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Documentation / bpf / bpf_design_QA.txt




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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

1	BPF extensibility and applicability to networking, tracing, security
2	in the linux kernel and several user space implementations of BPF
3	virtual machine led to a number of misunderstanding on what BPF actually is.
4	This short QA is an attempt to address that and outline a direction
5	of where BPF is heading long term.
6	
7	Q: Is BPF a generic instruction set similar to x64 and arm64?
8	A: NO.
9	
10	Q: Is BPF a generic virtual machine ?
11	A: NO.
12	
13	BPF is generic instruction set _with_ C calling convention.
14	
15	Q: Why C calling convention was chosen?
16	A: Because BPF programs are designed to run in the linux kernel
17	   which is written in C, hence BPF defines instruction set compatible
18	   with two most used architectures x64 and arm64 (and takes into
19	   consideration important quirks of other architectures) and
20	   defines calling convention that is compatible with C calling
21	   convention of the linux kernel on those architectures.
22	
23	Q: can multiple return values be supported in the future?
24	A: NO. BPF allows only register R0 to be used as return value.
25	
26	Q: can more than 5 function arguments be supported in the future?
27	A: NO. BPF calling convention only allows registers R1-R5 to be used
28	   as arguments. BPF is not a standalone instruction set.
29	   (unlike x64 ISA that allows msft, cdecl and other conventions)
30	
31	Q: can BPF programs access instruction pointer or return address?
32	A: NO.
33	
34	Q: can BPF programs access stack pointer ?
35	A: NO. Only frame pointer (register R10) is accessible.
36	   From compiler point of view it's necessary to have stack pointer.
37	   For example LLVM defines register R11 as stack pointer in its
38	   BPF backend, but it makes sure that generated code never uses it.
39	
40	Q: Does C-calling convention diminishes possible use cases?
41	A: YES. BPF design forces addition of major functionality in the form
42	   of kernel helper functions and kernel objects like BPF maps with
43	   seamless interoperability between them. It lets kernel call into
44	   BPF programs and programs call kernel helpers with zero overhead.
45	   As all of them were native C code. That is particularly the case
46	   for JITed BPF programs that are indistinguishable from
47	   native kernel C code.
48	
49	Q: Does it mean that 'innovative' extensions to BPF code are disallowed?
50	A: Soft yes. At least for now until BPF core has support for
51	   bpf-to-bpf calls, indirect calls, loops, global variables,
52	   jump tables, read only sections and all other normal constructs
53	   that C code can produce.
54	
55	Q: Can loops be supported in a safe way?
56	A: It's not clear yet. BPF developers are trying to find a way to
57	   support bounded loops where the verifier can guarantee that
58	   the program terminates in less than 4096 instructions.
59	
60	Q: How come LD_ABS and LD_IND instruction are present in BPF whereas
61	   C code cannot express them and has to use builtin intrinsics?
62	A: This is artifact of compatibility with classic BPF. Modern
63	   networking code in BPF performs better without them.
64	   See 'direct packet access'.
65	
66	Q: It seems not all BPF instructions are one-to-one to native CPU.
67	   For example why BPF_JNE and other compare and jumps are not cpu-like?
68	A: This was necessary to avoid introducing flags into ISA which are
69	   impossible to make generic and efficient across CPU architectures.
70	
71	Q: why BPF_DIV instruction doesn't map to x64 div?
72	A: Because if we picked one-to-one relationship to x64 it would have made
73	   it more complicated to support on arm64 and other archs. Also it
74	   needs div-by-zero runtime check.
75	
76	Q: why there is no BPF_SDIV for signed divide operation?
77	A: Because it would be rarely used. llvm errors in such case and
78	   prints a suggestion to use unsigned divide instead
79	
80	Q: Why BPF has implicit prologue and epilogue?
81	A: Because architectures like sparc have register windows and in general
82	   there are enough subtle differences between architectures, so naive
83	   store return address into stack won't work. Another reason is BPF has
84	   to be safe from division by zero (and legacy exception path
85	   of LD_ABS insn). Those instructions need to invoke epilogue and
86	   return implicitly.
