1 Objtool 1 Objtool
2 ======= 2 =======
3 3
4 The kernel CONFIG_OBJTOOL option enables a hos 4 The kernel CONFIG_OBJTOOL option enables a host tool named 'objtool'
5 which runs at compile time. It can do various 5 which runs at compile time. It can do various validations and
6 transformations on .o files. 6 transformations on .o files.
7 7
8 Objtool has become an integral part of the x86 8 Objtool has become an integral part of the x86-64 kernel toolchain. The
9 kernel depends on it for a variety of security 9 kernel depends on it for a variety of security and performance features
10 (and other types of features as well). 10 (and other types of features as well).
11 11
12 12
13 Features 13 Features
14 -------- 14 --------
15 15
16 Objtool has the following features: 16 Objtool has the following features:
17 17
18 - Stack unwinding metadata validation -- usefu 18 - Stack unwinding metadata validation -- useful for helping to ensure
19 stack traces are reliable for live patching 19 stack traces are reliable for live patching
20 20
21 - ORC unwinder metadata generation -- a faster 21 - ORC unwinder metadata generation -- a faster and more precise
22 alternative to frame pointer based unwinding 22 alternative to frame pointer based unwinding
23 23
24 - Retpoline validation -- ensures that all ind 24 - Retpoline validation -- ensures that all indirect calls go through
25 retpoline thunks, for Spectre v2 mitigations 25 retpoline thunks, for Spectre v2 mitigations
26 26
27 - Retpoline call site annotation -- annotates 27 - Retpoline call site annotation -- annotates all retpoline thunk call
28 sites, enabling the kernel to patch them inl 28 sites, enabling the kernel to patch them inline, to prevent "thunk
29 funneling" for both security and performance 29 funneling" for both security and performance reasons
30 30
31 - Non-instrumentation validation -- validates 31 - Non-instrumentation validation -- validates non-instrumentable
32 ("noinstr") code rules, preventing instrumen 32 ("noinstr") code rules, preventing instrumentation in low-level C
33 entry code 33 entry code
34 34
35 - Static call annotation -- annotates static c 35 - Static call annotation -- annotates static call sites, enabling the
36 kernel to implement inline static calls, a f 36 kernel to implement inline static calls, a faster alternative to some
37 indirect branches 37 indirect branches
38 38
39 - Uaccess validation -- validates uaccess rule 39 - Uaccess validation -- validates uaccess rules for a proper
40 implementation of Supervisor Mode Access Pro 40 implementation of Supervisor Mode Access Protection (SMAP)
41 41
42 - Straight Line Speculation validation -- vali 42 - Straight Line Speculation validation -- validates certain SLS
43 mitigations 43 mitigations
44 44
45 - Indirect Branch Tracking validation -- valid 45 - Indirect Branch Tracking validation -- validates Intel CET IBT rules
46 to ensure that all functions referenced by f 46 to ensure that all functions referenced by function pointers have
47 corresponding ENDBR instructions 47 corresponding ENDBR instructions
48 48
49 - Indirect Branch Tracking annotation -- annot 49 - Indirect Branch Tracking annotation -- annotates unused ENDBR
50 instruction sites, enabling the kernel to "s 50 instruction sites, enabling the kernel to "seal" them (replace them
51 with NOPs) to further harden IBT 51 with NOPs) to further harden IBT
52 52
53 - Function entry annotation -- annotates funct 53 - Function entry annotation -- annotates function entries, enabling
54 kernel function tracing 54 kernel function tracing
55 55
56 - Other toolchain hacks which will go unmentio 56 - Other toolchain hacks which will go unmentioned at this time...
57 57
58 Each feature can be enabled individually or in 58 Each feature can be enabled individually or in combination using the
59 objtool cmdline. 59 objtool cmdline.
60 60
61 61
62 Objects 62 Objects
63 ------- 63 -------
64 64
65 Typically, objtool runs on every translation u 65 Typically, objtool runs on every translation unit (TU, aka ".o file") in
66 the kernel. If a TU is part of a kernel modul 66 the kernel. If a TU is part of a kernel module, the '--module' option
67 is added. 67 is added.
