Message-ID: <3a71bd4a-dc3c-eb66-6555-2f96877499f4@linux.microsoft.com>
Date:   Thu, 12 Aug 2021 13:31:48 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     mark.rutland@....com, broonie@...nel.org, jpoimboe@...hat.com,
        ardb@...nel.org, nobuta.keiya@...itsu.com,
        sjitindarsingh@...il.com, catalin.marinas@....com, will@...nel.org,
        jmorris@...ei.org, pasha.tatashin@...een.com, jthierry@...hat.com,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v7 0/4] arm64: Reorganize the unwinder and implement
 stack trace reliability checks

The messages are not threaded properly.

I will resend the whole series with proper threading.

I apologize.

Madhavan

On 8/12/21 8:24 AM, madvenka@...ux.microsoft.com wrote:
> From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
> 
> Make all stack walking functions use arch_stack_walk()
> ======================================================
> 
> Currently, there are multiple functions in ARM64 code that walk the
> stack using start_backtrace() and unwind_frame(). Convert all of
> them to use arch_stack_walk(). This makes maintenance easier.
> 
> Reorganize the unwinder code for better consistency and maintenance
> ===================================================================
> 
> Rename unwinder functions to unwind_*() similar to other architectures
> for naming consistency.
> 
> Annotate all of the unwind_*() functions with notrace so they cannot be
> ftraced and NOKPROBE_SYMBOL() so they cannot be kprobed. Ftrace and Kprobe
> code can call the unwinder.
> 
> Redefine the unwinder loop and make it similar to other architectures.
> Define the following:
> 
> 	unwind_start(&frame, task, fp, pc);
> 	while (unwind_consume(&frame, consume_entry, cookie))
> 		unwind_next(&frame);
> 	return !unwind_failed(&frame);
> 
> unwind_start()
> 	Same as the original start_backtrace().
> 
> unwind_consume()
> 	This new function does two things:
> 
> 	- Calls consume_entry() to consume the return PC.
> 
> 	- Implements checks to determine whether the unwind should continue
> 	  or terminate.
> 
> unwind_next()
> 	Same as the original unwind_frame() except:
> 
> 	- the stack trace termination check has been moved from here to
> 	  unwind_consume(). So, unwind_next() assumes that the fp is valid.
> 
> 	- unwind_frame() used to return an error value. This function only
> 	  sets internal state and does not return anything. The state is
> 	  retrieved via a helper. See next.
> 
> unwind_failed()
> 	Return a boolean to indicate whether the stack trace completed
> 	successfully or failed. arch_stack_walk() ignores the return
> 	value. But arch_stack_walk_reliable() in the future will look
> 	at the return value.
> 
> Unwind status
> 	Introduce a new flag called "failed" in struct stackframe. Set this
> 	flag when an error is encountered. If this flag is set, terminate
> 	the unwind. Also, let the unwinder return the status to the caller.
> 
> 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_is_reliable() that will detect
> these cases and return a boolean.
> 
> Introduce a new argument to unwind() called "need_reliable" so a caller
> can tell unwind() that it requires a reliable stack trace. For such a
> caller, any unreliability in the stack trace must be treated as a fatal
> error and the unwind must be aborted.
> 
> Call unwind_is_reliable() from unwind_consume() like this:
> 
> 	if (frame->need_reliable && !unwind_is_reliable(frame)) {
> 		frame->failed = true;
> 		return false;
> 	}
> 
> arch_stack_walk() passes "false" for need_reliable because its callers
> don't care about reliability. arch_stack_walk() is used for debug and
> test purposes.
> 
> Introduce arch_stack_walk_reliable() for ARM64. This works like
> arch_stack_walk() except for two things:
> 
> 	- It passes "true" for need_reliable.
> 
> 	- It returns -EINVAL if unwind() aborts.
> 
> Introduce the first reliability check in unwind_is_reliable() - 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. Until all of the
> checks are in place, arch_stack_walk_reliable() may not be used by
> livepatch. But it may be used by debug and test code.
> 
> SYM_CODE check
> ==============
> 
> 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_is_reliable(), check the return PC against these ranges. If a
> match is found, then consider the stack trace unreliable. This is the
> second reliability check introduced by this work.
> 
> 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().
> ---
> Changelog:
> 
> v7:
> 	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
> 		 */
> 
> 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
> ================================
> 
> 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 (4):
>   arm64: Make all stack walking functions use arch_stack_walk()
>   arm64: Reorganize the unwinder code for better consistency and
>     maintenance
>   arm64: Introduce stack trace reliability checks in the unwinder
>   arm64: Create a list of SYM_CODE functions, check return PC against
>     list
> 
>  arch/arm64/include/asm/linkage.h    |  12 ++
>  arch/arm64/include/asm/sections.h   |   1 +
>  arch/arm64/include/asm/stacktrace.h |  16 +-
>  arch/arm64/kernel/perf_callchain.c  |   5 +-
>  arch/arm64/kernel/process.c         |  39 ++--
>  arch/arm64/kernel/return_address.c  |   6 +-
>  arch/arm64/kernel/stacktrace.c      | 291 ++++++++++++++++++++--------
>  arch/arm64/kernel/time.c            |  22 ++-
>  arch/arm64/kernel/vmlinux.lds.S     |  10 +
>  9 files changed, 277 insertions(+), 125 deletions(-)
> 
> 
> base-commit: 36a21d51725af2ce0700c6ebcb6b9594aac658a6
> 

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