Message-ID: <f3e313cc-57d3-bac1-f4a6-d953c3a026b4@linux.microsoft.com>
Date:   Thu, 12 Aug 2021 13:45:22 -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

My mailer is screwing up.

I will resend the whole series as version 8 instead of version 7 to avoid further
confusion.

Thunderbird, sometimes! Again, I am so sorry.

Madhavan

On 8/12/21 1:31 PM, Madhavan T. Venkataraman wrote:
> 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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