Per David's description of the IETF standardization process, this document will form the basis for an informational eBPF ABI. The work in this commit is a slightly more complete skeleton for the work that we will do. Everything in this document (from formatting to topics to details) is open for change and feedback. --- Documentation/bpf/standardization/abi.rst | 259 +++++++++++++++++++++- 1 file changed, 250 insertions(+), 9 deletions(-) Changelog: v1 -> v2: - Address David's comments - Reflow to a max of 80 character lines (Christoph's feedback) v2 -> v3: - Update Java Language Specification reference to version 21 - Make author's notes into comments. - Fix minor typographical errors in References section. diff --git a/Documentation/bpf/standardization/abi.rst b/Documentation/bpf/standardization/abi.rst index 0c2e10eeb89a..8bb7383688bc 100644 --- a/Documentation/bpf/standardization/abi.rst +++ b/Documentation/bpf/standardization/abi.rst @@ -1,18 +1,159 @@ -.. contents:: -.. sectnum:: - =================================================== BPF ABI Recommended Conventions and Guidelines v1.0 =================================================== -This is version 1.0 of an informational document containing recommended -conventions and guidelines for producing portable BPF program binaries. +An application binary interface (ABI) defines the requirements that one or more +binary software objects must meet in order to guarantee that they can +interoperate and/or use the resources provided by operating systems/hardware +combinations. (For alternate definitions of ABI, see [SYSVABI]_, [POWERPCABI]_) + +The purpose of this document is to define an ABI which will define the extent +to which compiled BPF programs are compatible with each other and the BPF +machine/processor [#]_ on which they are executing. + +The ABI is specified in two parts: a generic part and a processor-specific part. +A pairing of generic ABI with the processor-specific ABI for a certain +instantiation of a BPF machine represents a complete binary interface for BPF +programs executing on that machine. + +This document is the generic ABI and specifies the parameters and behavior +common to all instantiations of BPF machines. In addition, it defines the +details that must be specified by each processor-specific ABI. + +These psABIs are the second part of the ABI. Each instantiation of a BPF +machine must describe the mechanism through which binary interface +compatibility is maintained with respect to the issues highlighted by this +document. However, the details that must be defined by a psABI are a minimum -- +a psABI may specify additional requirements for binary interface compatibility +on a platform. + +.. contents:: +.. sectnum:: + +How To Use This ABI +=================== + +Conformance +=========== +.. + Red Hat specifies different levels of conformance over time [RHELABI]_. We + could use information from that document here, if we want. + +Related Work +============ +BPF programs are not unique for the way that they operate on a virtualized +machine and processor. There are many programming languages that compile to an +ISA that is specific to a virtual machine. Like the specification presented +herein, those languages and virtual machines also have ABIs. + +For example, the Go programming language and the runtime included statically +with each program compiled from Go source code have a defined ABI [GOABI]_. +Java programs compiled to bytecode follow a well-defined ABI for +interoperability with other compiled Java programs and libraries [JAVAABI]_. +Programs compiled to bytecode for execution as user applications on the Android +operating system (OS) adhere to a bytecode specification that shares much in +common with an ABI [DALVIKABI]_. Finally, the Common Language Runtime (CLR) +designed to execute programs compiled to the Microsoft Intermediate Language +(MSIL) has a fully specified ABI [CLRABI]_. + +Vocabulary +========== + +#. Program: A BPF Program is an ordered set of BPF instructions, with exactly + one entry instruction where the program begins, and one or more exit + instructions where program execution can end. +#. Program Type: Every BPF program has an associated type. The program type + defines, among other things, a program's possible attach types. +#. Attach Type: An attach type defines the set of BPF hook points to which a BPF + program can attach. +#. BPF Hook Points: Places in a BPF-enabled component (e.g., the Linux Kernel, + the Windows kernel) where a BPF program may be attached. +#. ABI-conforming BPF Machine Instantiation: A physical or logical realization + of a computer system capable of executing BPF programs consistently with the + specifications outlined in this document. +#. ABI-conforming BPF program: A BPF program written to include only the system + routines, commands, and other resources included in this ABI; or a BPF + program compiled into an executable file that has the formats and + characteristics specified for such files in this ABI; or a BPF program whose + behavior complies with the rules given in the ABI [SYSVABI]_. +#. ABI-nonconforming program: A BPF program that is not ABI conforming. +#. Undefined Behavior: Behavior that may vary from instance to instance or may + change at some time in the future. Some undesirable programming practices + are marked in this ABI as yielding undefined behavior [SYSVABI]_. +#. Unspecified Property: A property of an entity defined in this document that + is not explicitly included, defined or referenced in this specification, and + may change at some time in the future. In general, it is not good practice + to make a program depend on an unspecified property [SYSVABI]_. + +Program Execution Environment +============================= + +A loaded BPF program is executed in a freestanding or hosted environment. [#]_. + +BPF Program Freestanding Execution Environment +---------------------------------------------- + +BPF Program Hosted Execution Environment +---------------------------------------- + +A hosted execution environment is one in which a BPF machine instantiation is +embedded within another computer system known as a BPF-enabled application +(e.g., a user application or an operating system kernel). A loaded BPF program +can be attached to a BPF hook