68 68
69 However: 69 However:
70 70
71 - If noinstr validation is enabled, it also ru 71 - If noinstr validation is enabled, it also runs on vmlinux.o, with all
72 options removed and '--noinstr' added. 72 options removed and '--noinstr' added.
73 73
74 - If IBT or LTO is enabled, it doesn't run on 74 - If IBT or LTO is enabled, it doesn't run on TUs at all. Instead it
75 runs on vmlinux.o and linked modules, with a 75 runs on vmlinux.o and linked modules, with all options.
76 76
77 In summary: 77 In summary:
78 78
79 A) Legacy mode: 79 A) Legacy mode:
80 TU: objtool [--module] <options> 80 TU: objtool [--module] <options>
81 vmlinux: N/A 81 vmlinux: N/A
82 module: N/A 82 module: N/A
83 83
84 B) CONFIG_NOINSTR_VALIDATION=y && !(CONFIG_X 84 B) CONFIG_NOINSTR_VALIDATION=y && !(CONFIG_X86_KERNEL_IBT=y || CONFIG_LTO=y):
85 TU: objtool [--module] <options> 85 TU: objtool [--module] <options> // no --noinstr
86 vmlinux: objtool --noinstr 86 vmlinux: objtool --noinstr // other options removed
87 module: N/A 87 module: N/A
88 88
89 C) CONFIG_X86_KERNEL_IBT=y || CONFIG_LTO=y: 89 C) CONFIG_X86_KERNEL_IBT=y || CONFIG_LTO=y:
90 TU: N/A 90 TU: N/A
91 vmlinux: objtool --noinstr <options> 91 vmlinux: objtool --noinstr <options>
92 module: objtool --module --noinstr <o 92 module: objtool --module --noinstr <options>
93 93
94 94
95 Stack validation 95 Stack validation
96 ---------------- 96 ----------------
97 97
98 Objtool's stack validation feature analyzes ev 98 Objtool's stack validation feature analyzes every .o file and ensures
99 the validity of its stack metadata. It enforc 99 the validity of its stack metadata. It enforces a set of rules on asm
100 code and C inline assembly code so that stack 100 code and C inline assembly code so that stack traces can be reliable.
101 101
102 For each function, it recursively follows all 102 For each function, it recursively follows all possible code paths and
103 validates the correct frame pointer state at e 103 validates the correct frame pointer state at each instruction.
104 104
105 It also follows code paths involving special s 105 It also follows code paths involving special sections, like
106 .altinstructions, __jump_table, and __ex_table 106 .altinstructions, __jump_table, and __ex_table, which can add
107 alternative execution paths to a given instruc 107 alternative execution paths to a given instruction (or set of
108 instructions). Similarly, it knows how to fol 108 instructions). Similarly, it knows how to follow switch statements, for
109 which gcc sometimes uses jump tables. 109 which gcc sometimes uses jump tables.
110 110
111 Here are some of the benefits of validating st 111 Here are some of the benefits of validating stack metadata:
112 112
113 a) More reliable stack traces for frame pointe 113 a) More reliable stack traces for frame pointer enabled kernels
114 114
115 Frame pointers are used for debugging purpo 115 Frame pointers are used for debugging purposes. They allow runtime
116 code and debug tools to be able to walk the 116 code and debug tools to be able to walk the stack to determine the
117 chain of function call sites that led to th 117 chain of function call sites that led to the currently executing
118 code. 118 code.
119 119
120 For some architectures, frame pointers are 120 For some architectures, frame pointers are enabled by
121 CONFIG_FRAME_POINTER. For some other archi 121 CONFIG_FRAME_POINTER. For some other architectures they may be
122 required by the ABI (sometimes referred to 122 required by the ABI (sometimes referred to as "backchain pointers").
123 123
124 For C code, gcc automatically generates ins 124 For C code, gcc automatically generates instructions for setting up
125 frame pointers when the -fno-omit-frame-poi 125 frame pointers when the -fno-omit-frame-pointer option is used.
