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1 | ============== |
2 | BPF Design Q&A | |
3 | ============== | |
4 | ||
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5 | BPF extensibility and applicability to networking, tracing, security |
6 | in the linux kernel and several user space implementations of BPF | |
7 | virtual machine led to a number of misunderstanding on what BPF actually is. | |
8 | This short QA is an attempt to address that and outline a direction | |
9 | of where BPF is heading long term. | |
10 | ||
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11 | .. contents:: |
12 | :local: | |
13 | :depth: 3 | |
14 | ||
15 | Questions and Answers | |
16 | ===================== | |
17 | ||
2e39748a | 18 | Q: Is BPF a generic instruction set similar to x64 and arm64? |
1a6ac1d5 | 19 | ------------------------------------------------------------- |
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20 | A: NO. |
21 | ||
22 | Q: Is BPF a generic virtual machine ? | |
1a6ac1d5 | 23 | ------------------------------------- |
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24 | A: NO. |
25 | ||
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26 | BPF is generic instruction set *with* C calling convention. |
27 | ----------------------------------------------------------- | |
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28 | |
29 | Q: Why C calling convention was chosen? | |
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30 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
31 | ||
2e39748a | 32 | A: Because BPF programs are designed to run in the linux kernel |
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33 | which is written in C, hence BPF defines instruction set compatible |
34 | with two most used architectures x64 and arm64 (and takes into | |
35 | consideration important quirks of other architectures) and | |
36 | defines calling convention that is compatible with C calling | |
37 | convention of the linux kernel on those architectures. | |
2e39748a | 38 | |
46604676 | 39 | Q: Can multiple return values be supported in the future? |
1a6ac1d5 | 40 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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41 | A: NO. BPF allows only register R0 to be used as return value. |
42 | ||
46604676 | 43 | Q: Can more than 5 function arguments be supported in the future? |
1a6ac1d5 | 44 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
2e39748a | 45 | A: NO. BPF calling convention only allows registers R1-R5 to be used |
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46 | as arguments. BPF is not a standalone instruction set. |
47 | (unlike x64 ISA that allows msft, cdecl and other conventions) | |
2e39748a | 48 | |
46604676 | 49 | Q: Can BPF programs access instruction pointer or return address? |
1a6ac1d5 | 50 | ----------------------------------------------------------------- |
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51 | A: NO. |
52 | ||
46604676 | 53 | Q: Can BPF programs access stack pointer ? |
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54 | ------------------------------------------ |
55 | A: NO. | |
56 | ||
57 | Only frame pointer (register R10) is accessible. | |
58 | From compiler point of view it's necessary to have stack pointer. | |
46604676 | 59 | For example, LLVM defines register R11 as stack pointer in its |
1a6ac1d5 | 60 | BPF backend, but it makes sure that generated code never uses it. |
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61 | |
62 | Q: Does C-calling convention diminishes possible use cases? | |
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63 | ----------------------------------------------------------- |
64 | A: YES. | |
65 | ||
66 | BPF design forces addition of major functionality in the form | |
67 | of kernel helper functions and kernel objects like BPF maps with | |
68 | seamless interoperability between them. It lets kernel call into | |
46604676 AN |
69 | BPF programs and programs call kernel helpers with zero overhead, |
70 | as all of them were native C code. That is particularly the case | |
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71 | for JITed BPF programs that are indistinguishable from |
72 | native kernel C code. | |
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73 | |
74 | Q: Does it mean that 'innovative' extensions to BPF code are disallowed? | |
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75 | ------------------------------------------------------------------------ |
76 | A: Soft yes. | |
77 | ||
46604676 | 78 | At least for now, until BPF core has support for |
1a6ac1d5 | 79 | bpf-to-bpf calls, indirect calls, loops, global variables, |
46604676 | 80 | jump tables, read-only sections, and all other normal constructs |
1a6ac1d5 | 81 | that C code can produce. |
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82 | |
83 | Q: Can loops be supported in a safe way? | |
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84 | ---------------------------------------- |
85 | A: It's not clear yet. | |
86 | ||
87 | BPF developers are trying to find a way to | |
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88 | support bounded loops. |
