Re: [RFC PATCH v15 0/6] arm64: Reorganize the unwinder and implement stack trace reliability checks

Madhavan T. Venkataraman
2022-06-17T21:11:10Z

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