377 lines
14 KiB
ReStructuredText
377 lines
14 KiB
ReStructuredText
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===================
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Classic BPF vs eBPF
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===================
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eBPF is designed to be JITed with one to one mapping, which can also open up
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the possibility for GCC/LLVM compilers to generate optimized eBPF code through
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an eBPF backend that performs almost as fast as natively compiled code.
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Some core changes of the eBPF format from classic BPF:
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- Number of registers increase from 2 to 10:
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The old format had two registers A and X, and a hidden frame pointer. The
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new layout extends this to be 10 internal registers and a read-only frame
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pointer. Since 64-bit CPUs are passing arguments to functions via registers
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the number of args from eBPF program to in-kernel function is restricted
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to 5 and one register is used to accept return value from an in-kernel
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function. Natively, x86_64 passes first 6 arguments in registers, aarch64/
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sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved
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registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers.
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Thus, all eBPF registers map one to one to HW registers on x86_64, aarch64,
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etc, and eBPF calling convention maps directly to ABIs used by the kernel on
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64-bit architectures.
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On 32-bit architectures JIT may map programs that use only 32-bit arithmetic
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and may let more complex programs to be interpreted.
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R0 - R5 are scratch registers and eBPF program needs spill/fill them if
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necessary across calls. Note that there is only one eBPF program (== one
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eBPF main routine) and it cannot call other eBPF functions, it can only
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call predefined in-kernel functions, though.
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- Register width increases from 32-bit to 64-bit:
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Still, the semantics of the original 32-bit ALU operations are preserved
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via 32-bit subregisters. All eBPF registers are 64-bit with 32-bit lower
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subregisters that zero-extend into 64-bit if they are being written to.
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That behavior maps directly to x86_64 and arm64 subregister definition, but
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makes other JITs more difficult.
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32-bit architectures run 64-bit eBPF programs via interpreter.
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Their JITs may convert BPF programs that only use 32-bit subregisters into
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native instruction set and let the rest being interpreted.
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Operation is 64-bit, because on 64-bit architectures, pointers are also
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64-bit wide, and we want to pass 64-bit values in/out of kernel functions,
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so 32-bit eBPF registers would otherwise require to define register-pair
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ABI, thus, there won't be able to use a direct eBPF register to HW register
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mapping and JIT would need to do combine/split/move operations for every
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register in and out of the function, which is complex, bug prone and slow.
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Another reason is the use of atomic 64-bit counters.
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- Conditional jt/jf targets replaced with jt/fall-through:
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While the original design has constructs such as ``if (cond) jump_true;
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else jump_false;``, they are being replaced into alternative constructs like
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``if (cond) jump_true; /* else fall-through */``.
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- Introduces bpf_call insn and register passing convention for zero overhead
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calls from/to other kernel functions:
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Before an in-kernel function call, the eBPF program needs to
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place function arguments into R1 to R5 registers to satisfy calling
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convention, then the interpreter will take them from registers and pass
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to in-kernel function. If R1 - R5 registers are mapped to CPU registers
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that are used for argument passing on given architecture, the JIT compiler
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doesn't need to emit extra moves. Function arguments will be in the correct
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registers and BPF_CALL instruction will be JITed as single 'call' HW
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instruction. This calling convention was picked to cover common call
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situations without performance penalty.
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After an in-kernel function call, R1 - R5 are reset to unreadable and R0 has
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a return value of the function. Since R6 - R9 are callee saved, their state
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is preserved across the call.
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For example, consider three C functions::
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u64 f1() { return (*_f2)(1); }
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u64 f2(u64 a) { return f3(a + 1, a); }
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u64 f3(u64 a, u64 b) { return a - b; }
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GCC can compile f1, f3 into x86_64::
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f1:
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movl $1, %edi
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movq _f2(%rip), %rax
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jmp *%rax
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f3:
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movq %rdi, %rax
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subq %rsi, %rax
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ret
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Function f2 in eBPF may look like::
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f2:
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bpf_mov R2, R1
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bpf_add R1, 1
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bpf_call f3
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bpf_exit
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If f2 is JITed and the pointer stored to ``_f2``. The calls f1 -> f2 -> f3 and
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returns will be seamless. Without JIT, __bpf_prog_run() interpreter needs to
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be used to call into f2.
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For practical reasons all eBPF programs have only one argument 'ctx' which is
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already placed into R1 (e.g. on __bpf_prog_run() startup) and the programs
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can call kernel functions with up to 5 arguments. Calls with 6 or more arguments
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are currently not supported, but these restrictions can be lifted if necessary
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in the future.
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On 64-bit architectures all register map to HW registers one to one. For
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example, x86_64 JIT compiler can map them as ...
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::
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R0 - rax
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R1 - rdi
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R2 - rsi
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R3 - rdx
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R4 - rcx
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R5 - r8
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R6 - rbx
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R7 - r13
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R8 - r14
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R9 - r15
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R10 - rbp
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... since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing
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and rbx, r12 - r15 are callee saved.