89 | ||
90 | Q: What are the verifier limits? | |
91 | -------------------------------- | |
92 | A: The only limit known to the user space is BPF_MAXINSNS (4096). | |
93 | It's the maximum number of instructions that the unprivileged bpf | |
94 | program can have. The verifier has various internal limits. | |
95 | Like the maximum number of instructions that can be explored during | |
96 | program analysis. Currently, that limit is set to 1 million. | |
97 | Which essentially means that the largest program can consist | |
98 | of 1 million NOP instructions. There is a limit to the maximum number | |
99 | of subsequent branches, a limit to the number of nested bpf-to-bpf | |
100 | calls, a limit to the number of the verifier states per instruction, | |
101 | a limit to the number of maps used by the program. | |
102 | All these limits can be hit with a sufficiently complex program. | |
103 | There are also non-numerical limits that can cause the program | |
104 | to be rejected. The verifier used to recognize only pointer + constant | |
105 | expressions. Now it can recognize pointer + bounded_register. | |
106 | bpf_lookup_map_elem(key) had a requirement that 'key' must be | |
107 | a pointer to the stack. Now, 'key' can be a pointer to map value. | |
108 | The verifier is steadily getting 'smarter'. The limits are | |
109 | being removed. The only way to know that the program is going to | |
110 | be accepted by the verifier is to try to load it. | |
111 | The bpf development process guarantees that the future kernel | |
112 | versions will accept all bpf programs that were accepted by | |
113 | the earlier versions. | |
114 | ||
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115 | |
116 | Instruction level questions | |
117 | --------------------------- | |
118 | ||
119 | Q: LD_ABS and LD_IND instructions vs C code | |
120 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
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121 | |
122 | Q: How come LD_ABS and LD_IND instruction are present in BPF whereas | |
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123 | C code cannot express them and has to use builtin intrinsics? |
124 | ||
2e39748a | 125 | A: This is artifact of compatibility with classic BPF. Modern |
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126 | networking code in BPF performs better without them. |
127 | See 'direct packet access'. | |
2e39748a | 128 | |
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129 | Q: BPF instructions mapping not one-to-one to native CPU |
130 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
2e39748a | 131 | Q: It seems not all BPF instructions are one-to-one to native CPU. |
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132 | For example why BPF_JNE and other compare and jumps are not cpu-like? |
133 | ||
2e39748a | 134 | A: This was necessary to avoid introducing flags into ISA which are |
1a6ac1d5 | 135 | impossible to make generic and efficient across CPU architectures. |
2e39748a | 136 | |
46604676 | 137 | Q: Why BPF_DIV instruction doesn't map to x64 div? |
1a6ac1d5 | 138 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
2e39748a | 139 | A: Because if we picked one-to-one relationship to x64 it would have made |
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140 | it more complicated to support on arm64 and other archs. Also it |
141 | needs div-by-zero runtime check. | |
2e39748a | 142 | |
46604676 | 143 | Q: Why there is no BPF_SDIV for signed divide operation? |
1a6ac1d5 | 144 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
2e39748a | 145 | A: Because it would be rarely used. llvm errors in such case and |
46604676 | 146 | prints a suggestion to use unsigned divide instead. |
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147 | |
148 | Q: Why BPF has implicit prologue and epilogue? | |
1a6ac1d5 | 149 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
2e39748a | 150 | A: Because architectures like sparc have register windows and in general |
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151 | there are enough subtle differences between architectures, so naive |
152 | store return address into stack won't work. Another reason is BPF has | |
153 | to be safe from division by zero (and legacy exception path | |
154 | of LD_ABS insn). Those instructions need to invoke epilogue and | |
155 | return implicitly. | |
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156 | |
157 | Q: Why BPF_JLT and BPF_JLE instructions were not introduced in the beginning? | |
1a6ac1d5 | 158 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
2e39748a | 159 | A: Because classic BPF didn't have them and BPF authors felt that compiler |
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160 | workaround would be acceptable. Turned out that programs lose performance |
161 | due to lack of these compare instructions and they were added. | |
162 | These two instructions is a perfect example what kind of new BPF | |
163 | instructions are acceptable and can be added in the future. | |
164 | These two already had equivalent instructions in native CPUs. | |
165 | New instructions that don't have one-to-one mapping to HW instructions | |
