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Date:   Fri, 17 Jun 2022 15:50:39 -0500
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Subject: Re: [RFC PATCH v15 0/6] arm64: Reorganize the unwinder and implement
 stack trace reliability checks
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To:     broonie@kernel.org, mark.rutland@arm.com, jpoimboe@redhat.com,
        ardb@kernel.org, nobuta.keiya@fujitsu.com,
        sjitindarsingh@gmail.com, catalin.marinas@arm.com, will@kernel.org,
        jamorris@linux.microsoft.com, linux-arm-kernel@lists.infradead.org,
        live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
References: <ff68fb850d42e1adaa6a0a6c9c258acabb898b24>
 <20220617180219.20352-1-madvenka@linux.microsoft.com>
From:   "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
In-Reply-To: <20220617180219.20352-1-madvenka@linux.microsoft.com>
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Sorry for the word "RFC" in the Subject line. It is my silly script.
Please ignore the word. This is not an RFC.

I will resend with the word deleted from the subject line.

Madhavan

On 6/17/22 13:02, madvenka@linux.microsoft.com wrote:
> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
> 
> I have synced this patch series to v5.19-rc2.
> I have also removed the following patch.
> 
> 	[PATCH v14 7/7] arm64: Select HAVE_RELIABLE_STACKTRACE
> 
> as HAVE_RELIABLE_STACKTRACE depends on STACK_VALIDATION which is not present
> yet. This patch will be added in the future once Objtool is enhanced to
> provide stack validation in some form.
> 
> Split unwind_init()
> ===================
> 
> Unwind initialization has 3 cases. Accordingly, define 3 separate init
> functions as follows:
> 
> 	- unwind_init_from_regs()
> 	- unwind_init_from_current()
> 	- unwind_init_from_task()
> 
> This makes it easier to understand and add specialized code to each case
> in the future.
> 
> Copy task argument
> ==================
> 
> Copy the task argument passed to arch_stack_walk() to unwind_state so that
> it can be passed to unwind functions via unwind_state rather than as a
> separate argument. The task is a fundamental part of the unwind state.
> 
> Redefine the unwinder loop
> ==========================
> 
> Redefine the unwinder loop and make it simple and somewhat similar to other
> architectures. Define the following:
> 
> 	while (unwind_continue(&state, consume_entry, cookie))
> 		unwind_next(&state);
> 
> unwind_continue()
> 	This new function implements checks to determine whether the
> 	unwind should continue or terminate.
> 
> Reliability checks
> ==================
> 
> There are some kernel features and conditions that make a stack trace
> unreliable. Callers may require the unwinder to detect these cases.
> E.g., livepatch.
> 
> Introduce a new function called unwind_check_reliability() that will detect
> these cases and set a boolean "reliable" in the stackframe. Call
> unwind_check_reliability() for every frame.
> 
> Introduce the first reliability check in unwind_check_reliability() - If
> a return PC is not a valid kernel text address, consider the stack
> trace unreliable. It could be some generated code.
> 
> Other reliability checks will be added in the future.
> 
> Make unwind() return a boolean to indicate reliability of the stack trace.
> 
> SYM_CODE check
> ==============
> 
> This is the second reliability check implemented.
> 
> SYM_CODE functions do not follow normal calling conventions. They cannot
> be unwound reliably using the frame pointer. Collect the address ranges
> of these functions in a special section called "sym_code_functions".
> 
> In unwind_check_reliability(), check the return PC against these ranges. If
> a match is found, then mark the stack trace unreliable.
> 
> Last stack frame
> ================
> 
> If a SYM_CODE function occurs in the very last frame in the stack trace,
> then the stack trace is not considered unreliable. This is because there
> is no more unwinding to do. Examples:
> 
> 	- EL0 exception stack traces end in the top level EL0 exception
> 	  handlers.
> 
> 	- All kernel thread stack traces end in ret_from_fork().
> 
> arch_stack_walk_reliable()
> ==========================
> 
> Introduce arch_stack_walk_reliable() for ARM64. This works like
> arch_stack_walk() except that it returns an error if the stack trace is
> found to be unreliable.
> 
> Until all of the reliability checks are in place in
> unwind_check_reliability(), arch_stack_walk_reliable() may not be used by
> livepatch. But it may be used by debug and test code.