point in such a BPF-enabled application +compatible with the attach type of its program type. When the BPF-enabled +application's execution reaches a BPF hook point to which a BPF program is +attached, that BPF program begins execution on the embedded BPF machine at the +program's first instruction. The contents of the embedded BPF machine's +registers and memory at the time it starts execution of the BPF program are +defined by the BPF program's type and attach point. + +Processor Architecture +====================== + +This section describes the processor architecture available +to programs. It also defines the reference language data types, giving the +foundation for system interface specifications [SYSVABI]_ + +Registers +--------- + +General Purpose Registers +^^^^^^^^^^^^^^^^^^^^^^^^^ +BPF has 11 64-bit wide registers, `r0` - `r10`. There exists a single +32-bit wide subregister for each one of the 11 64-bit wide registers. Those +registers do not have their own names -- they are accessible indirectly +through the 32-bit ALU instructions. + +The contents of the registers at the beginning of a BPF program's +execution depend on the program's type. + +Frame Pointer Register +^^^^^^^^^^^^^^^^^^^^^^ +The use of a frame pointer by programs is not required. If, however, a BPF +program does use a frame pointer, it must be stored in register `r10` and +must be read only. + +Data Types +---------- -Registers and calling convention -================================ +Numeric Types +^^^^^^^^^^^^^ -BPF has 10 general purpose registers and a read-only frame pointer register, -all of which are 64-bits wide. +The BPF machine supports 32- and 64-bit signed and unsigned integers. It does +not support floating-point data types. All signed integers are represented in +twos-complement format where the sign bit is stored in the most-significant bit. + +Pointers +^^^^^^^^ + +Function Calling Sequence +========================= +This section defines the standard function calling sequence in a way that +accommodates exceptions, stack management, register (non)volatility, and access +to capabilities of the hosting environment (where applicable). + +Functions in BPF may define between 0 and 5 parameters. Each of the arguments in +a function call are passed in registers. The BPF calling convention is defined as: @@ -23,3 +164,103 @@ The BPF calling convention is defined as: R0 - R5 are scratch registers and BPF programs needs to spill/fill them if necessary across calls. + +Every function invocation proceeds as if it has exclusive access to an +implementation-defined amount of stack space. R10 is a pointer to the byte of +memory with the highest address in that stack space. The contents +of a function invocation's stack space do not persist between invocations. + +.. + Discuss manufactured prologue and epilogue. Take language from the design FAQ. + +Execution Environment Interface +=============================== + +When a BPF program executes in a hosted environment, the hosted environment +may make available to BPF programs certain capabilities. This section +describes those capabilities and the mechanism for accessing them. + + +Program Execution +================= + +Program Return Values +--------------------- + +.. + libbpf currently defines the return value of a bpf program as a 32-bit unsigned + integer. ubpf currently defines the return value of a bpf program. + +Program Loading and Dynamic Linking +----------------------------------- +This section describes the object file information and system actions that +create running programs. Some information here applies to all systems; +information specific to one processor resides in sections marked accordingly +[SYSVABI]_. + +BPF programs saved in ELF files must be loaded from storage and properly +configured before they can be executed on a BPF machine. + +Program Loading (Processor-Specific) +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Dynamic Linking +^^^^^^^^^^^^^^^ + +Global Offset Table (Processor-Specific) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Procedure Linkage Table (Processor-Specific) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Exception Handling +================== + +BPF Program Types +================== +.. This information may end up as a subsection somewhere else. + +BPF Maps +========= +.. This information may end up as a subsection somewhere else. + +System Calls +============ + +**TODO** + +C Programming Language Support +============================== + +.. + This section could be included in order to define the contents of standardized + processor-specific header files that would make it easier for programmers to + write programs. + +Notes +===== +.. [#] The BPF machine does not need to be a physical instantiation of a processor. + In fact, many instantiations of BPF machines are virtual. +.. [#] See the [CSTD]_ for the inspiration for this distinction. + +References +========== + +.. [SYSVABI] System V Application Binary Interface - Edition 4.1. SCO Developer Specs. + The Santa Cruz Operation. 1997. + https://www.sco.com/developers/devspecs/gabi41.pdf. +.. [POWERPCABI] Developing PowerPC Embedded Application Binary Interface (EABI) + Compliant Programs. PowerPC Embedded Processors Application Note. IBM. 1998. + http://class.ece.iastate.edu/arun/Cpre381_Sp06/lab/labw12a/eabi_app.pdf. +.. [GOABI] Go internal ABI specification. Go Source Code. No authors. 2023. + https://go.googlesource.com/go/+/refs/heads/master/src/cmd/compile/abi-internal.md. +.. [JAVAABI] The Java (r) Language Specification - Java SE 21 Edition. Gosling, James et. al. + Oracle. 2023. https://docs.oracle.com/javase/specs/jls/se21/html/index.html. +.. [DALVIKABI] Dalvik Bytecode. Android Core Runtime Documentation. No authors. Google. + 2022. https://source.android.com/docs/core/runtime/dalvik-bytecode. +.. [CLRABI] CLR ABI. The Book of the Runtime. No authors. Microsoft. 2023. + https://github.com/dotnet/coreclr/blob/master/Documentation/botr/clr-abi.md. +.. [CSTD] International Standard: Programming Languages - C. ISO/IEC. 2018. + https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf. +.. [RHELABI] Red Hat Enterprise Linux 8: Application Compatibility Guide. Red Hat. + 2023. https://access.redhat.com/articles/rhel8-abi-compatibility. -- 2.41.0