mirror of https://gitee.com/openkylin/linux.git
222 lines
9.0 KiB
ReStructuredText
222 lines
9.0 KiB
ReStructuredText
==============
|
|
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.
|
|
-----------------------------------------------------------
|
|
|
|
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.
|
|
|
|
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)
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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?
|
|
|
|
A: This is artifact of compatibility with classic BPF. Modern
|
|
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?
|
|
|
|
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?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
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
|
|
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.
|
|
|
|
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?
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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: NO.
|