126 126
127 But for asm code, the frame setup instructi 127 But for asm code, the frame setup instructions have to be written by
128 hand, which most people don't do. So the e 128 hand, which most people don't do. So the end result is that
129 CONFIG_FRAME_POINTER is honored for C code 129 CONFIG_FRAME_POINTER is honored for C code but not for most asm code.
130 130
131 For stack traces based on frame pointers to 131 For stack traces based on frame pointers to be reliable, all
132 functions which call other functions must f 132 functions which call other functions must first create a stack frame
133 and update the frame pointer. If a first f 133 and update the frame pointer. If a first function doesn't properly
134 create a stack frame before calling a secon 134 create a stack frame before calling a second function, the *caller*
135 of the first function will be skipped on th 135 of the first function will be skipped on the stack trace.
136 136
137 For example, consider the following example 137 For example, consider the following example backtrace with frame
138 pointers enabled: 138 pointers enabled:
139 139
140 [<ffffffff81812584>] dump_stack+0x4b/0x63 140 [<ffffffff81812584>] dump_stack+0x4b/0x63
141 [<ffffffff812d6dc2>] cmdline_proc_show+0x 141 [<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
142 [<ffffffff8127f568>] seq_read+0x108/0x3e0 142 [<ffffffff8127f568>] seq_read+0x108/0x3e0
143 [<ffffffff812cce62>] proc_reg_read+0x42/0 143 [<ffffffff812cce62>] proc_reg_read+0x42/0x70
144 [<ffffffff81256197>] __vfs_read+0x37/0x10 144 [<ffffffff81256197>] __vfs_read+0x37/0x100
145 [<ffffffff81256b16>] vfs_read+0x86/0x130 145 [<ffffffff81256b16>] vfs_read+0x86/0x130
146 [<ffffffff81257898>] SyS_read+0x58/0xd0 146 [<ffffffff81257898>] SyS_read+0x58/0xd0
147 [<ffffffff8181c1f2>] entry_SYSCALL_64_fas 147 [<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76
148 148
149 It correctly shows that the caller of cmdli 149 It correctly shows that the caller of cmdline_proc_show() is
150 seq_read(). 150 seq_read().
151 151
152 If we remove the frame pointer logic from c 152 If we remove the frame pointer logic from cmdline_proc_show() by
153 replacing the frame pointer related instruc 153 replacing the frame pointer related instructions with nops, here's
154 what it looks like instead: 154 what it looks like instead:
155 155
156 [<ffffffff81812584>] dump_stack+0x4b/0x63 156 [<ffffffff81812584>] dump_stack+0x4b/0x63
157 [<ffffffff812d6dc2>] cmdline_proc_show+0x 157 [<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
158 [<ffffffff812cce62>] proc_reg_read+0x42/0 158 [<ffffffff812cce62>] proc_reg_read+0x42/0x70
159 [<ffffffff81256197>] __vfs_read+0x37/0x10 159 [<ffffffff81256197>] __vfs_read+0x37/0x100
160 [<ffffffff81256b16>] vfs_read+0x86/0x130 160 [<ffffffff81256b16>] vfs_read+0x86/0x130
161 [<ffffffff81257898>] SyS_read+0x58/0xd0 161 [<ffffffff81257898>] SyS_read+0x58/0xd0
162 [<ffffffff8181c1f2>] entry_SYSCALL_64_fas 162 [<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76
163 163
164 Notice that cmdline_proc_show()'s caller, s 164 Notice that cmdline_proc_show()'s caller, seq_read(), has been
165 skipped. Instead the stack trace seems to 165 skipped. Instead the stack trace seems to show that
166 cmdline_proc_show() was called by proc_reg_ 166 cmdline_proc_show() was called by proc_reg_read().
167 167
168 The benefit of objtool here is that because 168 The benefit of objtool here is that because it ensures that *all*
169 functions honor CONFIG_FRAME_POINTER, no fu 169 functions honor CONFIG_FRAME_POINTER, no functions will ever[*] be
170 skipped on a stack trace. 170 skipped on a stack trace.
171 171
172 [*] unless an interrupt or exception has oc 172 [*] unless an interrupt or exception has occurred at the very
173 beginning of a function before the stac 173 beginning of a function before the stack frame has been created,
174 or at the very end of the function afte 174 or at the very end of the function after the stack frame has been
175 destroyed. This is an inherent limitat 175 destroyed. This is an inherent limitation of frame pointers.