87	
88	Q: Why BPF_JLT and BPF_JLE instructions were not introduced in the beginning?
89	A: Because classic BPF didn't have them and BPF authors felt that compiler
90	   workaround would be acceptable. Turned out that programs lose performance
91	   due to lack of these compare instructions and they were added.
92	   These two instructions is a perfect example what kind of new BPF
93	   instructions are acceptable and can be added in the future.
94	   These two already had equivalent instructions in native CPUs.
95	   New instructions that don't have one-to-one mapping to HW instructions
96	   will not be accepted.
97	
98	Q: BPF 32-bit subregisters have a requirement to zero upper 32-bits of BPF
99	   registers which makes BPF inefficient virtual machine for 32-bit
100	   CPU architectures and 32-bit HW accelerators. Can true 32-bit registers
101	   be added to BPF in the future?
102	A: NO. The first thing to improve performance on 32-bit archs is to teach
103	   LLVM to generate code that uses 32-bit subregisters. Then second step
104	   is to teach verifier to mark operations where zero-ing upper bits
105	   is unnecessary. Then JITs can take advantage of those markings and
106	   drastically reduce size of generated code and improve performance.
107	
108	Q: Does BPF have a stable ABI?
109	A: YES. BPF instructions, arguments to BPF programs, set of helper
110	   functions and their arguments, recognized return codes are all part
111	   of ABI. However when tracing programs are using bpf_probe_read() helper
112	   to walk kernel internal datastructures and compile with kernel
113	   internal headers these accesses can and will break with newer
114	   kernels. The union bpf_attr -> kern_version is checked at load time
115	   to prevent accidentally loading kprobe-based bpf programs written
116	   for a different kernel. Networking programs don't do kern_version check.
117	
118	Q: How much stack space a BPF program uses?
119	A: Currently all program types are limited to 512 bytes of stack
120	   space, but the verifier computes the actual amount of stack used
121	   and both interpreter and most JITed code consume necessary amount.
122	
123	Q: Can BPF be offloaded to HW?
124	A: YES. BPF HW offload is supported by NFP driver.
125	
126	Q: Does classic BPF interpreter still exist?
127	A: NO. Classic BPF programs are converted into extend BPF instructions.
128	
129	Q: Can BPF call arbitrary kernel functions?
130	A: NO. BPF programs can only call a set of helper functions which
131	   is defined for every program type.
132	
133	Q: Can BPF overwrite arbitrary kernel memory?
134	A: NO. Tracing bpf programs can _read_ arbitrary memory with bpf_probe_read()
135	   and bpf_probe_read_str() helpers. Networking programs cannot read
136	   arbitrary memory, since they don't have access to these helpers.
137	   Programs can never read or write arbitrary memory directly.
138	
139	Q: Can BPF overwrite arbitrary user memory?
140	A: Sort-of. Tracing BPF programs can overwrite the user memory
141	   of the current task with bpf_probe_write_user(). Every time such
142	   program is loaded the kernel will print warning message, so
143	   this helper is only useful for experiments and prototypes.
144	   Tracing BPF programs are root only.
145	
146	Q: When bpf_trace_printk() helper is used the kernel prints nasty
147	   warning message. Why is that?
148	A: This is done to nudge program authors into better interfaces when
149	   programs need to pass data to user space. Like bpf_perf_event_output()
150	   can be used to efficiently stream data via perf ring buffer.
151	   BPF maps can be used for asynchronous data sharing between kernel
152	   and user space. bpf_trace_printk() should only be used for debugging.
153	
154	Q: Can BPF functionality such as new program or map types, new
155	   helpers, etc be added out of kernel module code?
156	A: NO.
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