Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+===================
+Reliable Stacktrace
+===================
+
+This document outlines basic information about reliable stacktracing.
+
+.. Table of Contents:
+
+.. contents:: :local:
+
+1. Introduction
+===============
+
+The kernel livepatch consistency model relies on accurately identifying which
+functions may have live state and therefore may not be safe to patch. One way
+to identify which functions are live is to use a stacktrace.
+
+Existing stacktrace code may not always give an accurate picture of all
+functions with live state, and best-effort approaches which can be helpful for
+debugging are unsound for livepatching. Livepatching depends on architectures
+to provide a *reliable* stacktrace which ensures it never omits any live
+functions from a trace.
+
+
+2. Requirements
+===============
+
+Architectures must implement one of the reliable stacktrace functions.
+Architectures using CONFIG_ARCH_STACKWALK must implement
+'arch_stack_walk_reliable', and other architectures must implement
+'save_stack_trace_tsk_reliable'.
+
+Principally, the reliable stacktrace function must ensure that either:
+
+* The trace includes all functions that the task may be returned to, and the
+  return code is zero to indicate that the trace is reliable.
+
+* The return code is non-zero to indicate that the trace is not reliable.
+
+.. note::
+   In some cases it is legitimate to omit specific functions from the trace,
+   but all other functions must be reported. These cases are described in
+   futher detail below.
+
+Secondly, the reliable stacktrace function must be robust to cases where
+the stack or other unwind state is corrupt or otherwise unreliable. The
+function should attempt to detect such cases and return a non-zero error
+code, and should not get stuck in an infinite loop or access memory in
+an unsafe way.  Specific cases are described in further detail below.
+
+
+3. Compile-time analysis
+========================
+
+To ensure that kernel code can be correctly unwound in all cases,
+architectures may need to verify that code has been compiled in a manner
+expected by the unwinder. For example, an unwinder may expect that
+functions manipulate the stack pointer in a limited way, or that all
+functions use specific prologue and epilogue sequences. Architectures
+with such requirements should verify the kernel compilation using
+objtool.
+
+In some cases, an unwinder may require metadata to correctly unwind.
+Where necessary, this metadata should be generated at build time using
+objtool.
+
+
+4. Considerations
+=================
+
+The unwinding process varies across architectures, their respective procedure
+call standards, and kernel configurations. This section describes common
+details that architectures should consider.
+
+4.1 Identifying successful termination
+--------------------------------------
+
+Unwinding may terminate early for a number of reasons, including:
+
+* Stack or frame pointer corruption.
+
+* Missing unwind support for an uncommon scenario, or a bug in the unwinder.
+
+* Dynamically generated code (e.g. eBPF) or foreign code (e.g. EFI runtime
+  services) not following the conventions expected by the unwinder.
+
+To ensure that this does not result in functions being omitted from the trace,
+even if not caught by other checks, it is strongly recommended that
+architectures verify that a stacktrace ends at an expected location, e.g.
+
+* Within a specific function that is an entry point to the kernel.
+
+* At a specific location on a stack expected for a kernel entry point.
+
+* On a specific stack expected for a kernel entry point (e.g. if the
+  architecture has separate task and IRQ stacks).
+
+4.2 Identifying unwindable code
+-------------------------------
+
+Unwinding typically relies on code following specific conventions (e.g.
+manipulating a frame pointer), but there can be code which may not follow these
+conventions and may require special handling in the unwinder, e.g.
+
+* Exception vectors and entry assembly.
+
+* Procedure Linkage Table (PLT) entries and veneer functions.
+
+* Trampoline assembly (e.g. ftrace, kprobes).
+
+* Dynamically generated code (e.g. eBPF, optprobe trampolines).
+
+* Foreign code (e.g. EFI runtime services).
+
+To ensure that such cases do not result in functions being omitted from a
+trace, it is strongly recommended that architectures positively identify code
+which is known to be reliable to unwind from, and reject unwinding from all
+other code.
+
+Kernel code including modules and eBPF can be distinguished from foreign code
+using '__kernel_text_address()'. Checking for this also helps to detect stack
+corruption.
+
+There are several ways an architecture may identify kernel code which is deemed
+unreliable to unwind from, e.g.
+
+* Placing such code into special linker sections, and rejecting unwinding from
+  any code in these sections.
+
+* Identifying specific portions of code using bounds information.
+
+4.3 Unwinding across interrupts and exceptions
+----------------------------------------------
+
+At function call boundaries the stack and other unwind state is expected to be
+in a consistent state suitable for reliable unwinding, but this may not be the
+case part-way through a function. For example, during a function prologue or
+epilogue a frame pointer may be transiently invalid, or during the function
+body the return address may be held in an arbitrary general purpose register.
+For some architectures this may change at runtime as a result of dynamic
+instrumentation.
+
+If an interrupt or other exception is taken while the stack or other unwind
+state is in an inconsistent state, it may not be possible to reliably unwind,
+and it may not be possible to identify whether such unwinding will be reliable.
+See below for examples.
+
+Architectures which cannot identify when it is reliable to unwind such cases
+(or where it is never reliable) must reject unwinding across exception
+boundaries. Note that it may be reliable to unwind across certain
+exceptions (e.g. IRQ) but unreliable to unwind across other exceptions
+(e.g. NMI).
+
+Architectures which can identify when it is reliable to unwind such cases (or
+have no such cases) should attempt to unwind across exception boundaries, as
+doing so can prevent unnecessarily stalling livepatch consistency checks and
+permits livepatch transitions to complete more quickly.
