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Diffstat (limited to 'Documentation/bpf/bpf_design_QA.rst')
| -rw-r--r-- | Documentation/bpf/bpf_design_QA.rst | 201 |
1 files changed, 166 insertions, 35 deletions
diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst index 7cc9e368c1e9..eb19c945f4d5 100644 --- a/Documentation/bpf/bpf_design_QA.rst +++ b/Documentation/bpf/bpf_design_QA.rst @@ -36,27 +36,27 @@ 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? +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? +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) -Q: can BPF programs access instruction pointer or return address? +Q: Can BPF programs access instruction pointer or return address? ----------------------------------------------------------------- A: NO. -Q: can BPF programs access stack pointer ? +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 +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? @@ -66,8 +66,8 @@ 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 +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. @@ -75,9 +75,9 @@ 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 +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 +jump tables, read-only sections, and all other normal constructs that C code can produce. Q: Can loops be supported in a safe way? @@ -85,8 +85,33 @@ 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. +support bounded loops. + +Q: What are the verifier limits? +-------------------------------- +A: The only limit known to the user space is BPF_MAXINSNS (4096). +It's the maximum number of instructions that the unprivileged bpf +program can have. The verifier has various internal limits. +Like the maximum number of instructions that can be explored during +program analysis. Currently, that limit is set to 1 million. +Which essentially means that the largest program can consist +of 1 million NOP instructions. There is a limit to the maximum number +of subsequent branches, a limit to the number of nested bpf-to-bpf +calls, a limit to the number of the verifier states per instruction, +a limit to the number of maps used by the program. +All these limits can be hit with a sufficiently complex program. +There are also non-numerical limits that can cause the program +to be rejected. The verifier used to recognize only pointer + constant +expressions. Now it can recognize pointer + bounded_register. +bpf_lookup_map_elem(key) had a requirement that 'key' must be +a pointer to the stack. Now, 'key' can be a pointer to map value. +The verifier is steadily getting 'smarter'. The limits are +being removed. The only way to know that the program is going to +be accepted by the verifier is to try to load it. +The bpf development process guarantees that the future kernel +versions will accept all bpf programs that were accepted by +the earlier versions. + Instruction level questions --------------------------- @@ -109,17 +134,12 @@ 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. -Q: why BPF_DIV instruction doesn't map to x64 div? +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. -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 - Q: Why BPF has implicit prologue and epilogue? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A: Because architectures like sparc have register windows and in general @@ -147,11 +167,31 @@ 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. +A: NO. + +But some optimizations on zero-ing the upper 32 bits for BPF registers are +available, and can be leveraged to improve the performance of JITed BPF +programs for 32-bit architectures. + +Starting with version 7, LLVM is able to generate instructions that operate +on 32-bit subregisters, provided the option -mattr=+alu32 is passed for +compiling a program. Furthermore, the verifier can now mark the +instructions for which zero-ing the upper bits of the destination register +is required, and insert an explicit zero-extension (zext) instruction +(a mov32 variant). This means that for architectures without zext hardware +support, the JIT back-ends do not need to clear the upper bits for +subregisters written by alu32 instructions or narrow loads. Instead, the +back-ends simply need to support code generation for that mov32 variant, +and to overwrite bpf_jit_needs_zext() to make it return "true" (in order to +enable zext insertion in the verifier). + +Note that it is possible for a JIT back-end to have partial hardware +support for zext. In that case, if verifier zext insertion is enabled, +it could lead to the insertion of unnecessary zext instructions. Such +instructions could be removed by creating a simple peephole inside the JIT +back-end: if one instruction has hardware support for zext and if the next +instruction is an explicit zext, then the latter can be skipped when doing +the code generation. Q: Does BPF have a stable ABI? ------------------------------ @@ -163,6 +203,22 @@ data structures and compile with kernel internal headers. Both of these kernel internals are subject to change and can break with newer kernels such that the program needs to be adapted accordingly. +New BPF functionality is generally added through the use of kfuncs instead of +new helpers. Kfuncs are not considered part of the stable API, and have their own +lifecycle expectations as described in :ref:`BPF_kfunc_lifecycle_expectations`. + +Q: Are tracepoints part of the stable ABI? +------------------------------------------ +A: NO. Tracepoints are tied to internal implementation