1=================== 2Reliable Stacktrace 3=================== 4 5This document outlines basic information about reliable stacktracing. 6 7.. Table of Contents: 8 9.. contents:: :local: 10 111. Introduction 12=============== 13 14The kernel livepatch consistency model relies on accurately identifying which 15functions may have live state and therefore may not be safe to patch. One way 16to identify which functions are live is to use a stacktrace. 17 18Existing stacktrace code may not always give an accurate picture of all 19functions with live state, and best-effort approaches which can be helpful for 20debugging are unsound for livepatching. Livepatching depends on architectures 21to provide a *reliable* stacktrace which ensures it never omits any live 22functions from a trace. 23 24 252. Requirements 26=============== 27 28Architectures must implement one of the reliable stacktrace functions. 29Architectures using CONFIG_ARCH_STACKWALK must implement 30'arch_stack_walk_reliable', and other architectures must implement 31'save_stack_trace_tsk_reliable'. 32 33Principally, the reliable stacktrace function must ensure that either: 34 35* The trace includes all functions that the task may be returned to, and the 36 return code is zero to indicate that the trace is reliable. 37 38* The return code is non-zero to indicate that the trace is not reliable. 39 40.. note:: 41 In some cases it is legitimate to omit specific functions from the trace, 42 but all other functions must be reported. These cases are described in 43 further detail below. 44 45Secondly, the reliable stacktrace function must be robust to cases where 46the stack or other unwind state is corrupt or otherwise unreliable. The 47function should attempt to detect such cases and return a non-zero error 48code, and should not get stuck in an infinite loop or access memory in 49an unsafe way. Specific cases are described in further detail below. 50 51 523. Compile-time analysis 53======================== 54 55To ensure that kernel code can be correctly unwound in all cases, 56architectures may need to verify that code has been compiled in a manner 57expected by the unwinder. For example, an unwinder may expect that 58functions manipulate the stack pointer in a limited way, or that all 59functions use specific prologue and epilogue sequences. Architectures 60with such requirements should verify the kernel compilation using 61objtool. 62 63In some cases, an unwinder may require metadata to correctly unwind. 64Where necessary, this metadata should be generated at build time using 65objtool. 66 67 684. Considerations 69================= 70 71The unwinding process varies across architectures, their respective procedure 72call standards, and kernel configurations. This section describes common 73details that architectures should consider. 74 754.1 Identifying successful termination 76-------------------------------------- 77 78Unwinding may terminate early for a number of reasons, including: 79 80* Stack or frame pointer corruption. 81 82* Missing unwind support for an uncommon scenario, or a bug in the unwinder. 83 84* Dynamically generated code (e.g. eBPF) or foreign code (e.g. EFI runtime 85 services) not following the conventions expected by the unwinder. 86 87To ensure that this does not result in functions being omitted from the trace, 88even if not caught by other checks, it is strongly recommended that 89architectures verify that a stacktrace ends at an expected location, e.g. 90 91* Within a specific function that is an entry point to the kernel. 92 93* At a specific location on a stack expected for a kernel entry point. 94 95* On a specific stack expected for a kernel entry point (e.g. if the 96 architecture has separate task and IRQ stacks). 97 984.2 Identifying unwindable code 99------------------------------- 100 101Unwinding typically relies on code following specific conventions (e.g. 102manipulating a frame pointer), but there can be code which may not follow these 103conventions and may require special handling in the unwinder, e.g. 104 105* Exception vectors and entry assembly. 106 107* Procedure Linkage Table (PLT) entries and veneer functions. 108 109* Trampoline assembly (e.g. ftrace, kprobes). 110 111* Dynamically generated code (e.g. eBPF, optprobe trampolines). 112 113* Foreign code (e.g. EFI runtime services). 114 115To ensure that such cases do not result in functions being omitted from a 116trace, it is strongly recommended that architectures positively identify code 117which is known to be reliable to unwind from, and reject unwinding from all 118other code. 119 120Kernel code including modules and eBPF can be distinguished from foreign code 121using '__kernel_text_address()'. Checking for this also helps to detect stack 122corruption. 123 124There are several ways an architecture may identify kernel code which is deemed 125unreliable to unwind from, e.g. 126 127* Placing such code into special linker sections, and rejecting unwinding from 128 any code in these sections. 129 130* Identifying specific portions of code using bounds information. 131 1324.3 Unwinding across interrupts and exceptions 133---------------------------------------------- 134 135At function call boundaries the stack and other unwind state is expected to be 136in a consistent state suitable for reliable unwinding, but this may not be the 137case part-way through a function. For example, during a function prologue or 138epilogue a frame pointer may be transiently invalid, or during the function 139body the return address may be held in an arbitrary general purpose register. 140For some architectures this may change at runtime as a result of dynamic 141instrumentation. 142 143If an interrupt or other exception is taken while the stack or other unwind 144state is in an inconsistent state, it may not be possible to reliably unwind, 145and it may not be possible to identify whether such unwinding will be reliable. 146See below for examples. 147 148Architectures which cannot identify when it is reliable to unwind such cases 149(or where it is never reliable) must reject unwinding across exception 150boundaries. Note that it may be reliable to unwind across certain 151exceptions (e.g. IRQ) but unreliable to unwind across other exceptions 152(e.g. NMI). 153 154Architectures which can identify when it is reliable to unwind such cases (or 155have no such cases) should attempt to unwind across exception boundaries, as 156doing so can prevent unnecessarily stalling livepatch consistency checks and 157permits livepatch transitions to complete more quickly. 