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

madvenka
2021-10-15T02:34:54Z

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
--
2.25.1