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diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst index f3e458a0bb2f..6780a6d81745 100644 --- a/Documentation/bpf/bpf_design_QA.rst +++ b/Documentation/bpf/bpf_design_QA.rst @@ -1,156 +1,221 @@ +============== +BPF Design Q&A +============== + BPF extensibility and applicability to networking, tracing, security in the linux kernel and several user space implementations of BPF virtual machine led to a number of misunderstanding on what BPF actually is. This short QA is an attempt to address that and outline a direction of where BPF is heading long term. +.. contents:: + :local: + :depth: 3 + +Questions and Answers +===================== + Q: Is BPF a generic instruction set similar to x64 and arm64? +------------------------------------------------------------- A: NO. Q: Is BPF a generic virtual machine ? +------------------------------------- A: NO. -BPF is generic instruction set _with_ C calling convention. +BPF is generic instruction set *with* C calling convention. +----------------------------------------------------------- Q: Why C calling convention was chosen? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + A: Because BPF programs are designed to run in the linux kernel - which is written in C, hence BPF defines instruction set compatible - with two most used architectures x64 and arm64 (and takes into - consideration important quirks of other architectures) and - defines calling convention that is compatible with C calling - convention of the linux kernel on those architectures. +which is written in C, hence BPF defines instruction set compatible +with two most used architectures x64 and arm64 (and takes into +consideration important quirks of other architectures) and +defines calling convention that is compatible with C calling +convention of the linux kernel on those architectures. Q: can multiple return values be supported in the future? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A: NO. BPF allows only register R0 to be used as return value. Q: can more than 5 function arguments be supported in the future? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A: NO. BPF calling convention only allows registers R1-R5 to be used - as arguments. BPF is not a standalone instruction set. - (unlike x64 ISA that allows msft, cdecl and other conventions) +as arguments. BPF is not a standalone instruction set. +(unlike x64 ISA that allows msft, cdecl and other conventions) Q: can BPF programs access instruction pointer or return address? +----------------------------------------------------------------- A: NO. Q: can BPF programs access stack pointer ? -A: NO. Only frame pointer (register R10) is accessible. - From compiler point of view it's necessary to have stack pointer. - For example LLVM defines register R11 as stack pointer in its - BPF backend, but it makes sure that generated code never uses it. +------------------------------------------ +A: NO. + +Only frame pointer (register R10) is accessible. +From compiler point of view it's necessary to have stack pointer. +For example LLVM defines register R11 as stack pointer in its +BPF backend, but it makes sure that generated code never uses it. Q: Does C-calling convention diminishes possible use cases? -A: YES. BPF design forces addition of major functionality in the form - of kernel helper functions and kernel objects like BPF maps with - seamless interoperability between them. It lets kernel call into - BPF programs and programs call kernel helpers with zero overhead. - As all of them were native C code. That is particularly the case - for JITed BPF programs that are indistinguishable from - native kernel C code. +----------------------------------------------------------- +A: YES. + +BPF design forces addition of major functionality in the form +of kernel helper functions and kernel objects like BPF maps with +seamless interoperability between them. It lets kernel call into +BPF programs and programs call kernel helpers with zero overhead. +As all of them were native C code. That is particularly the case +for JITed BPF programs that are indistinguishable from +native kernel C code. Q: Does it mean that 'innovative' extensions to BPF code are disallowed? -A: Soft yes. At least for now until BPF core has support for - bpf-to-bpf calls, indirect calls, loops, global variables, - jump tables, read only sections and all other normal constructs - that C code can produce. +------------------------------------------------------------------------ +A: Soft yes. + +At least for now until BPF core has support for +bpf-to-bpf calls, indirect calls, loops, global variables, +jump tables, read only sections and all other normal constructs +that C code can produce. Q: Can loops be supported in a safe way? -A: It's not clear yet. BPF developers are trying to find a way to - support bounded loops where the verifier can guarantee that - the program terminates in less than 4096 instructions. +---------------------------------------- +A: It's not clear yet. + +BPF developers are trying to find a way to +support bounded loops where the verifier can guarantee that +the program terminates in less than 4096 instructions. + +Instruction level questions +--------------------------- + +Q: LD_ABS and LD_IND instructions vs C code +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Q: How come LD_ABS and LD_IND instruction are present in BPF whereas - C code cannot express them and has to use builtin intrinsics? +C code cannot express them and has to use builtin intrinsics? + A: This is artifact of compatibility with classic BPF. Modern - networking code in BPF performs better without them. - See 'direct packet access'. +networking code in BPF performs better without them. +See 'direct packet access'. +Q: BPF instructions mapping not one-to-one to native CPU +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Q: It seems not all BPF instructions are one-to-one to native CPU. - For example why BPF_JNE and other compare and jumps are not cpu-like? +For example why BPF_JNE and other compare and jumps are not cpu-like? + A: This was necessary to avoid introducing flags into ISA which are - impossible to make generic and efficient across CPU architectures. +impossible to make generic and efficient across CPU architectures. Q: why BPF_DIV instruction doesn't map to x64 div? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A: Because if we picked one-to-one relationship to x64 it would have made - it more complicated to support on arm64 and other archs. Also it - needs div-by-zero runtime check. +it more complicated to support on arm64 and other archs. Also it +needs div-by-zero runtime check. Q: why there is no BPF_SDIV for signed divide operation? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A: Because it would be rarely used. llvm errors in such case and - prints a suggestion to use unsigned divide instead +prints a suggestion to use unsigned divide instead Q: Why BPF has implicit prologue and epilogue? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A: Because architectures like sparc have register windows and in general - there are enough subtle differences between architectures, so naive - store return address into stack won't work. Another reason is BPF has - to be safe from division by zero (and legacy exception path - of LD_ABS insn). Those instructions need to invoke epilogue and - return implicitly. +there are enough subtle differences between architectures, so naive +store return address into stack won't work. Another reason is BPF has +to be safe from division by zero (and legacy exception path +of LD_ABS insn). Those instructions need to invoke epilogue and +return implicitly. Q: Why BPF_JLT and BPF_JLE instructions were not introduced in the beginning? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A: Because classic BPF didn't have them and BPF authors felt that compiler - workaround would be acceptable. Turned out that programs lose performance - due to lack of these compare instructions and they were added. - These two instructions is a perfect example what kind of new BPF - instructions are acceptable and can be added in the future. - These two already had equivalent instructions in native CPUs. - New instructions that don't have one-to-one mapping to HW instructions - will not be accepted. - +workaround would be acceptable. Turned out that programs lose performance +due to lack of these compare instructions and they were added. +These two instructions is a perfect example what kind of new BPF +instructions are acceptable and can be added in the future. +These two already had equivalent instructions in native CPUs. +New instructions that don't have one-to-one mapping to HW instructions +will not be accepted. + +Q: BPF 32-bit subregister requirements +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Q: BPF 32-bit subregisters have a requirement to zero upper 32-bits of BPF - registers which makes BPF inefficient virtual machine for 32-bit - CPU architectures and 32-bit HW accelerators. Can true 32-bit registers - be added to BPF in the future? +registers which makes BPF inefficient virtual machine for 32-bit +CPU architectures and 32-bit HW accelerators. Can true 32-bit registers +be added to BPF in the future? + A: NO. The first thing to improve performance on 32-bit archs is to teach - LLVM to generate code that uses 32-bit subregisters. Then second step - is to teach verifier to mark operations where zero-ing upper bits - is unnecessary. Then JITs can take advantage of those markings and - drastically reduce size of generated code and improve performance. +LLVM to generate code that uses 32-bit subregisters. Then second step +is to teach verifier to mark operations where zero-ing upper bits +is unnecessary. Then JITs can take advantage of those markings and +drastically reduce size of generated code and improve performance. Q: Does BPF have a stable ABI? +------------------------------ A: YES. BPF instructions, arguments to BPF programs, set of helper - functions and their arguments, recognized return codes are all part - of ABI. However when tracing programs are using bpf_probe_read() helper - to walk kernel internal datastructures and compile with kernel - internal headers these accesses can and will break with newer - kernels. The union bpf_attr -> kern_version is checked at load time - to prevent accidentally loading kprobe-based bpf programs written - for a different kernel. Networking programs don't do kern_version check. +functions and their arguments, recognized return codes are all part +of ABI. However when tracing programs are using bpf_probe_read() helper +to walk kernel internal datastructures and compile with kernel +internal headers these accesses can and will break with newer +kernels. The union bpf_attr -> kern_version is checked at load time +to prevent accidentally loading kprobe-based bpf programs written +for a different kernel. Networking programs don't do kern_version check. Q: How much stack space a BPF program uses? +------------------------------------------- A: Currently all program types are limited to 512 bytes of stack - space, but the verifier computes the actual amount of stack used - and both interpreter and most JITed code consume necessary amount. +space, but the verifier computes the actual amount of stack used +and both interpreter and most JITed code consume necessary amount. Q: Can BPF be offloaded to HW? +------------------------------ A: YES. BPF HW offload is supported by NFP driver. Q: Does classic BPF interpreter still exist? +-------------------------------------------- A: NO. Classic BPF programs are converted into extend BPF instructions. Q: Can BPF call arbitrary kernel functions? +------------------------------------------- A: NO. BPF programs can only call a set of helper functions which - is defined for every program type. +is defined for every program type. Q: Can BPF overwrite arbitrary kernel memory? -A: NO. Tracing bpf programs can _read_ arbitrary memory with bpf_probe_read() - and bpf_probe_read_str() helpers. Networking programs cannot read - arbitrary memory, since they don't have access to these helpers. - Programs can never read or write arbitrary memory directly. +--------------------------------------------- +A: NO. + +Tracing bpf programs can *read* arbitrary memory with bpf_probe_read() +and bpf_probe_read_str() helpers. Networking programs cannot read +arbitrary memory, since they don't have access to these helpers. +Programs can never read or write arbitrary memory directly. Q: Can BPF overwrite arbitrary user memory? -A: Sort-of. Tracing BPF programs can overwrite the user memory - of the current task with bpf_probe_write_user(). Every time such - program is loaded the kernel will print warning message, so - this helper is only useful for experiments and prototypes. - Tracing BPF programs are root only. +------------------------------------------- +A: Sort-of. + +Tracing BPF programs can overwrite the user memory +of the current task with bpf_probe_write_user(). Every time such +program is loaded the kernel will print warning message, so +this helper is only useful for experiments and prototypes. +Tracing BPF programs are root only. +Q: bpf_trace_printk() helper warning +------------------------------------ Q: When bpf_trace_printk() helper is used the kernel prints nasty - warning message. Why is that? +warning message. Why is that? + A: This is done to nudge program authors into better interfaces when - programs need to pass data to user space. Like bpf_perf_event_output() - can be used to efficiently stream data via perf ring buffer. - BPF maps can be used for asynchronous data sharing between kernel - and user space. bpf_trace_printk() should only be used for debugging. +programs need to pass data to user space. Like bpf_perf_event_output() +can be used to efficiently stream data via perf ring buffer. +BPF maps can be used for asynchronous data sharing between kernel +and user space. bpf_trace_printk() should only be used for debugging. +Q: New functionality via kernel modules? +---------------------------------------- Q: Can BPF functionality such as new program or map types, new - helpers, etc be added out of kernel module code? +helpers, etc be added out of kernel module code? + A: NO. |