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Then the following eBPF pseudo-program::
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bpf_mov R6, R1 /* save ctx */
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bpf_mov R2, 2
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bpf_mov R3, 3
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bpf_mov R4, 4
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bpf_mov R5, 5
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bpf_call foo
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bpf_mov R7, R0 /* save foo() return value */
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bpf_mov R1, R6 /* restore ctx for next call */
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bpf_mov R2, 6
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bpf_mov R3, 7
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bpf_mov R4, 8
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bpf_mov R5, 9
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bpf_call bar
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bpf_add R0, R7
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bpf_exit
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After JIT to x86_64 may look like::
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push %rbp
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mov %rsp,%rbp
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sub $0x228,%rsp
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mov %rbx,-0x228(%rbp)
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mov %r13,-0x220(%rbp)
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mov %rdi,%rbx
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mov $0x2,%esi
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mov $0x3,%edx
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mov $0x4,%ecx
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mov $0x5,%r8d
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callq foo
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mov %rax,%r13
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mov %rbx,%rdi
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mov $0x6,%esi
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mov $0x7,%edx
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mov $0x8,%ecx
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mov $0x9,%r8d
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callq bar
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add %r13,%rax
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mov -0x228(%rbp),%rbx
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mov -0x220(%rbp),%r13
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leaveq
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retq
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Which is in this example equivalent in C to::
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u64 bpf_filter(u64 ctx)
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{
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return foo(ctx, 2, 3, 4, 5) + bar(ctx, 6, 7, 8, 9);
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}
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In-kernel functions foo() and bar() with prototype: u64 (*)(u64 arg1, u64
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arg2, u64 arg3, u64 arg4, u64 arg5); will receive arguments in proper
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registers and place their return value into ``%rax`` which is R0 in eBPF.
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Prologue and epilogue are emitted by JIT and are implicit in the
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interpreter. R0-R5 are scratch registers, so eBPF program needs to preserve
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them across the calls as defined by calling convention.
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For example the following program is invalid::
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bpf_mov R1, 1
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bpf_call foo
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bpf_mov R0, R1
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bpf_exit
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After the call the registers R1-R5 contain junk values and cannot be read.
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An in-kernel verifier.rst is used to validate eBPF programs.
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Also in the new design, eBPF is limited to 4096 insns, which means that any
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program will terminate quickly and will only call a fixed number of kernel
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functions. Original BPF and eBPF are two operand instructions,
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which helps to do one-to-one mapping between eBPF insn and x86 insn during JIT.
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The input context pointer for invoking the interpreter function is generic,
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its content is defined by a specific use case. For seccomp register R1 points
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to seccomp_data, for converted BPF filters R1 points to a skb.
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A program, that is translated internally consists of the following elements::
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op:16, jt:8, jf:8, k:32 ==> op:8, dst_reg:4, src_reg:4, off:16, imm:32
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So far 87 eBPF instructions were implemented. 8-bit 'op' opcode field
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has room for new instructions. Some of them may use 16/24/32 byte encoding. New
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instructions must be multiple of 8 bytes to preserve backward compatibility.
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eBPF is a general purpose RISC instruction set. Not every register and
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every instruction are used during translation from original BPF to eBPF.
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For example, socket filters are not using ``exclusive add`` instruction, but
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tracing filters may do to maintain counters of events, for example. Register R9
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is not used by socket filters either, but more complex filters may be running
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out of registers and would have to resort to spill/fill to stack.
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eBPF can be used as a generic assembler for last step performance
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optimizations, socket filters and seccomp are using it as assembler. Tracing
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filters may use it as assembler to generate code from kernel. In kernel usage
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may not be bounded by security considerations, since generated eBPF code
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may be optimizing internal code path and not being exposed to the user space.
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Safety of eBPF can come from the verifier.rst. In such use cases as
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described, it may be used as safe instruction set.
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Just like the original BPF, eBPF runs within a controlled environment,
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is deterministic and the kernel can easily prove that. The safety of the program
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can be determined in two steps: first step does depth-first-search to disallow
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loops and other CFG validation; second step starts from the first insn and
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descends all possible paths. It simulates execution of every insn and observes
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the state change of registers and stack.
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opcode encoding
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===============
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eBPF is reusing most of the opcode encoding from classic to simplify conversion
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of classic BPF to eBPF.
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For arithmetic and jump instructions the 8-bit 'code' field is divided into three
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parts::
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+----------------+--------+--------------------+
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| 4 bits | 1 bit | 3 bits |
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| operation code | source | instruction class |
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+----------------+--------+--------------------+
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(MSB) (LSB)
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Three LSB bits store instruction class which is one of:
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=================== ===============
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Classic BPF classes eBPF classes
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=================== ===============
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BPF_LD 0x00 BPF_LD 0x00
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BPF_LDX 0x01 BPF_LDX 0x01
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BPF_ST 0x02 BPF_ST 0x02
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BPF_STX 0x03 BPF_STX 0x03
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BPF_ALU 0x04 BPF_ALU 0x04
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BPF_JMP 0x05 BPF_JMP 0x05
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BPF_RET 0x06 BPF_JMP32 0x06
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BPF_MISC 0x07 BPF_ALU64 0x07
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=================== ===============
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The 4th bit encodes the source operand ...
