ABS``
accesses packet data at an absolute offset specified by the immediate data
and ``BPF_IND`` access packet data at an offset that includes the value of
a register in addition to the immediate data.

These instructions have seven implicit operands:

* Register R6 is an implicit input that must contain a pointer to a
struct sk_buff.
* Register R0 is an implicit output which contains the data fetched from
the packet.
* Registers R1-R5 are scratch registers that are clobbered by the
instruction.

These instructions have an implicit program exit condition as well. If an
eBPF program attempts access data beyond the packet boundary, the
program execution will be aborted.

``BPF_ABS | BPF_W | BPF_LD`` (0x20) means::

R0 = ntohl(*(u32 *) ((struct sk_buff *) R6->data + imm))

where ``ntohl()`` converts a 32-bit value from network byte order to host byte order.

``BPF_IND | BPF_W | BPF_LD`` (0x40) means::

R0 = ntohl(*(u32 *) ((struct sk_buff *) R6->data + src + imm))

Appendix
========

For reference, the following table lists legacy Linux-specific opcodes in order by value.

====== ==== =================================================== =============
opcode imm description reference
====== ==== =================================================== =============
0x20 any dst = ntohl(\*(uint32_t \*)(R6->data + imm)) `Legacy BPF Packet access instructions`_
0x28 any dst = ntohs(\*(uint16_t \*)(R6->data + imm)) `Legacy BPF Packet access instructions`_
0x30 any dst = (\*(uint8_t \*)(R6->data + imm)) `Legacy BPF Packet access instructions`_
0x38 any dst = ntohll(\*(uint64_t \*)(R6->data + imm)) `Legacy BPF Packet access instructions`_
0x40 any dst = ntohl(\*(uint32_t \*)(R6->data + src + imm)) `Legacy BPF Packet access instructions`_
0x48 any dst = ntohs(\*(uint16_t \*)(R6->data + src + imm)) `Legacy BPF Packet access instructions`_
0x50 any dst = \*(uint8_t \*)(R6->data + src + imm)) `Legacy BPF Packet access instructions`_
0x58 any dst = ntohll(\*(uint64_t \*)(R6->data + src + imm)) `Legacy BPF Packet access instructions`_

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