Re: [PATCH v9 00/11] arm64: Reorganize the unwinder and implement stack trace reliability checks

Madhavan T. Venkataraman
2021-10-15T02:53:39Z

My mailer screwed up the threading again. I am having Thunderbird woes.

I will correct and resend as version 10. Please delete version 9 of this
patch series.

I profusely apologize for this annoyance.

Madhavan

On 10/14/21 9:34 PM, madvenka@linux.microsoft.com wrote:
> From: "Madhavan T. Venkataraman" <madvenka@linux.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() or start_backtrace()
> and walk_stackframe(). Convert all of them to use arch_stack_walk().
> This makes maintenance easier.
>
> This means that arch_stack_walk() needs to be always defined. So,
> select CONFIG_STACKTRACE in the ARM64 Kconfig file.
>
> Consolidate the unwinder
> ========================
>
> Currently, start_backtrace() and walk_stackframe() are called separately.
> There is no need to do that. Move the call to start_backtrace() into
> walk_stackframe() so that walk_stackframe() is the only unwinder function
> a consumer needs to call.
>
> The consumers of walk_stackframe() are arch_stack_walk() and
> arch_stack_walk_reliable().
>
> Rename unwinder functions
> =========================
>
> Rename unwinder functions to unwind*() similar to other architectures
> for naming consistency.
>
> start_backtrace() ==> unwind_start()
> unwind_frame() ==> unwind_next()
> walk_stackframe() ==> unwind()
>
> Annotate unwinder functions
> ===========================
>
> 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
> ==========================
>
> Redefine the unwinder loop and make it similar to other architectures.
> Define the following:
>
> unwind_start(&frame, fp, pc);
> while (unwind_continue(task, &frame, consume_entry, cookie))
> unwind_next(task, &frame);
>
> unwind_continue()
> This new function 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_continue(). So, unwind_next() assumes that the fp is valid.
>
> - unwind_frame() used to return an error value. unwind_next() only
> sets an internal flag "failed" to indicate that an error was
> encountered. This flag is checked by unwind_continue().
>
> 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.
>
> unwind_check_reliability() will be called for every frame. That is, in
> unwind_start() as well as unwind_next().
>
> 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.
>
> 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.
>
> 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().
> ---
> Changelog:
>
> v9:
> 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
> ================================
>
> 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 (11):
> arm64: Select STACKTRACE in arch/arm64/Kconfig
> arm64: Make perf_callchain_kernel() use arch_stack_walk()
> arm64: Make get_wchan() use arch_stack_walk()
> arm64: Make return_address() use arch_stack_walk()
> arm64: Make dump_stacktrace() use arch_stack_walk()
> arm64: Make profile_pc() use arch_stack_walk()
> arm64: Call stack_backtrace() only from within walk_stackframe()
> arm64: Rename unwinder functions, prevent them from being traced and
> kprobed
> 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
>
> arch/arm64/Kconfig | 1 +
> arch/arm64/include/asm/linkage.h | 12 ++
> arch/arm64/include/asm/sections.h | 1 +
> arch/arm64/include/asm/stacktrace.h | 12 +-
> arch/arm64/kernel/perf_callchain.c | 8 +-
> arch/arm64/kernel/process.c | 38 ++--
> arch/arm64/kernel/return_address.c | 6 +-
> arch/arm64/kernel/stacktrace.c | 274 +++++++++++++++++++---------
> arch/arm64/kernel/time.c | 22 ++-
> arch/arm64/kernel/vmlinux.lds.S | 10 +
> 10 files changed, 257 insertions(+), 127 deletions(-)
>
>
> base-commit: 36a21d51725af2ce0700c6ebcb6b9594aac658a6
>