176 176
177 b) ORC (Oops Rewind Capability) unwind table g 177 b) ORC (Oops Rewind Capability) unwind table generation
178 178
179 An alternative to frame pointers and DWARF, 179 An alternative to frame pointers and DWARF, ORC unwind data can be
180 used to walk the stack. Unlike frame point 180 used to walk the stack. Unlike frame pointers, ORC data is out of
181 band. So it doesn't affect runtime perform 181 band. So it doesn't affect runtime performance and it can be
182 reliable even when interrupts or exceptions 182 reliable even when interrupts or exceptions are involved.
183 183
184 For more details, see Documentation/arch/x8 !! 184 For more details, see Documentation/x86/orc-unwinder.rst.
185 185
186 c) Higher live patching compatibility rate 186 c) Higher live patching compatibility rate
187 187
188 Livepatch has an optional "consistency mode 188 Livepatch has an optional "consistency model", which is needed for
189 more complex patches. In order for the con 189 more complex patches. In order for the consistency model to work,
190 stack traces need to be reliable (or an unr 190 stack traces need to be reliable (or an unreliable condition needs to
191 be detectable). Objtool makes that possibl 191 be detectable). Objtool makes that possible.
192 192
193 For more details, see the livepatch documen 193 For more details, see the livepatch documentation in the Linux kernel
194 source tree at Documentation/livepatch/live 194 source tree at Documentation/livepatch/livepatch.rst.
195 195
196 To achieve the validation, objtool enforces th 196 To achieve the validation, objtool enforces the following rules:
197 197
198 1. Each callable function must be annotated as 198 1. Each callable function must be annotated as such with the ELF
199 function type. In asm code, this is typica 199 function type. In asm code, this is typically done using the
200 ENTRY/ENDPROC macros. If objtool finds a r 200 ENTRY/ENDPROC macros. If objtool finds a return instruction
201 outside of a function, it flags an error si 201 outside of a function, it flags an error since that usually indicates
202 callable code which should be annotated acc 202 callable code which should be annotated accordingly.
203 203
204 This rule is needed so that objtool can pro 204 This rule is needed so that objtool can properly identify each
205 callable function in order to analyze its s 205 callable function in order to analyze its stack metadata.
206 206
207 2. Conversely, each section of code which is * 207 2. Conversely, each section of code which is *not* callable should *not*
208 be annotated as an ELF function. The ENDPR 208 be annotated as an ELF function. The ENDPROC macro shouldn't be used
209 in this case. 209 in this case.
210 210
211 This rule is needed so that objtool can ign 211 This rule is needed so that objtool can ignore non-callable code.
212 Such code doesn't have to follow any of the 212 Such code doesn't have to follow any of the other rules.
213 213
214 3. Each callable function which calls another 214 3. Each callable function which calls another function must have the
215 correct frame pointer logic, if required by 215 correct frame pointer logic, if required by CONFIG_FRAME_POINTER or
216 the architecture's back chain rules. This 216 the architecture's back chain rules. This can by done in asm code
217 with the FRAME_BEGIN/FRAME_END macros. 217 with the FRAME_BEGIN/FRAME_END macros.
218 218
219 This rule ensures that frame pointer based 219 This rule ensures that frame pointer based stack traces will work as
220 designed. If function A doesn't create a s 220 designed. If function A doesn't create a stack frame before calling
221 function B, the _caller_ of function A will 221 function B, the _caller_ of function A will be skipped on the stack
222 trace. 222 trace.
223 223
224 4. Dynamic jumps and jumps to undefined symbol 224 4. Dynamic jumps and jumps to undefined symbols are only allowed if:
225 225
226 a) the jump is part of a switch statement; 226 a) the jump is part of a switch statement; or
227 227
228 b) the jump matches sibling call semantics 228 b) the jump matches sibling call semantics and the frame pointer has
229 the same value it had on function entry. 229 the same value it had on function entry.