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::
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BPF_K 0x00
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BPF_X 0x08
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* in classic BPF, this means::
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BPF_SRC(code) == BPF_X - use register X as source operand
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BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
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* in eBPF, this means::
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BPF_SRC(code) == BPF_X - use 'src_reg' register as source operand
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BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
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... and four MSB bits store operation code.
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If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of::
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BPF_ADD 0x00
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BPF_SUB 0x10
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BPF_MUL 0x20
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BPF_DIV 0x30
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BPF_OR 0x40
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BPF_AND 0x50
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BPF_LSH 0x60
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BPF_RSH 0x70
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BPF_NEG 0x80
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BPF_MOD 0x90
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BPF_XOR 0xa0
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BPF_MOV 0xb0 /* eBPF only: mov reg to reg */
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BPF_ARSH 0xc0 /* eBPF only: sign extending shift right */
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BPF_END 0xd0 /* eBPF only: endianness conversion */
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If BPF_CLASS(code) == BPF_JMP or BPF_JMP32 [ in eBPF ], BPF_OP(code) is one of::
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BPF_JA 0x00 /* BPF_JMP only */
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BPF_JEQ 0x10
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BPF_JGT 0x20
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BPF_JGE 0x30
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BPF_JSET 0x40
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BPF_JNE 0x50 /* eBPF only: jump != */
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BPF_JSGT 0x60 /* eBPF only: signed '>' */
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BPF_JSGE 0x70 /* eBPF only: signed '>=' */
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BPF_CALL 0x80 /* eBPF BPF_JMP only: function call */
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BPF_EXIT 0x90 /* eBPF BPF_JMP only: function return */
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BPF_JLT 0xa0 /* eBPF only: unsigned '<' */
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BPF_JLE 0xb0 /* eBPF only: unsigned '<=' */
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BPF_JSLT 0xc0 /* eBPF only: signed '<' */
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BPF_JSLE 0xd0 /* eBPF only: signed '<=' */
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So BPF_ADD | BPF_X | BPF_ALU means 32-bit addition in both classic BPF
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and eBPF. There are only two registers in classic BPF, so it means A += X.
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In eBPF it means dst_reg = (u32) dst_reg + (u32) src_reg; similarly,
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BPF_XOR | BPF_K | BPF_ALU means A ^= imm32 in classic BPF and analogous
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src_reg = (u32) src_reg ^ (u32) imm32 in eBPF.
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Classic BPF is using BPF_MISC class to represent A = X and X = A moves.
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eBPF is using BPF_MOV | BPF_X | BPF_ALU code instead. Since there are no
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BPF_MISC operations in eBPF, the class 7 is used as BPF_ALU64 to mean
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exactly the same operations as BPF_ALU, but with 64-bit wide operands
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instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition, i.e.:
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dst_reg = dst_reg + src_reg
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Classic BPF wastes the whole BPF_RET class to represent a single ``ret``
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operation. Classic BPF_RET | BPF_K means copy imm32 into return register
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and perform function exit. eBPF is modeled to match CPU, so BPF_JMP | BPF_EXIT
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in eBPF means function exit only. The eBPF program needs to store return
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value into register R0 before doing a BPF_EXIT. Class 6 in eBPF is used as
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BPF_JMP32 to mean exactly the same operations as BPF_JMP, but with 32-bit wide
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operands for the comparisons instead.
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For load and store instructions the 8-bit 'code' field is divided as::
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+--------+--------+-------------------+
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| 3 bits | 2 bits | 3 bits |
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| mode | size | instruction class |
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+--------+--------+-------------------+
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(MSB) (LSB)
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Size modifier is one of ...
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::
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BPF_W 0x00 /* word */
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BPF_H 0x08 /* half word */
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BPF_B 0x10 /* byte */
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BPF_DW 0x18 /* eBPF only, double word */
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... which encodes size of load/store operation::
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B - 1 byte
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H - 2 byte
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W - 4 byte
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DW - 8 byte (eBPF only)
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Mode modifier is one of::
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BPF_IMM 0x00 /* used for 32-bit mov in classic BPF and 64-bit in eBPF */
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BPF_ABS 0x20
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BPF_IND 0x40
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BPF_MEM 0x60
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BPF_LEN 0x80 /* classic BPF only, reserved in eBPF */
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BPF_MSH 0xa0 /* classic BPF only, reserved in eBPF */
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BPF_ATOMIC 0xc0 /* eBPF only, atomic operations */
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