Messages in this thread Patch in this message | | | Date | Thu, 18 Mar 2021 17:12:07 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH 2/2] arm64: stacktrace: Add skip when task == current |
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On Thu, Mar 18, 2021 at 04:17:24PM +0000, Catalin Marinas wrote: > On Wed, Mar 17, 2021 at 07:34:16PM +0000, Mark Rutland wrote: > > On Wed, Mar 17, 2021 at 06:36:36PM +0000, Catalin Marinas wrote: > > > On Wed, Mar 17, 2021 at 02:20:50PM +0000, Chen Jun wrote: > > > > 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 <chenjun102@huawei.com> > > > > --- > > > > 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 __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?
> > 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. > > > 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?
So if this is only off-by-one, I agree it's the same problem.
Thanks, Mark.
> So save_stack_trace() only need to skip 1 and I think that's in line > with the original report where the entries[0] is __set_page_owner(). We > only need to skip one. Another untested quick hack (we should probably > add the skip argument to start_backtrace()): > > diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h > index eb29b1fe8255..0d32d932ac89 100644 > --- a/arch/arm64/include/asm/stacktrace.h > +++ b/arch/arm64/include/asm/stacktrace.h > @@ -56,6 +56,7 @@ struct stackframe { > DECLARE_BITMAP(stacks_done, __NR_STACK_TYPES); > unsigned long prev_fp; > enum stack_type prev_type; > + int skip; > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > int graph; > #endif > @@ -153,6 +154,7 @@ static inline void start_backtrace(struct stackframe *frame, > { > frame->fp = fp; > frame->pc = pc; > + frame->skip = 0; > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > frame->graph = 0; > #endif > diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c > index ad20981dfda4..a89b2ecbf3de 100644 > --- a/arch/arm64/kernel/stacktrace.c > +++ b/arch/arm64/kernel/stacktrace.c > @@ -118,7 +118,9 @@ void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame, > while (1) { > int ret; > > - if (!fn(data, frame->pc)) > + if (frame->skip > 0) > + frame->skip--; > + else if (!fn(data, frame->pc)) > break; > ret = unwind_frame(tsk, frame); > if (ret < 0) > @@ -201,11 +203,12 @@ 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) { > start_backtrace(&frame, > (unsigned long)__builtin_frame_address(0), > (unsigned long)arch_stack_walk); > - else > + frame.skip = 1; > + } else > start_backtrace(&frame, thread_saved_fp(task), > thread_saved_pc(task)); > > > -- > Catalin
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