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linux/Documentation/bpf/classic_vs_extended.rst
Christoph Hellwig 41db511a3a bpf, docs: Split the comparism to classic BPF from instruction-set.rst 
Split the introductory that explain eBPF vs classic BPF and how it maps
to hardware from the instruction set specification into a standalone
document.  This duplicates a little bit of information but gives us a
useful reference for the eBPF instrution set that is not encumbered by
classic BPF.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211223101906.977624-3-hch@lst.de
2021-12-30 16:31:44 -08:00

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===================
Classic BPF vs eBPF
===================
eBPF is designed to be JITed with one to one mapping, which can also open up
the possibility for GCC/LLVM compilers to generate optimized eBPF code through
an eBPF backend that performs almost as fast as natively compiled code.
Some core changes of the eBPF format from classic BPF:
- Number of registers increase from 2 to 10:
The old format had two registers A and X, and a hidden frame pointer. The
new layout extends this to be 10 internal registers and a read-only frame
pointer. Since 64-bit CPUs are passing arguments to functions via registers
the number of args from eBPF program to in-kernel function is restricted
to 5 and one register is used to accept return value from an in-kernel
function. Natively, x86_64 passes first 6 arguments in registers, aarch64/
sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved
registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers.
Thus, all eBPF registers map one to one to HW registers on x86_64, aarch64,
etc, and eBPF calling convention maps directly to ABIs used by the kernel on
64-bit architectures.
On 32-bit architectures JIT may map programs that use only 32-bit arithmetic
and may let more complex programs to be interpreted.
R0 - R5 are scratch registers and eBPF program needs spill/fill them if
necessary across calls. Note that there is only one eBPF program (== one
eBPF main routine) and it cannot call other eBPF functions, it can only
call predefined in-kernel functions, though.
- Register width increases from 32-bit to 64-bit:
Still, the semantics of the original 32-bit ALU operations are preserved
via 32-bit subregisters. All eBPF registers are 64-bit with 32-bit lower
subregisters that zero-extend into 64-bit if they are being written to.
That behavior maps directly to x86_64 and arm64 subregister definition, but
makes other JITs more difficult.
32-bit architectures run 64-bit eBPF programs via interpreter.
Their JITs may convert BPF programs that only use 32-bit subregisters into
native instruction set and let the rest being interpreted.
Operation is 64-bit, because on 64-bit architectures, pointers are also
64-bit wide, and we want to pass 64-bit values in/out of kernel functions,
so 32-bit eBPF registers would otherwise require to define register-pair
ABI, thus, there won't be able to use a direct eBPF register to HW register
mapping and JIT would need to do combine/split/move operations for every
register in and out of the function, which is complex, bug prone and slow.
Another reason is the use of atomic 64-bit counters.
- Conditional jt/jf targets replaced with jt/fall-through:
While the original design has constructs such as ``if (cond) jump_true;
else jump_false;``, they are being replaced into alternative constructs like
``if (cond) jump_true; /* else fall-through */``.
- Introduces bpf_call insn and register passing convention for zero overhead
calls from/to other kernel functions:
Before an in-kernel function call, the eBPF program needs to
place function arguments into R1 to R5 registers to satisfy calling
convention, then the interpreter will take them from registers and pass
to in-kernel function. If R1 - R5 registers are mapped to CPU registers
that are used for argument passing on given architecture, the JIT compiler
doesn't need to emit extra moves. Function arguments will be in the correct
registers and BPF_CALL instruction will be JITed as single 'call' HW
instruction. This calling convention was picked to cover common call
situations without performance penalty.
After an in-kernel function call, R1 - R5 are reset to unreadable and R0 has
a return value of the function. Since R6 - R9 are callee saved, their state
is preserved across the call.
For example, consider three C functions::
u64 f1() { return (*_f2)(1); }
u64 f2(u64 a) { return f3(a + 1, a); }
u64 f3(u64 a, u64 b) { return a - b; }
GCC can compile f1, f3 into x86_64::
f1:
movl $1, %edi
movq _f2(%rip), %rax
jmp *%rax
f3:
movq %rdi, %rax
subq %rsi, %rax
ret
Function f2 in eBPF may look like::
f2:
bpf_mov R2, R1
bpf_add R1, 1
bpf_call f3
bpf_exit
If f2 is JITed and the pointer stored to ``_f2``. The calls f1 -> f2 -> f3 and
returns will be seamless. Without JIT, __bpf_prog_run() interpreter needs to
be used to call into f2.
For practical reasons all eBPF programs have only one argument 'ctx' which is
already placed into R1 (e.g. on __bpf_prog_run() startup) and the programs
can call kernel functions with up to 5 arguments. Calls with 6 or more arguments
are currently not supported, but these restrictions can be lifted if necessary
in the future.
On 64-bit architectures all register map to HW registers one to one. For
example, x86_64 JIT compiler can map them as ...