> 
> HAVE_RELIABLE_STACKTRACE
> ========================
> 
> Select this config for arm64. However, make it conditional on
> STACK_VALIDATION. When objtool is enhanced to implement stack
> validation for arm64, STACK_VALIDATION will be defined.
> 
> ---
> Changelog:
> v15:
> 	From Mark Brown:
> 
> 	- Sync this patch series to v5.19-rc2.
> 
> 	From Madhavan T. Venkataraman:
> 
> 	- Remove the following patch from the series:
> 
> 	[PATCH v14 7/7] arm64: Select HAVE_RELIABLE_STACKTRACE
> 
> 	  as HAVE_RELIABLE_STACKTRACE depends on STACK_VALIDATION which is
> 	  not present yet. This patch will be added in the future once
> 	  Objtool is enhanced to provide stack validation in some form.
> 
> v14:
> 	From Mark Rutland, Mark Brown:
> 
> 	- Add requirements for the three helper functions that init a stack
> 	  trace.
> 
> 	From Mark Rutland:
> 
> 	- Change the comment for the task field in struct stackframe.
> 
> 	- Hard code the task to current in unwind_init_from_regs(). Add a
> 	  sanity check task == current.
> 
> 	- Rename unwind_init_from_current() to unwind_init_from_caller().
> 
> 	- Remove task argument from unwind_init_from_caller().
> 
> 	From Mark Brown:
> 
> 	- Reviewed-By: for:
> 
> 	[PATCH v13 05/11] arm64: Copy the task argument to unwind_state
> v13:
> 	From Mark Brown:
> 
> 	- Reviewed-by for the following:
> 
> 	[PATCH v12 03/10] arm64: Rename stackframe to unwind_state
> 	[PATCH v11 05/10] arm64: Copy unwind arguments to unwind_state
> 	[PATCH v11 07/10] arm64: Introduce stack trace reliability checks
> 	                  in the unwinder
> 	[PATCH v11 5/5] arm64: Create a list of SYM_CODE functions, check
> 	                return PC against list
> 
> 	From Mark Rutland:
> 
> 	- Reviewed-by for the following:
> 
> 	[PATCH v12 01/10] arm64: Remove NULL task check from unwind_frame()
> 	[PATCH v12 02/10] arm64: Rename unwinder functions
> 	[PATCH v12 03/10] arm64: Rename stackframe to unwind_state
> 
> 	- For each of the 3 cases of unwind initialization, have a separate
> 	  init function. Call the common init from each of these init
> 	  functions rather than call it separately.
> 
> 	- Only copy the task argument to arch_stack_walk() into
> 	  unwind state. Pass the rest of the arguments as arguments to
> 	  unwind functions.
> 
> v12:
> 	From Mark Brown:
> 
> 	- Reviewed-by for the following:
> 
> 	[PATCH v11 1/5] arm64: Call stack_backtrace() only from within
> 	                walk_stackframe()
> 	[PATCH v11 2/5] arm64: Rename unwinder functions
> 	[PATCH v11 3/5] arm64: Make the unwind loop in unwind() similar to
> 	                other architectures
> 	[PATCH v11 5/5] arm64: Create a list of SYM_CODE functions, check
> 	                return PC against list
> 
> 	- Add an extra patch at the end to select HAVE_RELIABLE_STACKTRACE
> 	  just as a place holder for the review. I have added it and made
> 	  it conditional on STACK_VALIDATION which has not yet been
> 	  implemented.
> 
> 	- Mark had a concern about the code for the check for the final
> 	  frame being repeated in two places. I have now added a new
> 	  field called "final_fp" in struct stackframe which I compute
> 	  once in stacktrace initialization. I have added an explicit
> 	  comment that the stacktrace must terminate at the final_fp.
> 
> 	- Place the implementation of arch_stack_walk_reliable() in a
> 	  separate patch after all the reliability checks have been
> 	  implemented.
> 
> 	From Mark Rutland:
> 
> 	- Place the removal of the NULL task check in unwind_frame() in
> 	  a separate patch.
> 
> 	- Add a task field to struct stackframe so the task pointer can be
> 	  passed around via the frame instead of as a separate argument. I have
> 	  taken this a step further by copying all of the arguments to
> 	  arch_stack_walk() into struct stackframe so that only that
> 	  struct needs to be passed to unwind functions.
> 
> 	- Rename start_backtrace() to unwind_init() instead of unwind_start().