230 230
231 This rule is needed so that objtool can rel 231 This rule is needed so that objtool can reliably analyze all of a
232 function's code paths. If a function jumps 232 function's code paths. If a function jumps to code in another file,
233 and it's not a sibling call, objtool has no 233 and it's not a sibling call, objtool has no way to follow the jump
234 because it only analyzes a single file at a 234 because it only analyzes a single file at a time.
235 235
236 5. A callable function may not execute kernel 236 5. A callable function may not execute kernel entry/exit instructions.
237 The only code which needs such instructions 237 The only code which needs such instructions is kernel entry code,
238 which shouldn't be be in callable functions 238 which shouldn't be be in callable functions anyway.
239 239
240 This rule is just a sanity check to ensure 240 This rule is just a sanity check to ensure that callable functions
241 return normally. 241 return normally.
242 242
243 243
244 Objtool warnings 244 Objtool warnings
245 ---------------- 245 ----------------
246 246
247 NOTE: When requesting help with an objtool war <<
248 OBJTOOL_VERBOSE=1 (e.g., "make OBJTOOL_VERBOSE <<
249 output, including any disassembly or backtrace <<
250 objtool maintainers. <<
251 <<
252 For asm files, if you're getting an error whic 247 For asm files, if you're getting an error which doesn't make sense,
253 first make sure that the affected code follows 248 first make sure that the affected code follows the above rules.
254 249
255 For C files, the common culprits are inline as 250 For C files, the common culprits are inline asm statements and calls to
256 "noreturn" functions. See below for more deta 251 "noreturn" functions. See below for more details.
257 252
258 Another possible cause for errors in C code is 253 Another possible cause for errors in C code is if the Makefile removes
259 -fno-omit-frame-pointer or adds -fomit-frame-p 254 -fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options.
260 255
261 Here are some examples of common warnings repo 256 Here are some examples of common warnings reported by objtool, what
262 they mean, and suggestions for how to fix them 257 they mean, and suggestions for how to fix them. When in doubt, ping
263 the objtool maintainers. 258 the objtool maintainers.
264 259
265 260
266 1. file.o: warning: objtool: func()+0x128: cal 261 1. file.o: warning: objtool: func()+0x128: call without frame pointer save/setup
267 262
268 The func() function made a function call wi 263 The func() function made a function call without first saving and/or
269 updating the frame pointer, and CONFIG_FRAM 264 updating the frame pointer, and CONFIG_FRAME_POINTER is enabled.
270 265
271 If the error is for an asm file, and func() 266 If the error is for an asm file, and func() is indeed a callable
272 function, add proper frame pointer logic us 267 function, add proper frame pointer logic using the FRAME_BEGIN and
273 FRAME_END macros. Otherwise, if it's not a 268 FRAME_END macros. Otherwise, if it's not a callable function, remove
274 its ELF function annotation by changing END 269 its ELF function annotation by changing ENDPROC to END, and instead
275 use the manual unwind hint macros in asm/un 270 use the manual unwind hint macros in asm/unwind_hints.h.
276 271
277 If it's a GCC-compiled .c file, the error m 272 If it's a GCC-compiled .c file, the error may be because the function
278 uses an inline asm() statement which has a 273 uses an inline asm() statement which has a "call" instruction. An
279 asm() statement with a call instruction mus 274 asm() statement with a call instruction must declare the use of the
280 stack pointer in its output operand. On x8 275 stack pointer in its output operand. On x86_64, this means adding
281 the ASM_CALL_CONSTRAINT as an output constr 276 the ASM_CALL_CONSTRAINT as an output constraint:
282 277
283 asm volatile("call func" : ASM_CALL_CONST 278 asm volatile("call func" : ASM_CALL_CONSTRAINT);