158 1594.4 Rewriting of return addresses 160--------------------------------- 161 162Some trampolines temporarily modify the return address of a function in order 163to intercept when that function returns with a return trampoline, e.g. 164 165* An ftrace trampoline may modify the return address so that function graph 166 tracing can intercept returns. 167 168* A kprobes (or optprobes) trampoline may modify the return address so that 169 kretprobes can intercept returns. 170 171When this happens, the original return address will not be in its usual 172location. For trampolines which are not subject to live patching, where an 173unwinder can reliably determine the original return address and no unwind state 174is altered by the trampoline, the unwinder may report the original return 175address in place of the trampoline and report this as reliable. Otherwise, an 176unwinder must report these cases as unreliable. 177 178Special care is required when identifying the original return address, as this 179information is not in a consistent location for the duration of the entry 180trampoline or return trampoline. For example, considering the x86_64 181'return_to_handler' return trampoline: 182 183.. code-block:: none 184 185 SYM_CODE_START(return_to_handler) 186 UNWIND_HINT_UNDEFINED 187 subq $24, %rsp 188 189 /* Save the return values */ 190 movq %rax, (%rsp) 191 movq %rdx, 8(%rsp) 192 movq %rbp, %rdi 193 194 call ftrace_return_to_handler 195 196 movq %rax, %rdi 197 movq 8(%rsp), %rdx 198 movq (%rsp), %rax 199 addq $24, %rsp 200 JMP_NOSPEC rdi 201 SYM_CODE_END(return_to_handler) 202 203While the traced function runs its return address on the stack points to 204the start of return_to_handler, and the original return address is stored in 205the task's cur_ret_stack. During this time the unwinder can find the return 206address using ftrace_graph_ret_addr(). 207 208When the traced function returns to return_to_handler, there is no longer a 209return address on the stack, though the original return address is still stored 210in the task's cur_ret_stack. Within ftrace_return_to_handler(), the original 211return address is removed from cur_ret_stack and is transiently moved 212arbitrarily by the compiler before being returned in rax. The return_to_handler 213trampoline moves this into rdi before jumping to it. 214 215Architectures might not always be able to unwind such sequences, such as when 216ftrace_return_to_handler() has removed the address from cur_ret_stack, and the 217location of the return address cannot be reliably determined. 218 219It is recommended that architectures unwind cases where return_to_handler has 220not yet been returned to, but architectures are not required to unwind from the 221middle of return_to_handler and can report this as unreliable. Architectures 222are not required to unwind from other trampolines which modify the return 223address. 224 2254.5 Obscuring of return addresses 226--------------------------------- 227 228Some trampolines do not rewrite the return address in order to intercept 229returns, but do transiently clobber the return address or other unwind state. 230 231For example, the x86_64 implementation of optprobes patches the probed function 232with a JMP instruction which targets the associated optprobe trampoline. When 233the probe is hit, the CPU will branch to the optprobe trampoline, and the 234address of the probed function is not held in any register or on the stack. 235 236Similarly, the arm64 implementation of DYNAMIC_FTRACE_WITH_REGS patches traced 237functions with the following: 238 239.. code-block:: none 240 241 MOV X9, X30 242 BL <trampoline> 243 244The MOV saves the link register (X30) into X9 to preserve the return address 245before the BL clobbers the link register and branches to the trampoline. At the 246start of the trampoline, the address of the traced function is in X9 rather 247than the link register as would usually be the case. 248 249Architectures must either ensure that unwinders either reliably unwind 250such cases, or report the unwinding as unreliable. 251 2524.6 Link register unreliability 253------------------------------- 254 255On some other architectures, 'call' instructions place the return address into a 256link register, and 'return' instructions consume the return address from the 257link register without modifying the register. On these architectures software 258must save the return address to the stack prior to making a function call. Over 259the duration of a function call, the return address may be held in the link 260register alone, on the stack alone, or in both locations. 261 262Unwinders typically assume the link register is always live, but this 263assumption can lead to unreliable stack traces. For example, consider the 264following arm64 assembly for a simple function: 265 266.. code-block:: none 267 268 function: 269 STP X29, X30, [SP, -16]! 270 MOV X29, SP 271 BL <other_function> 272 LDP X29, X30, [SP], #16 273 RET 274 275At entry to the function, the link register (x30) points to the caller, and the 276frame pointer (X29) points to the caller's frame including the caller's return 277address. The first two instructions create a new stackframe and update the 278frame pointer, and at this point the link register and the frame pointer both 279describe this function's return address. A trace at this point may describe 280this function twice, and if the function return is being traced, the unwinder 281may consume two entries from the fgraph return stack rather than one entry. 282 283The BL invokes 'other_function' with the link register pointing to this 284function's LDR and the frame pointer pointing to this function's stackframe. 285When 'other_function' returns, the link register is left pointing at the BL, 286and so a trace at this point could result in 'function' appearing twice in the 287backtrace. 288 289Similarly, a function may deliberately clobber the LR, e.g. 290 291.. code-block:: none 292 293 caller: 294 STP X29, X30, [SP, -16]! 295 MOV X29, SP 296 ADR LR, <callee> 297 BLR LR 298 LDP X29, X30, [SP], #16 299 RET 300 301The ADR places the address of 'callee' into the LR, before the BLR branches to 302this address. If a trace is made immediately after the ADR, 'callee' will 303appear to be the parent of 'caller', rather than the child. 304 305Due to cases such as the above, it may only be possible to reliably consume a 306link register value at a function call boundary. Architectures where this is 307the case must reject unwinding across exception boundaries unless they can 308reliably identify when the LR or stack value should be used (e.g. using 309metadata generated by objtool). 310