net: filter: document internal instruction encoding

This patch adds a description of eBPFs instruction encoding in order
to bring the documentation in line with the implementation.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/filter.txt

@@ -834,6 +834,167 @@ loops and other CFG validation; second step starts from the first insn and
 descends all possible paths. It simulates execution of every insn and observes
 the state change of registers and stack.
 
+eBPF opcode encoding
+--------------------
+
+eBPF is reusing most of the opcode encoding from classic to simplify conversion
+of classic BPF to eBPF. For arithmetic and jump instructions the 8-bit 'code'
+field is divided into three parts:
+
+  +----------------+--------+--------------------+
+  |   4 bits       |  1 bit |   3 bits           |
+  | operation code | source | instruction class  |
+  +----------------+--------+--------------------+
+  (MSB)                                      (LSB)
+
+Three LSB bits store instruction class which is one of:
+
+  Classic BPF classes:    eBPF classes:
+
+  BPF_LD    0x00          BPF_LD    0x00
+  BPF_LDX   0x01          BPF_LDX   0x01
+  BPF_ST    0x02          BPF_ST    0x02
+  BPF_STX   0x03          BPF_STX   0x03
+  BPF_ALU   0x04          BPF_ALU   0x04
+  BPF_JMP   0x05          BPF_JMP   0x05
+  BPF_RET   0x06          [ class 6 unused, for future if needed ]
+  BPF_MISC  0x07          BPF_ALU64 0x07
+
+When BPF_CLASS(code) == BPF_ALU or BPF_JMP, 4th bit encodes source operand ...
+
+  BPF_K     0x00
+  BPF_X     0x08
+
+ * in classic BPF, this means:
+
+  BPF_SRC(code) == BPF_X - use register X as source operand
+  BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
+
+ * in eBPF, this means:
+
+  BPF_SRC(code) == BPF_X - use 'src_reg' register as source operand
+  BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
+
+... and four MSB bits store operation code.
+
+If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of:
+
+  BPF_ADD   0x00
+  BPF_SUB   0x10
+  BPF_MUL   0x20
+  BPF_DIV   0x30
+  BPF_OR    0x40
+  BPF_AND   0x50
+  BPF_LSH   0x60
+  BPF_RSH   0x70
+  BPF_NEG   0x80
+  BPF_MOD   0x90
+  BPF_XOR   0xa0
+  BPF_MOV   0xb0  /* eBPF only: mov reg to reg */
+  BPF_ARSH  0xc0  /* eBPF only: sign extending shift right */
+  BPF_END   0xd0  /* eBPF only: endianness conversion */
+
+If BPF_CLASS(code) == BPF_JMP, BPF_OP(code) is one of:
+
+  BPF_JA    0x00
+  BPF_JEQ   0x10
+  BPF_JGT   0x20
+  BPF_JGE   0x30
+  BPF_JSET  0x40
+  BPF_JNE   0x50  /* eBPF only: jump != */
+  BPF_JSGT  0x60  /* eBPF only: signed '>' */
+  BPF_JSGE  0x70  /* eBPF only: signed '>=' */
+  BPF_CALL  0x80  /* eBPF only: function call */
+  BPF_EXIT  0x90  /* eBPF only: function return */
+
+So BPF_ADD | BPF_X | BPF_ALU means 32-bit addition in both classic BPF
+and eBPF. There are only two registers in classic BPF, so it means A += X.
+In eBPF it means dst_reg = (u32) dst_reg + (u32) src_reg; similarly,
+BPF_XOR | BPF_K | BPF_ALU means A ^= imm32 in classic BPF and analogous
+src_reg = (u32) src_reg ^ (u32) imm32 in eBPF.
+
+Classic BPF is using BPF_MISC class to represent A = X and X = A moves.
+eBPF is using BPF_MOV | BPF_X | BPF_ALU code instead. Since there are no
+BPF_MISC operations in eBPF, the class 7 is used as BPF_ALU64 to mean
+exactly the same operations as BPF_ALU, but with 64-bit wide operands
+instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition, i.e.:
+dst_reg = dst_reg + src_reg
+
+Classic BPF wastes the whole BPF_RET class to represent a single 'ret'
+operation. Classic BPF_RET | BPF_K means copy imm32 into return register
+and perform function exit. eBPF is modeled to match CPU, so BPF_JMP | BPF_EXIT
+in eBPF means function exit only. The eBPF program needs to store return
+value into register R0 before doing a BPF_EXIT. Class 6 in eBPF is currently
+unused and reserved for future use.
+
+For load and store instructions the 8-bit 'code' field is divided as:
+
+  +--------+--------+-------------------+
+  | 3 bits | 2 bits |   3 bits          |
+  |  mode  |  size  | instruction class |
+  +--------+--------+-------------------+
+  (MSB)                             (LSB)
+
+Size modifier is one of ...
+
+  BPF_W   0x00    /* word */
+  BPF_H   0x08    /* half word */
+  BPF_B   0x10    /* byte */
+  BPF_DW  0x18    /* eBPF only, double word */
+
+... which encodes size of load/store operation:
+
+ B  - 1 byte
+ H  - 2 byte
+ W  - 4 byte
+ DW - 8 byte (eBPF only)
+
+Mode modifier is one of:
+
+  BPF_IMM  0x00  /* classic BPF only, reserved in eBPF */
+  BPF_ABS  0x20
+  BPF_IND  0x40
+  BPF_MEM  0x60
+  BPF_LEN  0x80  /* classic BPF only, reserved in eBPF */
+  BPF_MSH  0xa0  /* classic BPF only, reserved in eBPF */
+  BPF_XADD 0xc0  /* eBPF only, exclusive add */
+
+eBPF has two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and
+(BPF_IND | <size> | BPF_LD) which are used to access packet data.
+
+They had to be carried over from classic to have strong performance of
+socket filters running in eBPF interpreter. These instructions can only
+be used when interpreter context is a pointer to 'struct sk_buff' and
+have seven implicit operands. Register R6 is an implicit input that must
+contain pointer to sk_buff. Register R0 is an implicit output which contains
+the data fetched from the packet. Registers R1-R5 are scratch registers
+and must not be used to store the data across BPF_ABS | BPF_LD or
+BPF_IND | BPF_LD instructions.
+
+These instructions have implicit program exit condition as well. When
+eBPF program is trying to access the data beyond the packet boundary,
+the interpreter will abort the execution of the program. JIT compilers
+therefore must preserve this property. src_reg and imm32 fields are
+explicit inputs to these instructions.
+
+For example:
+
+  BPF_IND | BPF_W | BPF_LD means:
+
+    R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32))
+    and R1 - R5 were scratched.
+
+Unlike classic BPF instruction set, eBPF has generic load/store operations:
+
+BPF_MEM | <size> | BPF_STX:  *(size *) (dst_reg + off) = src_reg
+BPF_MEM | <size> | BPF_ST:   *(size *) (dst_reg + off) = imm32
+BPF_MEM | <size> | BPF_LDX:  dst_reg = *(size *) (src_reg + off)
+BPF_XADD | BPF_W  | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg
+BPF_XADD | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg
+
+Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW. Note that 1 and
+2 byte atomic increments are not supported.
+
 Testing
 -------