> 
> 	- Acked-by for the following:
> 
> 	[PATCH v11 2/5] arm64: Rename unwinder functions
> 
> 	- Rename "struct stackframe" to "struct unwind_state".
> 
> 	- Define separate inline functions for initializing the starting
> 	  FP and PC from regs, or caller, or blocked task. Don't merge
> 	  unwind_init() into unwind().
> 
> v11:
> 	From Mark Rutland:
> 
> 	- Peter Zijlstra has submitted patches that make ARCH_STACKWALK
> 	  independent of STACKTRACE. Mark Rutland extracted some of the
> 	  patches from my v10 series and added his own patches and comments,
> 	  rebased it on top of Peter's changes and submitted the series.
> 	  
> 	  So, I have rebased the rest of the patches from v10 on top of
> 	  Mark Rutland's changes.
> 
> 	- Split the renaming of the unwinder functions and annotating them
> 	  with notrace and NOKPROBE_SYMBOL(). Also, there is currently no
> 	  need to annotate unwind_start() as its caller is already annotated
> 	  properly. So, I am removing the annotation patch from the series.
> 	  This can be done separately later if deemed necessary. Similarly,
> 	  I have removed the annotations from unwind_check_reliability() and
> 	  unwind_continue().
> 
> 	From Nobuta Keiya:
> 
> 	- unwind_start() should check for final frame and not mark the
> 	  final frame unreliable.
> 
> v9, v10:
> 	- v9 had a threading problem. So, I resent it as v10.
> 
> 	From me:
> 
> 	- Removed the word "RFC" from the subject line as I believe this
> 	  is mature enough to be a regular patch.
> 
> 	From Mark Brown, Mark Rutland:
> 
> 	- Split the patches into smaller, self-contained ones.
> 
> 	- Always enable STACKTRACE so that arch_stack_walk() is always
> 	  defined.
> 
> 	From Mark Rutland:
> 
> 	- Update callchain_trace() take the return value of
> 	  perf_callchain_store() into acount.
> 
> 	- Restore get_wchan() behavior to the original code.
> 
> 	- Simplify an if statement in dump_backtrace().
> 
> 	From Mark Brown:
> 
> 	- Do not abort the stack trace on the first unreliable frame.
> 
> 	
> v8:
> 	- Synced to v5.14-rc5.
> 
> 	From Mark Rutland:
> 
> 	- Make the unwinder loop similar to other architectures.
> 
> 	- Keep details to within the unwinder functions and return a simple
> 	  boolean to the caller.
> 
> 	- Convert some of the current code that contains unwinder logic to
> 	  simply use arch_stack_walk(). I have converted all of them.
> 
> 	- Do not copy sym_code_functions[]. Just place it in rodata for now.
> 
> 	- Have the main loop check for termination conditions rather than
> 	  having unwind_frame() check for them. In other words, let
> 	  unwind_frame() assume that the fp is valid.
> 
> 	- Replace the big comment for SYM_CODE functions with a shorter
> 	  comment.
> 
> 		/*
> 		 * As SYM_CODE functions don't follow the usual calling
> 		 * conventions, we assume by default that any SYM_CODE function
> 		 * cannot be unwound reliably.
> 		 *
> 		 * Note that this includes:
> 		 *
> 		 * - Exception handlers and entry assembly
> 		 * - Trampoline assembly (e.g., ftrace, kprobes)
> 		 * - Hypervisor-related assembly
> 		 * - Hibernation-related assembly
> 		 * - CPU start-stop, suspend-resume assembly
> 		 * - Kernel relocation assembly
> 		 */
> 
> v7:
> 	The Mailer screwed up the threading on this. So, I have resent this
> 	same series as version 8 with proper threading to avoid confusion.
> v6:
> 	From Mark Rutland:
> 
> 	- The per-frame reliability concept and flag are acceptable. But more
> 	  work is needed to make the per-frame checks more accurate and more
> 	  complete. E.g., some code reorg is being worked on that will help.
> 
> 	  I have now removed the frame->reliable flag and deleted the whole
> 	  concept of per-frame status. This is orthogonal to this patch series.
> 	  Instead, I have improved the unwinder to return proper return codes
> 	  so a caller can take appropriate action without needing per-frame
> 	  status.
> 
> 	- Remove the mention of PLTs and update the comment.