::
R0 - rax
R1 - rdi
R2 - rsi
R3 - rdx
R4 - rcx
R5 - r8
R6 - rbx
R7 - r13
R8 - r14
R9 - r15
R10 - rbp
... since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing
and rbx, r12 - r15 are callee saved.
Then the following eBPF pseudo-program::
bpf_mov R6, R1 /* save ctx */
bpf_mov R2, 2
bpf_mov R3, 3
bpf_mov R4, 4
bpf_mov R5, 5
bpf_call foo
bpf_mov R7, R0 /* save foo() return value */
bpf_mov R1, R6 /* restore ctx for next call */
bpf_mov R2, 6
bpf_mov R3, 7
bpf_mov R4, 8
bpf_mov R5, 9
bpf_call bar
bpf_add R0, R7
bpf_exit
After JIT to x86_64 may look like::
push %rbp
mov %rsp,%rbp
sub $0x228,%rsp
mov %rbx,-0x228(%rbp)
mov %r13,-0x220(%rbp)
mov %rdi,%rbx
mov $0x2,%esi
mov $0x3,%edx
mov $0x4,%ecx
mov $0x5,%r8d
callq foo
mov %rax,%r13
mov %rbx,%rdi
mov $0x6,%esi
mov $0x7,%edx
mov $0x8,%ecx
mov $0x9,%r8d
callq bar
add %r13,%rax
mov -0x228(%rbp),%rbx
mov -0x220(%rbp),%r13
leaveq
retq
Which is in this example equivalent in C to::
u64 bpf_filter(u64 ctx)
{
return foo(ctx, 2, 3, 4, 5) + bar(ctx, 6, 7, 8, 9);
}
In-kernel functions foo() and bar() with prototype: u64 (*)(u64 arg1, u64
arg2, u64 arg3, u64 arg4, u64 arg5); will receive arguments in proper
registers and place their return value into ``%rax`` which is R0 in eBPF.
Prologue and epilogue are emitted by JIT and are implicit in the
interpreter. R0-R5 are scratch registers, so eBPF program needs to preserve
them across the calls as defined by calling convention.
For example the following program is invalid::
bpf_mov R1, 1
bpf_call foo
bpf_mov R0, R1
bpf_exit
After the call the registers R1-R5 contain junk values and cannot be read.
An in-kernel verifier.rst is used to validate eBPF programs.
Also in the new design, eBPF is limited to 4096 insns, which means that any
program will terminate quickly and will only call a fixed number of kernel
functions. Original BPF and eBPF are two operand instructions,
which helps to do one-to-one mapping between eBPF insn and x86 insn during JIT.
The input context pointer for invoking the interpreter function is generic,
its content is defined by a specific use case. For seccomp register R1 points
to seccomp_data, for converted BPF filters R1 points to a skb.
A program, that is translated internally consists of the following elements::
op:16, jt:8, jf:8, k:32 ==> op:8, dst_reg:4, src_reg:4, off:16, imm:32
So far 87 eBPF instructions were implemented. 8-bit 'op' opcode field
has room for new instructions. Some of them may use 16/24/32 byte encoding. New
instructions must be multiple of 8 bytes to preserve backward compatibility.
eBPF is a general purpose RISC instruction set. Not every register and
every instruction are used during translation from original BPF to eBPF.
For example, socket filters are not using ``exclusive add`` instruction, but
tracing filters may do to maintain counters of events, for example. Register R9
is not used by socket filters either, but more complex filters may be running
out of registers and would have to resort to spill/fill to stack.
eBPF can be used as a generic assembler for last step performance
optimizations, socket filters and seccomp are using it as assembler. Tracing
filters may use it as assembler to generate code from kernel. In kernel usage
may not be bounded by security considerations, since generated eBPF code
may be optimizing internal code path and not being exposed to the user space.
Safety of eBPF can come from the verifier.rst. In such use cases as
described, it may be used as safe instruction set.
Just like the original BPF, eBPF runs within a controlled environment,
is deterministic and the kernel can easily prove that. The safety of the program
can be determined in two steps: first step does depth-first-search to disallow
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.
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 BPF_JMP32 0x06
BPF_MISC 0x07 BPF_ALU64 0x07
=================== ===============
The 4th bit encodes the 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 or BPF_JMP32 [ in eBPF ], BPF_OP(code) is one of::
BPF_JA 0x00 /* BPF_JMP only */
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 BPF_JMP only: function call */
BPF_EXIT 0x90 /* eBPF BPF_JMP only: function return */
BPF_JLT 0xa0 /* eBPF only: unsigned '<' */
BPF_JLE 0xb0 /* eBPF only: unsigned '<=' */
BPF_JSLT 0xc0 /* eBPF only: signed '<' */
BPF_JSLE 0xd0 /* eBPF only: signed '<=' */
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 used as
BPF_JMP32 to mean exactly the same operations as BPF_JMP, but with 32-bit wide
operands for the comparisons instead.
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 /* used for 32-bit mov in classic BPF and 64-bit 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_ATOMIC 0xc0 /* eBPF only, atomic operations */
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