284 279
285 Otherwise the stack frame may not get creat 280 Otherwise the stack frame may not get created before the call.
286 281
287 objtool can help with pinpointing the exact <<
288 <<
289 $ OBJTOOL_ARGS="--verbose" make arch/x86/kv <<
290 <<
291 arch/x86/kvm/kvm.o: warning: objtool: .alti <<
292 arch/x86/kvm/kvm.o: warning: objtool: em_ <<
293 arch/x86/kvm/kvm.o: warning: objtool: em_ <<
294 LD [M] arch/x86/kvm/kvm-intel.o <<
295 0000 0000000000028220 <em_loop.part.0>: <<
296 0000 28220: 0f b6 47 61 mov <<
297 0004 28224: 3c e2 cmp <<
298 0006 28226: 74 2c je <<
299 0008 28228: 48 8b 57 10 mov <<
300 000c 2822c: 83 f0 05 xor <<
301 000f 2822f: 48 c1 e0 04 shl <<
302 0013 28233: 25 f0 00 00 00 and <<
303 0018 28238: 81 e2 d5 08 00 00 and <<
304 001e 2823e: 80 ce 02 or <<
305 ... <<
306 282
307 2. file.o: warning: objtool: .text+0x53: unrea 283 2. file.o: warning: objtool: .text+0x53: unreachable instruction
308 284
309 Objtool couldn't find a code path to reach 285 Objtool couldn't find a code path to reach the instruction.
310 286
311 If the error is for an asm file, and the in 287 If the error is for an asm file, and the instruction is inside (or
312 reachable from) a callable function, the fu 288 reachable from) a callable function, the function should be annotated
313 with the ENTRY/ENDPROC macros (ENDPROC is t 289 with the ENTRY/ENDPROC macros (ENDPROC is the important one).
314 Otherwise, the code should probably be anno 290 Otherwise, the code should probably be annotated with the unwind hint
315 macros in asm/unwind_hints.h so objtool and 291 macros in asm/unwind_hints.h so objtool and the unwinder can know the
316 stack state associated with the code. 292 stack state associated with the code.
317 293
318 If you're 100% sure the code won't affect s 294 If you're 100% sure the code won't affect stack traces, or if you're
319 a just a bad person, you can tell objtool t 295 a just a bad person, you can tell objtool to ignore it. See the
320 "Adding exceptions" section below. 296 "Adding exceptions" section below.
321 297
322 If it's not actually in a callable function 298 If it's not actually in a callable function (e.g. kernel entry code),
323 change ENDPROC to END. 299 change ENDPROC to END.
324 300
325 3. file.o: warning: objtool: foo+0x48c: bar() <<
326 <<
327 The call from foo() to bar() doesn't return <<
328 __noreturn annotation. NOTE: In addition t <<
329 with __noreturn, please also add it to tool <<
330 301
331 4. file.o: warning: objtool: func(): can't fin 302 4. file.o: warning: objtool: func(): can't find starting instruction
332 or 303 or
333 file.o: warning: objtool: func()+0x11dd: ca 304 file.o: warning: objtool: func()+0x11dd: can't decode instruction
334 305
335 Does the file have data in a text section? 306 Does the file have data in a text section? If so, that can confuse
336 objtool's instruction decoder. Move the da 307 objtool's instruction decoder. Move the data to a more appropriate
337 section like .data or .rodata. 308 section like .data or .rodata.
338 309
339 310
340 5. file.o: warning: objtool: func()+0x6: unsup 311 5. file.o: warning: objtool: func()+0x6: unsupported instruction in callable function
341 312
342 This is a kernel entry/exit instruction lik 313 This is a kernel entry/exit instruction like sysenter or iret. Such
343 instructions aren't allowed in a callable f 314 instructions aren't allowed in a callable function, and are most
344 likely part of the kernel entry code. They 315 likely part of the kernel entry code. They should usually not have
345 the callable function annotation (ENDPROC) 316 the callable function annotation (ENDPROC) and should always be
346 annotated with the unwind hint macros in as 317 annotated with the unwind hint macros in asm/unwind_hints.h.
347 318
348 319
349 6. file.o: warning: objtool: func()+0x26: sibl 320 6. file.o: warning: objtool: func()+0x26: sibling call from callable instruction with modified stack frame
350 321
351 This is a dynamic jump or a jump to an unde 322 This is a dynamic jump or a jump to an undefined symbol. Objtool
352 assumed it's a sibling call and detected th 323 assumed it's a sibling call and detected that the frame pointer
353 wasn't first restored to its original state 324 wasn't first restored to its original state.
354 325
355 If it's not really a sibling call, you may 326 If it's not really a sibling call, you may need to move the
356 destination code to the local file. 327 destination code to the local file.