> 
> 	  I have replaced the comment above the call to __kernel_text_address()
> 	  with the comment suggested by Mark Rutland.
> 
> 	Other comments:
> 
> 	- Other comments on the per-frame stuff are not relevant because
> 	  that approach is not there anymore.
> 
> v5:
> 	From Keiya Nobuta:
> 	
> 	- The term blacklist(ed) is not to be used anymore. I have changed it
> 	  to unreliable. So, the function unwinder_blacklisted() has been
> 	  changed to unwinder_is_unreliable().
> 
> 	From Mark Brown:
> 
> 	- Add a comment for the "reliable" flag in struct stackframe. The
> 	  reliability attribute is not complete until all the checks are
> 	  in place. Added a comment above struct stackframe.
> 
> 	- Include some of the comments in the cover letter in the actual
> 	  code so that we can compare it with the reliable stack trace
> 	  requirements document for completeness. I have added a comment:
> 
> 	  	- above unwinder_is_unreliable() that lists the requirements
> 		  that are addressed by the function.
> 
> 		- above the __kernel_text_address() call about all the cases
> 		  the call covers.
> 
> v4:
> 	From Mark Brown:
> 
> 	- I was checking the return PC with __kernel_text_address() before
> 	  the Function Graph trace handling. Mark Brown felt that all the
> 	  reliability checks should be performed on the original return PC
> 	  once that is obtained. So, I have moved all the reliability checks
> 	  to after the Function Graph Trace handling code in the unwinder.
> 	  Basically, the unwinder should perform PC translations first (for
> 	  rhe return trampoline for Function Graph Tracing, Kretprobes, etc).
> 	  Then, the reliability checks should be applied to the resulting
> 	  PC.
> 
> 	- Mark said to improve the naming of the new functions so they don't
> 	  collide with existing ones. I have used a prefix "unwinder_" for
> 	  all the new functions.
> 
> 	From Josh Poimboeuf:
> 
> 	- In the error scenarios in the unwinder, the reliable flag in the
> 	  stack frame should be set. Implemented this.
> 
> 	- Some of the other comments are not relevant to the new code as
> 	  I have taken a different approach in the new code. That is why
> 	  I have not made those changes. E.g., Ard wanted me to add the
> 	  "const" keyword to the global section array. That array does not
> 	  exist in v4. Similarly, Mark Brown said to use ARRAY_SIZE() for
> 	  the same array in a for loop.
> 
> 	Other changes:
> 
> 	- Add a new definition for SYM_CODE_END() that adds the address
> 	  range of the function to a special section called
> 	  "sym_code_functions".
> 
> 	- Include the new section under initdata in vmlinux.lds.S.
> 
> 	- Define an early_initcall() to copy the contents of the
> 	  "sym_code_functions" section to an array by the same name.
> 
> 	- Define a function unwinder_blacklisted() that compares a return
> 	  PC against sym_code_sections[]. If there is a match, mark the
> 	  stack trace unreliable. Call this from unwind_frame().
> 
> v3:
> 	- Implemented a sym_code_ranges[] array to contains sections bounds
> 	  for text sections that contain SYM_CODE_*() functions. The unwinder
> 	  checks each return PC against the sections. If it falls in any of
> 	  the sections, the stack trace is marked unreliable.
> 
> 	- Moved SYM_CODE functions from .text and .init.text into a new
> 	  text section called ".code.text". Added this section to
> 	  vmlinux.lds.S and sym_code_ranges[].
> 
> 	- Fixed the logic in the unwinder that handles Function Graph
> 	  Tracer return trampoline.
> 
> 	- Removed all the previous code that handles:
> 		- ftrace entry code for traced function
> 		- special_functions[] array that lists individual functions
> 		- kretprobe_trampoline() special case
> 
> v2
> 	- Removed the terminating entry { 0, 0 } in special_functions[]
> 	  and replaced it with the idiom { /* sentinel */ }.
> 
> 	- Change the ftrace trampoline entry ftrace_graph_call in
> 	  special_functions[] to ftrace_call + 4 and added explanatory
> 	  comments.
> 
> 	- Unnested #ifdefs in special_functions[] for FTRACE.
> 
> v1
> 	- Define a bool field in struct stackframe. This will indicate if
> 	  a stack trace is reliable.
