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aee1720eeb
In commit 4d496be9ca ("bpf,docs: Create new standardization
subdirectory"), I added a standardization/ directory to the BPF
documentation, which will contain the docs that will be standardized
as part of the effort with the IETF.
I included linux-notes.rst in that directory, but I shouldn't have. It
doesn't contain anything that will be standardized. Let's move it back
to Documentation/bpf.
Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230828155948.123405-2-void@manifault.com
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.. contents::
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.. sectnum::
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==========================
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Linux implementation notes
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==========================
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This document provides more details specific to the Linux kernel implementation of the eBPF instruction set.
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Byte swap instructions
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======================
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``BPF_FROM_LE`` and ``BPF_FROM_BE`` exist as aliases for ``BPF_TO_LE`` and ``BPF_TO_BE`` respectively.
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Jump instructions
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=================
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``BPF_CALL | BPF_X | BPF_JMP`` (0x8d), where the helper function
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integer would be read from a specified register, is not currently supported
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by the verifier. Any programs with this instruction will fail to load
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until such support is added.
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Maps
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====
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Linux only supports the 'map_val(map)' operation on array maps with a single element.
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Linux uses an fd_array to store maps associated with a BPF program. Thus,
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map_by_idx(imm) uses the fd at that index in the array.
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Variables
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=========
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The following 64-bit immediate instruction specifies that a variable address,
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which corresponds to some integer stored in the 'imm' field, should be loaded:
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========================= ====== === ========================================= =========== ==============
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opcode construction opcode src pseudocode imm type dst type
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========================= ====== === ========================================= =========== ==============
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BPF_IMM | BPF_DW | BPF_LD 0x18 0x3 dst = var_addr(imm) variable id data pointer
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========================= ====== === ========================================= =========== ==============
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On Linux, this integer is a BTF ID.
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Legacy BPF Packet access instructions
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=====================================
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As mentioned in the `ISA standard documentation
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<instruction-set.html#legacy-bpf-packet-access-instructions>`_,
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Linux has special eBPF instructions for access to packet data that have been
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carried over from classic BPF to retain the performance of legacy socket
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filters running in the eBPF interpreter.
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The instructions come in two forms: ``BPF_ABS | <size> | BPF_LD`` and
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``BPF_IND | <size> | BPF_LD``.
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These instructions are used to access packet data and can only be used when
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the program context is a pointer to a networking packet. ``BPF_ABS``
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accesses packet data at an absolute offset specified by the immediate data
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and ``BPF_IND`` access packet data at an offset that includes the value of
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a register in addition to the immediate data.
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These instructions have seven implicit operands:
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* Register R6 is an implicit input that must contain a pointer to a
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struct sk_buff.
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* Register R0 is an implicit output which contains the data fetched from
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the packet.
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* Registers R1-R5 are scratch registers that are clobbered by the
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instruction.
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These instructions have an implicit program exit condition as well. If an
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eBPF program attempts access data beyond the packet boundary, the
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program execution will be aborted.
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``BPF_ABS | BPF_W | BPF_LD`` (0x20) means::
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R0 = ntohl(*(u32 *) ((struct sk_buff *) R6->data + imm))
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where ``ntohl()`` converts a 32-bit value from network byte order to host byte order.
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``BPF_IND | BPF_W | BPF_LD`` (0x40) means::
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R0 = ntohl(*(u32 *) ((struct sk_buff *) R6->data + src + imm))
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