166 | will not be accepted. | |
167 | ||
168 | Q: BPF 32-bit subregister requirements | |
169 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
2e39748a | 170 | Q: BPF 32-bit subregisters have a requirement to zero upper 32-bits of BPF |
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171 | registers which makes BPF inefficient virtual machine for 32-bit |
172 | CPU architectures and 32-bit HW accelerators. Can true 32-bit registers | |
173 | be added to BPF in the future? | |
174 | ||
2e39748a | 175 | A: NO. The first thing to improve performance on 32-bit archs is to teach |
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176 | LLVM to generate code that uses 32-bit subregisters. Then second step |
177 | is to teach verifier to mark operations where zero-ing upper bits | |
178 | is unnecessary. Then JITs can take advantage of those markings and | |
179 | drastically reduce size of generated code and improve performance. | |
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180 | |
181 | Q: Does BPF have a stable ABI? | |
1a6ac1d5 | 182 | ------------------------------ |
2e39748a | 183 | A: YES. BPF instructions, arguments to BPF programs, set of helper |
1a6ac1d5 | 184 | functions and their arguments, recognized return codes are all part |
a769fa72 DB |
185 | of ABI. However there is one specific exception to tracing programs |
186 | which are using helpers like bpf_probe_read() to walk kernel internal | |
187 | data structures and compile with kernel internal headers. Both of these | |
188 | kernel internals are subject to change and can break with newer kernels | |
189 | such that the program needs to be adapted accordingly. | |
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190 | |
191 | Q: How much stack space a BPF program uses? | |
1a6ac1d5 | 192 | ------------------------------------------- |
2e39748a | 193 | A: Currently all program types are limited to 512 bytes of stack |
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194 | space, but the verifier computes the actual amount of stack used |
195 | and both interpreter and most JITed code consume necessary amount. | |
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196 | |
197 | Q: Can BPF be offloaded to HW? | |
1a6ac1d5 | 198 | ------------------------------ |
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199 | A: YES. BPF HW offload is supported by NFP driver. |
200 | ||
201 | Q: Does classic BPF interpreter still exist? | |
1a6ac1d5 | 202 | -------------------------------------------- |
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203 | A: NO. Classic BPF programs are converted into extend BPF instructions. |
204 | ||
205 | Q: Can BPF call arbitrary kernel functions? | |
1a6ac1d5 | 206 | ------------------------------------------- |
2e39748a | 207 | A: NO. BPF programs can only call a set of helper functions which |
1a6ac1d5 | 208 | is defined for every program type. |
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209 | |
210 | Q: Can BPF overwrite arbitrary kernel memory? | |
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211 | --------------------------------------------- |
212 | A: NO. | |
213 | ||
214 | Tracing bpf programs can *read* arbitrary memory with bpf_probe_read() | |
215 | and bpf_probe_read_str() helpers. Networking programs cannot read | |
216 | arbitrary memory, since they don't have access to these helpers. | |
217 | Programs can never read or write arbitrary memory directly. | |
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218 | |
219 | Q: Can BPF overwrite arbitrary user memory? | |
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220 | ------------------------------------------- |
221 | A: Sort-of. | |
222 | ||
223 | Tracing BPF programs can overwrite the user memory | |
224 | of the current task with bpf_probe_write_user(). Every time such | |
225 | program is loaded the kernel will print warning message, so | |
226 | this helper is only useful for experiments and prototypes. | |
227 | Tracing BPF programs are root only. | |
2e39748a | 228 | |
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229 | Q: bpf_trace_printk() helper warning |
230 | ------------------------------------ | |
2e39748a | 231 | Q: When bpf_trace_printk() helper is used the kernel prints nasty |
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232 | warning message. Why is that? |
233 | ||
2e39748a | 234 | A: This is done to nudge program authors into better interfaces when |
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235 | programs need to pass data to user space. Like bpf_perf_event_output() |
236 | can be used to efficiently stream data via perf ring buffer. | |
237 | BPF maps can be used for asynchronous data sharing between kernel | |
238 | and user space. bpf_trace_printk() should only be used for debugging. | |
2e39748a | 239 | |
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240 | Q: New functionality via kernel modules? |
241 | ---------------------------------------- | |
2e39748a | 242 | Q: Can BPF functionality such as new program or map types, new |
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243 | helpers, etc be added out of kernel module code? |
244 | ||
2e39748a | 245 | A: NO. |