> 
> 	- Implement a special_functions[] array that will be populated
> 	  with special functions in which the stack trace is considered
> 	  unreliable.
> 	
> 	- Using kallsyms_lookup(), get the address ranges for the special
> 	  functions and record them.
> 
> 	- Implement an is_reliable_function(pc). This function will check
> 	  if a given return PC falls in any of the special functions. If
> 	  it does, the stack trace is unreliable.
> 
> 	- Implement check_reliability() function that will check if a
> 	  stack frame is reliable. Call is_reliable_function() from
> 	  check_reliability().
> 
> 	- Before a return PC is checked against special_funtions[], it
> 	  must be validates as a proper kernel text address. Call
> 	  __kernel_text_address() from check_reliability().
> 
> 	- Finally, call check_reliability() from unwind_frame() for
> 	  each stack frame.
> 
> 	- Add EL1 exception handlers to special_functions[].
> 
> 		el1_sync();
> 		el1_irq();
> 		el1_error();
> 		el1_sync_invalid();
> 		el1_irq_invalid();
> 		el1_fiq_invalid();
> 		el1_error_invalid();
> 
> 	- The above functions are currently defined as LOCAL symbols.
> 	  Make them global so that they can be referenced from the
> 	  unwinder code.
> 
> 	- Add FTRACE trampolines to special_functions[]:
> 
> 		ftrace_graph_call()
> 		ftrace_graph_caller()
> 		return_to_handler()
> 
> 	- Add the kretprobe trampoline to special functions[]:
> 
> 		kretprobe_trampoline()
> 
> Previous versions and discussion
> ================================
> 
> v14: https://lore.kernel.org/linux-arm-kernel/20220413140528.3815-1-madvenka@linux.microsoft.com/T/#t
> v13: https://lore.kernel.org/linux-arm-kernel/20220117145608.6781-1-madvenka@linux.microsoft.com/T/#t
> v12: https://lore.kernel.org/linux-arm-kernel/20220103165212.9303-1-madvenka@linux.microsoft.com/T/#m21e86eecb9b8f0831196568f0bf62c3b56f65bf0
> v11: https://lore.kernel.org/linux-arm-kernel/20211123193723.12112-1-madvenka@linux.microsoft.com/T/#t
> v10: https://lore.kernel.org/linux-arm-kernel/4b3d5552-590c-e6a0-866b-9bc51da7bebf@linux.microsoft.com/T/#t
> v9: Mailer screwed up the threading. Sent the same as v10 with proper threading.
> v8: https://lore.kernel.org/linux-arm-kernel/20210812190603.25326-1-madvenka@linux.microsoft.com/
> v7: Mailer screwed up the threading. Sent the same as v8 with proper threading.
> v6: https://lore.kernel.org/linux-arm-kernel/20210630223356.58714-1-madvenka@linux.microsoft.com/
> v5: https://lore.kernel.org/linux-arm-kernel/20210526214917.20099-1-madvenka@linux.microsoft.com/
> v4: https://lore.kernel.org/linux-arm-kernel/20210516040018.128105-1-madvenka@linux.microsoft.com/
> v3: https://lore.kernel.org/linux-arm-kernel/20210503173615.21576-1-madvenka@linux.microsoft.com/
> v2: https://lore.kernel.org/linux-arm-kernel/20210405204313.21346-1-madvenka@linux.microsoft.com/
> v1: https://lore.kernel.org/linux-arm-kernel/20210330190955.13707-1-madvenka@linux.microsoft.com/
> 
> Madhavan T. Venkataraman (6):
>   arm64: Split unwind_init()
>   arm64: Copy the task argument to unwind_state
>   arm64: Make the unwind loop in unwind() similar to other architectures
>   arm64: Introduce stack trace reliability checks in the unwinder
>   arm64: Create a list of SYM_CODE functions, check return PC against
>     list
>   arm64: Introduce arch_stack_walk_reliable()
> 
>  arch/arm64/include/asm/linkage.h  |  11 ++
>  arch/arm64/include/asm/sections.h |   1 +
>  arch/arm64/kernel/stacktrace.c    | 266 +++++++++++++++++++++++++-----
>  arch/arm64/kernel/vmlinux.lds.S   |  10 ++
>  4 files changed, 246 insertions(+), 42 deletions(-)
> 
> 
> base-commit: b13baccc3850ca8b8cccbf8ed9912dbaa0fdf7f3