357 328
358 If the instruction is not actually in a cal 329 If the instruction is not actually in a callable function (e.g.
359 kernel entry code), change ENDPROC to END a 330 kernel entry code), change ENDPROC to END and annotate manually with
360 the unwind hint macros in asm/unwind_hints. 331 the unwind hint macros in asm/unwind_hints.h.
361 332
362 333
363 7. file: warning: objtool: func()+0x5c: stack 334 7. file: warning: objtool: func()+0x5c: stack state mismatch
364 335
365 The instruction's frame pointer state is in 336 The instruction's frame pointer state is inconsistent, depending on
366 which execution path was taken to reach the 337 which execution path was taken to reach the instruction.
367 338
368 Make sure that, when CONFIG_FRAME_POINTER i 339 Make sure that, when CONFIG_FRAME_POINTER is enabled, the function
369 pushes and sets up the frame pointer (for x 340 pushes and sets up the frame pointer (for x86_64, this means rbp) at
370 the beginning of the function and pops it a 341 the beginning of the function and pops it at the end of the function.
371 Also make sure that no other code in the fu 342 Also make sure that no other code in the function touches the frame
372 pointer. 343 pointer.
373 344
374 Another possibility is that the code has so 345 Another possibility is that the code has some asm or inline asm which
375 does some unusual things to the stack or th 346 does some unusual things to the stack or the frame pointer. In such
376 cases it's probably appropriate to use the 347 cases it's probably appropriate to use the unwind hint macros in
377 asm/unwind_hints.h. 348 asm/unwind_hints.h.
378 349
379 350
380 8. file.o: warning: objtool: funcA() falls thr 351 8. file.o: warning: objtool: funcA() falls through to next function funcB()
381 352
382 This means that funcA() doesn't end with a 353 This means that funcA() doesn't end with a return instruction or an
383 unconditional jump, and that objtool has de 354 unconditional jump, and that objtool has determined that the function
384 can fall through into the next function. T 355 can fall through into the next function. There could be different
385 reasons for this: 356 reasons for this:
386 357
387 1) funcA()'s last instruction is a call to 358 1) funcA()'s last instruction is a call to a "noreturn" function like
388 panic(). In this case the noreturn func 359 panic(). In this case the noreturn function needs to be added to
389 objtool's hard-coded global_noreturns ar 360 objtool's hard-coded global_noreturns array. Feel free to bug the
390 objtool maintainer, or you can submit a 361 objtool maintainer, or you can submit a patch.
391 362
392 2) funcA() uses the unreachable() annotatio 363 2) funcA() uses the unreachable() annotation in a section of code
393 that is actually reachable. 364 that is actually reachable.
394 365
395 3) If funcA() calls an inline function, the 366 3) If funcA() calls an inline function, the object code for funcA()
396 might be corrupt due to a gcc bug. For 367 might be corrupt due to a gcc bug. For more details, see:
397 https://gcc.gnu.org/bugzilla/show_bug.cg 368 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
398 369
399 9. file.o: warning: objtool: funcA() call to f 370 9. file.o: warning: objtool: funcA() call to funcB() with UACCESS enabled
400 371
401 This means that an unexpected call to a non 372 This means that an unexpected call to a non-whitelisted function exists
402 outside of arch-specific guards. 373 outside of arch-specific guards.
403 X86: SMAP (stac/clac): __uaccess_begin()/__ 374 X86: SMAP (stac/clac): __uaccess_begin()/__uaccess_end()
404 ARM: PAN: uaccess_enable()/uaccess_disable( 375 ARM: PAN: uaccess_enable()/uaccess_disable()
405 376
406 These functions should be called to denote 377 These functions should be called to denote a minimal critical section around
407 access to __user variables. See also: https 378 access to __user variables. See also: https://lwn.net/Articles/517475/
408 379
409 The intention of the warning is to prevent 380 The intention of the warning is to prevent calls to funcB() from eventually
410 calling schedule(), potentially leaking the 381 calling schedule(), potentially leaking the AC flags state, and not
411 restoring them correctly. 382 restoring them correctly.
412 383
413 It also helps verify that there are no unex 384 It also helps verify that there are no unexpected calls to funcB() which may
414 access user space pages with protections ag 385 access user space pages with protections against doing so disabled.
