Re: [PATCH 2/2] arm64: stacktrace: Add skip when task == current

On Thu, Mar 18, 2021 at 05:12:07PM +0000, Mark Rutland wrote:

On Thu, Mar 18, 2021 at 04:17:24PM +0000, Catalin Marinas wrote:

quoted

On Wed, Mar 17, 2021 at 07:34:16PM +0000, Mark Rutland wrote:

quoted

On Wed, Mar 17, 2021 at 06:36:36PM +0000, Catalin Marinas wrote:

quoted

On Wed, Mar 17, 2021 at 02:20:50PM +0000, Chen Jun wrote:

quoted

On ARM64, cat /sys/kernel/debug/page_owner, all pages return the same
stack:
 stack_trace_save+0x4c/0x78
 register_early_stack+0x34/0x70
 init_page_owner+0x34/0x230
 page_ext_init+0x1bc/0x1dc

The reason is that:
check_recursive_alloc always return 1 because that
entries[0] is always equal to ip (__set_page_owner+0x3c/0x60).

The root cause is that:
commit 5fc57df2f6fd ("arm64: stacktrace: Convert to ARCH_STACKWALK")
make the save_trace save 2 more entries.

Add skip in arch_stack_walk when task == current.

Fixes: 5fc57df2f6fd ("arm64: stacktrace: Convert to ARCH_STACKWALK")
Signed-off-by: Chen Jun <redacted>
---
 arch/arm64/kernel/stacktrace.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index ad20981..c26b0ac 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c

@@ -201,11 +201,12 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
 
 	if (regs)
 		start_backtrace(&frame, regs->regs[29], regs->pc);
-	else if (task == current)
+	else if (task == current) {
+		((struct stacktrace_cookie *)cookie)->skip += 2;
 		start_backtrace(&frame,
 				(unsigned long)__builtin_frame_address(0),
 				(unsigned long)arch_stack_walk);
-	else
+	} else
 		start_backtrace(&frame, thread_saved_fp(task),
 				thread_saved_pc(task));

I don't like abusing the cookie here. It's void * as it's meant to be an
opaque type. I'd rather skip the first two frames in walk_stackframe()
instead before invoking fn().

I agree that we shouldn't touch cookie here.

I don't think that it's right to bodge this inside walk_stackframe(),
since that'll add bogus skipping for the case starting with regs in the
current task. If we need a bodge, it has to live in arch_stack_walk()
where we set up the initial unwinding state.

Good point. However, instead of relying on __builtin_frame_address(1),
can we add a 'skip' value to struct stackframe via arch_stack_walk() ->
start_backtrace() that is consumed by walk_stackframe()?

We could, but I'd strongly prefer to use __builtin_frame_address(1) if
we can, as it's much simpler to read and keeps the logic constrained to
the starting function. I'd already hacked that up at:

https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/unwind&id=5811a76c1be1dcea7104a9a771fc2604bc2a90ef

... and I'm fairly confident that this works on arm64.

If it works with both clang and gcc (and various versions), it's cleaner
this way.

If __builtin_frame_address(1) is truly unreliable, then we could just
manually unwind one step within arch_stack_walk() when unwinding
current, which I think is cleaner than spreading this within
walk_stackframe().

I can clean up the commit message and post that as a real patch, if you
like?

Yes, please. Either variant is fine by me, with a preference for
__builtin_frame_address(1) (if we know it works).

quoted

quoted

In another thread, we came to the conclusion that arch_stack_walk()
should start at its parent, and its parent should add any skipping it
requires.

This makes sense.

quoted

Currently, arch_stack_walk() is off-by-one, and we can bodge that by
using __builtin_frame_address(1), though I'm waiting for some compiler
folk to confirm that's sound. Otherwise we need to add an assembly
trampoline to snapshot the FP, which is unfortunastely convoluted.

This report suggests that a caller of arch_stack_walk() is off-by-one
too, which suggests a larger cross-architecture semantic issue. I'll try
to take a look tomorrow.

I don't think the caller is off by one, at least not by the final skip
value. __set_page_owner() wants the trace to start at its caller. The
callee save_stack() in the same file adds a skip of 2.
save_stack_trace() increments the skip before invoking
arch_stack_walk(). So far, this assumes that arch_stack_walk() starts at
its parent, i.e. save_stack_trace().

FWIW, I had only assumed the caller was also off-by-one because the
commit message for this patch said the conversion to ARCH_STACKWALK
added two entries. Have I misunderstood, or is that incorrect?

I think the commit log is incorrect. Prior to the ARCH_STACKWALK
conversion, __save_stack_trace() was skipping 2 since it was creating
the initial stack_trace_data and called from save_stack_trace(). After
the conversion, the start frame is initialised by arch_stack_walk()
which doesn't have any other arch-specific caller it needs to skip.

-- 
Catalin

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