157 lines
7.5 KiB
Plaintext
157 lines
7.5 KiB
Plaintext
BPF extensibility and applicability to networking, tracing, security
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in the linux kernel and several user space implementations of BPF
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virtual machine led to a number of misunderstanding on what BPF actually is.
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This short QA is an attempt to address that and outline a direction
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of where BPF is heading long term.
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Q: Is BPF a generic instruction set similar to x64 and arm64?
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A: NO.
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Q: Is BPF a generic virtual machine ?
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A: NO.
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BPF is generic instruction set _with_ C calling convention.
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Q: Why C calling convention was chosen?
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A: Because BPF programs are designed to run in the linux kernel
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which is written in C, hence BPF defines instruction set compatible
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with two most used architectures x64 and arm64 (and takes into
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consideration important quirks of other architectures) and
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defines calling convention that is compatible with C calling
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convention of the linux kernel on those architectures.
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Q: can multiple return values be supported in the future?
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A: NO. BPF allows only register R0 to be used as return value.
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Q: can more than 5 function arguments be supported in the future?
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A: NO. BPF calling convention only allows registers R1-R5 to be used
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as arguments. BPF is not a standalone instruction set.
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(unlike x64 ISA that allows msft, cdecl and other conventions)
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Q: can BPF programs access instruction pointer or return address?
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A: NO.
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Q: can BPF programs access stack pointer ?
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A: NO. Only frame pointer (register R10) is accessible.
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From compiler point of view it's necessary to have stack pointer.
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For example LLVM defines register R11 as stack pointer in its
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BPF backend, but it makes sure that generated code never uses it.
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Q: Does C-calling convention diminishes possible use cases?
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A: YES. BPF design forces addition of major functionality in the form
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of kernel helper functions and kernel objects like BPF maps with
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seamless interoperability between them. It lets kernel call into
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BPF programs and programs call kernel helpers with zero overhead.
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As all of them were native C code. That is particularly the case
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for JITed BPF programs that are indistinguishable from
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native kernel C code.
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Q: Does it mean that 'innovative' extensions to BPF code are disallowed?
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A: Soft yes. At least for now until BPF core has support for
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bpf-to-bpf calls, indirect calls, loops, global variables,
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jump tables, read only sections and all other normal constructs
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that C code can produce.
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Q: Can loops be supported in a safe way?
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A: It's not clear yet. BPF developers are trying to find a way to
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support bounded loops where the verifier can guarantee that
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the program terminates in less than 4096 instructions.
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Q: How come LD_ABS and LD_IND instruction are present in BPF whereas
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C code cannot express them and has to use builtin intrinsics?
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A: This is artifact of compatibility with classic BPF. Modern
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networking code in BPF performs better without them.
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See 'direct packet access'.
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Q: It seems not all BPF instructions are one-to-one to native CPU.
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For example why BPF_JNE and other compare and jumps are not cpu-like?
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A: This was necessary to avoid introducing flags into ISA which are
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impossible to make generic and efficient across CPU architectures.
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Q: why BPF_DIV instruction doesn't map to x64 div?
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A: Because if we picked one-to-one relationship to x64 it would have made
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it more complicated to support on arm64 and other archs. Also it
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needs div-by-zero runtime check.
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Q: why there is no BPF_SDIV for signed divide operation?
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A: Because it would be rarely used. llvm errors in such case and
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prints a suggestion to use unsigned divide instead
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Q: Why BPF has implicit prologue and epilogue?
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A: Because architectures like sparc have register windows and in general
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there are enough subtle differences between architectures, so naive
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store return address into stack won't work. Another reason is BPF has
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to be safe from division by zero (and legacy exception path
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of LD_ABS insn). Those instructions need to invoke epilogue and
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return implicitly.
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Q: Why BPF_JLT and BPF_JLE instructions were not introduced in the beginning?
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A: Because classic BPF didn't have them and BPF authors felt that compiler
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workaround would be acceptable. Turned out that programs lose performance
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due to lack of these compare instructions and they were added.
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These two instructions is a perfect example what kind of new BPF
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instructions are acceptable and can be added in the future.
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These two already had equivalent instructions in native CPUs.
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New instructions that don't have one-to-one mapping to HW instructions
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will not be accepted.
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Q: BPF 32-bit subregisters have a requirement to zero upper 32-bits of BPF
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registers which makes BPF inefficient virtual machine for 32-bit
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CPU architectures and 32-bit HW accelerators. Can true 32-bit registers
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be added to BPF in the future?
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A: NO. The first thing to improve performance on 32-bit archs is to teach
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LLVM to generate code that uses 32-bit subregisters. Then second step
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is to teach verifier to mark operations where zero-ing upper bits
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is unnecessary. Then JITs can take advantage of those markings and
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drastically reduce size of generated code and improve performance.
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Q: Does BPF have a stable ABI?
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A: YES. BPF instructions, arguments to BPF programs, set of helper
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functions and their arguments, recognized return codes are all part
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of ABI. However when tracing programs are using bpf_probe_read() helper
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to walk kernel internal datastructures and compile with kernel
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internal headers these accesses can and will break with newer
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kernels. The union bpf_attr -> kern_version is checked at load time
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to prevent accidentally loading kprobe-based bpf programs written
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for a different kernel. Networking programs don't do kern_version check.
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Q: How much stack space a BPF program uses?
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A: Currently all program types are limited to 512 bytes of stack
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space, but the verifier computes the actual amount of stack used
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and both interpreter and most JITed code consume necessary amount.
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Q: Can BPF be offloaded to HW?
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A: YES. BPF HW offload is supported by NFP driver.
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Q: Does classic BPF interpreter still exist?
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A: NO. Classic BPF programs are converted into extend BPF instructions.
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Q: Can BPF call arbitrary kernel functions?
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A: NO. BPF programs can only call a set of helper functions which
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is defined for every program type.
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Q: Can BPF overwrite arbitrary kernel memory?
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A: NO. Tracing bpf programs can _read_ arbitrary memory with bpf_probe_read()
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and bpf_probe_read_str() helpers. Networking programs cannot read
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arbitrary memory, since they don't have access to these helpers.
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Programs can never read or write arbitrary memory directly.
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Q: Can BPF overwrite arbitrary user memory?
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A: Sort-of. Tracing BPF programs can overwrite the user memory
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of the current task with bpf_probe_write_user(). Every time such
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program is loaded the kernel will print warning message, so
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this helper is only useful for experiments and prototypes.
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Tracing BPF programs are root only.
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Q: When bpf_trace_printk() helper is used the kernel prints nasty
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warning message. Why is that?
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A: This is done to nudge program authors into better interfaces when
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programs need to pass data to user space. Like bpf_perf_event_output()
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can be used to efficiently stream data via perf ring buffer.
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BPF maps can be used for asynchronous data sharing between kernel
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and user space. bpf_trace_printk() should only be used for debugging.
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Q: Can BPF functionality such as new program or map types, new
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helpers, etc be added out of kernel module code?
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A: NO.
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