415 386
416 To fix, either: 387 To fix, either:
417 1) remove explicit calls to funcB() from fu 388 1) remove explicit calls to funcB() from funcA().
418 2) add the correct guards before and after 389 2) add the correct guards before and after calls to low level functions like
419 __get_user_size()/__put_user_size(). 390 __get_user_size()/__put_user_size().
420 3) add funcB to uaccess_safe_builtin whitel 391 3) add funcB to uaccess_safe_builtin whitelist in tools/objtool/check.c, if
421 funcB obviously does not call schedule() 392 funcB obviously does not call schedule(), and is marked notrace (since
422 function tracing inserts additional call 393 function tracing inserts additional calls, which is not obvious from the
423 sources). 394 sources).
424 395
425 10. file.o: warning: func()+0x5c: stack layout 396 10. file.o: warning: func()+0x5c: stack layout conflict in alternatives
426 397
427 This means that in the use of the alternat 398 This means that in the use of the alternative() or ALTERNATIVE()
428 macro, the code paths have conflicting mod 399 macro, the code paths have conflicting modifications to the stack.
429 The problem is that there is only one ORC 400 The problem is that there is only one ORC unwind table, which means
430 that the ORC unwind entries must be consis 401 that the ORC unwind entries must be consistent for all possible
431 instruction boundaries regardless of which 402 instruction boundaries regardless of which code has been patched.
432 This limitation can be overcome by massagi 403 This limitation can be overcome by massaging the alternatives with
433 NOPs to shift the stack changes around so 404 NOPs to shift the stack changes around so they no longer conflict.
434 405
435 11. file.o: warning: unannotated intra-functio 406 11. file.o: warning: unannotated intra-function call
436 407
437 This warning means that a direct call is do 408 This warning means that a direct call is done to a destination which
438 is not at the beginning of a function. If t 409 is not at the beginning of a function. If this is a legit call, you
439 can remove this warning by putting the ANNO 410 can remove this warning by putting the ANNOTATE_INTRA_FUNCTION_CALL
440 directive right before the call. 411 directive right before the call.
441 <<
442 12. file.o: warning: func(): not an indirect c <<
443 <<
444 This means that objtool is running with --i <<
445 to be an indirect call target is not. In pa <<
446 init_module() or cleanup_module() if a modu <<
447 names and does not use module_init() / modu <<
448 them. <<
449 412
450 413
451 If the error doesn't seem to make sense, it co 414 If the error doesn't seem to make sense, it could be a bug in objtool.
452 Feel free to ask the objtool maintainer for he 415 Feel free to ask the objtool maintainer for help.
453 416
454 417
455 Adding exceptions 418 Adding exceptions
456 ----------------- 419 -----------------
457 420
458 If you _really_ need objtool to ignore somethi 421 If you _really_ need objtool to ignore something, and are 100% sure
459 that it won't affect kernel stack traces, you 422 that it won't affect kernel stack traces, you can tell objtool to
460 ignore it: 423 ignore it:
461 424
462 - To skip validation of a function, use the ST 425 - To skip validation of a function, use the STACK_FRAME_NON_STANDARD
463 macro. 426 macro.
464 427
465 - To skip validation of a file, add 428 - To skip validation of a file, add
466 429
467 OBJECT_FILES_NON_STANDARD_filename.o := y 430 OBJECT_FILES_NON_STANDARD_filename.o := y
468 431
469 to the Makefile. 432 to the Makefile.
470 433
471 - To skip validation of a directory, add 434 - To skip validation of a directory, add
472 435
473 OBJECT_FILES_NON_STANDARD := y 436 OBJECT_FILES_NON_STANDARD := y
474 437
475 to the Makefile. 438 to the Makefile.
476 439
477 NOTE: OBJECT_FILES_NON_STANDARD doesn't work f 440 NOTE: OBJECT_FILES_NON_STANDARD doesn't work for link time validation of
478 vmlinux.o or a linked module. So it should on 441 vmlinux.o or a linked module. So it should only be used for files which
479 aren't linked into vmlinux or a module. 442 aren't linked into vmlinux or a module.
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