details hence they are +subject to change and can break with newer kernels. BPF programs need to change +accordingly when this happens. + +Q: Are places where kprobes can attach part of the stable ABI? +-------------------------------------------------------------- +A: NO. The places to which kprobes can attach are internal implementation +details, which means that they are subject to change and can break with +newer kernels. BPF programs need to change accordingly when this happens. + Q: How much stack space a BPF program uses? ------------------------------------------- A: Currently all program types are limited to 512 bytes of stack @@ -179,8 +235,8 @@ 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. +A: NO. BPF programs can only call specific functions exposed as BPF helpers or +kfuncs. The set of available functions is defined for every program type. Q: Can BPF overwrite arbitrary kernel memory? --------------------------------------------- @@ -201,20 +257,95 @@ 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? - -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. - 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? +A: Yes, through kfuncs and kptrs + +The core BPF functionality such as program types, maps and helpers cannot be +added to by modules. However, modules can expose functionality to BPF programs +by exporting kfuncs (which may return pointers to module-internal data +structures as kptrs). + +Q: Directly calling kernel function is an ABI? +---------------------------------------------- +Q: Some kernel functions (e.g. tcp_slow_start) can be called +by BPF programs. Do these kernel functions become an ABI? + A: NO. + +The kernel function protos will change and the bpf programs will be +rejected by the verifier. Also, for example, some of the bpf-callable +kernel functions have already been used by other kernel tcp +cc (congestion-control) implementations. If any of these kernel +functions has changed, both the in-tree and out-of-tree kernel tcp cc +implementations have to be changed. The same goes for the bpf +programs and they have to be adjusted accordingly. See +:ref:`BPF_kfunc_lifecycle_expectations` for details. + +Q: Attaching to arbitrary kernel functions is an ABI? +----------------------------------------------------- +Q: BPF programs can be attached to many kernel functions. Do these +kernel functions become part of the ABI? + +A: NO. + +The kernel function prototypes will change, and BPF programs attaching to +them will need to change. The BPF compile-once-run-everywhere (CO-RE) +should be used in order to make it easier to adapt your BPF programs to +different versions of the kernel. + +Q: Marking a function with BTF_ID makes that function an ABI? +------------------------------------------------------------- +A: NO. + +The BTF_ID macro does not cause a function to become part of the ABI +any more than does the EXPORT_SYMBOL_GPL macro. + +Q: What is the compatibility story for special BPF types in map values? +----------------------------------------------------------------------- +Q: Users are allowed to embed bpf_spin_lock, bpf_timer fields in their BPF map +values (when using BTF support for BPF maps). This allows to use helpers for +such objects on these fields inside map values. Users are also allowed to embed +pointers to some kernel types (with __kptr_untrusted and __kptr BTF tags). Will the +kernel preserve backwards compatibility for these features? + +A: It depends. For bpf_spin_lock, bpf_timer: YES, for kptr and everything else: +NO, but see below. + +For struct types that have been added already, like bpf_spin_lock and bpf_timer, +the kernel will preserve backwards compatibility, as they are part of UAPI. + +For kptrs, they are also part of UAPI, but only with respect to the kptr +mechanism. The types that you can use with a __kptr_untrusted and __kptr tagged +pointer in your struct are NOT part of the UAPI contract. The supported types can +and will change across kernel releases. However, operations like accessing kptr +fields and bpf_kptr_xchg() helper will continue to be supported across kernel +releases for the supported types. + +For any other supported struct type, unless explicitly stated in this document +and added to bpf.h UAPI header, such types can and will arbitrarily change their +size, type, and alignment, or any other user visible API or ABI detail across +kernel releases. The users must adapt their BPF programs to the new changes and +update them to make sure their programs continue to work correctly. + +NOTE: BPF subsystem specially reserves the 'bpf\_' prefix for type names, in +order to introduce more special fields in the future. Hence, user programs must +avoid defining types with 'bpf\_' prefix to not be broken in future releases. +In other words, no backwards compatibility is guaranteed if one using a type +in BTF with 'bpf\_' prefix. + +Q: What is the compatibility story for special BPF types in allocated objects? +------------------------------------------------------------------------------ +Q: Same as above, but for allocated objects (i.e. objects allocated using +bpf_obj_new for user defined types). Will the kernel preserve backwards +compatibility for these features? + +A: NO. + +Unlike map value types, the API to work with allocated objects and any support +for special fields inside them is exposed through kfuncs, and thus has the same +lifecycle expectations as the kfuncs themselves. See +:ref:`BPF_kfunc_lifecycle_expectations` for details. |