+
+4.4 Rewriting of return addresses
+---------------------------------
+
+Some trampolines temporarily modify the return address of a function in order
+to intercept when that function returns with a return trampoline, e.g.
+
+* An ftrace trampoline may modify the return address so that function graph
+  tracing can intercept returns.
+
+* A kprobes (or optprobes) trampoline may modify the return address so that
+  kretprobes can intercept returns.
+
+When this happens, the original return address will not be in its usual
+location. For trampolines which are not subject to live patching, where an
+unwinder can reliably determine the original return address and no unwind state
+is altered by the trampoline, the unwinder may report the original return
+address in place of the trampoline and report this as reliable. Otherwise, an
+unwinder must report these cases as unreliable.
+
+Special care is required when identifying the original return address, as this
+information is not in a consistent location for the duration of the entry
+trampoline or return trampoline. For example, considering the x86_64
+'return_to_handler' return trampoline:
+
+.. code-block:: none
+
+   SYM_CODE_START(return_to_handler)
+           UNWIND_HINT_EMPTY
+           subq  $24, %rsp
+
+           /* Save the return values */
+           movq %rax, (%rsp)
+           movq %rdx, 8(%rsp)
+           movq %rbp, %rdi
+
+           call ftrace_return_to_handler
+
+           movq %rax, %rdi
+           movq 8(%rsp), %rdx
+           movq (%rsp), %rax
+           addq $24, %rsp
+           JMP_NOSPEC rdi
+   SYM_CODE_END(return_to_handler)
+
+While the traced function runs its return address on the stack points to
+the start of return_to_handler, and the original return address is stored in
+the task's cur_ret_stack. During this time the unwinder can find the return
+address using ftrace_graph_ret_addr().
+
+When the traced function returns to return_to_handler, there is no longer a
+return address on the stack, though the original return address is still stored
+in the task's cur_ret_stack. Within ftrace_return_to_handler(), the original
+return address is removed from cur_ret_stack and is transiently moved
+arbitrarily by the compiler before being returned in rax. The return_to_handler
+trampoline moves this into rdi before jumping to it.
+
+Architectures might not always be able to unwind such sequences, such as when
+ftrace_return_to_handler() has removed the address from cur_ret_stack, and the
+location of the return address cannot be reliably determined.
+
+It is recommended that architectures unwind cases where return_to_handler has
+not yet been returned to, but architectures are not required to unwind from the
+middle of return_to_handler and can report this as unreliable. Architectures
+are not required to unwind from other trampolines which modify the return
+address.
+
+4.5 Obscuring of return addresses
+---------------------------------
+
+Some trampolines do not rewrite the return address in order to intercept
+returns, but do transiently clobber the return address or other unwind state.
+
+For example, the x86_64 implementation of optprobes patches the probed function
+with a JMP instruction which targets the associated optprobe trampoline. When
+the probe is hit, the CPU will branch to the optprobe trampoline, and the
+address of the probed function is not held in any register or on the stack.
+
+Similarly, the arm64 implementation of DYNAMIC_FTRACE_WITH_REGS patches traced
+functions with the following:
+
+.. code-block:: none
+
+   MOV X9, X30
+   BL <trampoline>
+
+The MOV saves the link register (X30) into X9 to preserve the return address
+before the BL clobbers the link register and branches to the trampoline. At the
+start of the trampoline, the address of the traced function is in X9 rather
+than the link register as would usually be the case.
+
+Architectures must either ensure that unwinders either reliably unwind
+such cases, or report the unwinding as unreliable.
+
+4.6 Link register unreliability
+-------------------------------
+
+On some other architectures, 'call' instructions place the return address into a
+link register, and 'return' instructions consume the return address from the
+link register without modifying the register. On these architectures software
+must save the return address to the stack prior to making a function call. Over
+the duration of a function call, the return address may be held in the link
+register alone, on the stack alone, or in both locations.
+
+Unwinders typically assume the link register is always live, but this
+assumption can lead to unreliable stack traces. For example, consider the
+following arm64 assembly for a simple function:
+
+.. code-block:: none
+
+   function:
+           STP X29, X30, [SP, -16]!
+           MOV X29, SP
+           BL <other_function>
+           LDP X29, X30, [SP], #16
+           RET
+
+At entry to the function, the link register (x30) points to the caller, and the
+frame pointer (X29) points to the caller's frame including the caller's return
+address. The first two instructions create a new stackframe and update the
+frame pointer, and at this point the link register and the frame pointer both
+describe this function's return address. A trace at this point may describe
+this function twice, and if the function return is being traced, the unwinder
+may consume two entries from the fgraph return stack rather than one entry.
+
+The BL invokes 'other_function' with the link register pointing to this
+function's LDR and the frame pointer pointing to this function's stackframe.
+When 'other_function' returns, the link register is left pointing at the BL,
+and so a trace at this point could result in 'function' appearing twice in the
+backtrace.
+
+Similarly, a function may deliberately clobber the LR, e.g.
+
+.. code-block:: none
+
+   caller:
+           STP X29, X30, [SP, -16]!
+           MOV X29, SP
+           ADR LR, <callee>
+           BLR LR
+           LDP X29, X30, [SP], #16
+           RET
+
+The ADR places the address of 'callee' into the LR, before the BLR branches to
+this address. If a trace is made immediately after the ADR, 'callee' will
+appear to be the parent of 'caller', rather than the child.
+
+Due to cases such as the above, it may only be possible to reliably consume a
+link register value at a function call boundary. Architectures where this is
+the case must reject unwinding across exception boundaries unless they can
+reliably identify when the LR or stack value should be used (e.g. using
+metadata generated by objtool).