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Message-ID: <52686cb6-573c-03ca-06c2-67ae07c91243@linux.microsoft.com>
Date: Thu, 29 Jul 2021 08:54:58 -0500
From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To: Mark Rutland <mark.rutland@....com>
Cc: broonie@...nel.org, jpoimboe@...hat.com, ardb@...nel.org,
nobuta.keiya@...itsu.com, sjitindarsingh@...il.com,
catalin.marinas@....com, will@...nel.org, jmorris@...ei.org,
pasha.tatashin@...een.com, jthierry@...hat.com,
linux-arm-kernel@...ts.infradead.org,
live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v6 1/3] arm64: Improve the unwinder return value
Thanks for the review. Responses inline...
On 7/28/21 11:56 AM, Mark Rutland wrote:
> On Wed, Jun 30, 2021 at 05:33:54PM -0500, madvenka@...ux.microsoft.com wrote:
>> From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
>>
>> Currently, the unwinder returns a tri-state return value:
>>
>> 0 means "continue with the unwind"
>> -ENOENT means "successful termination of the stack trace"
>> -EINVAL means "fatal error, abort the stack trace"
>>
>> This is confusing. To fix this, define an enumeration of different return
>> codes to make it clear. Handle the return codes in all of the unwind
>> consumers.
>
> I agree the tri-state is confusing, and I also generally agree that
> enums are preferabel to a set of error codes. However, I don't think
> this is quite the right abstraction; more on that below.
>
OK.
>>
>> Signed-off-by: Madhavan T. Venkataraman <madvenka@...ux.microsoft.com>
>> ---
>> arch/arm64/include/asm/stacktrace.h | 14 ++++++--
>> arch/arm64/kernel/perf_callchain.c | 5 ++-
>> arch/arm64/kernel/process.c | 8 +++--
>> arch/arm64/kernel/return_address.c | 10 ++++--
>> arch/arm64/kernel/stacktrace.c | 53 ++++++++++++++++-------------
>> arch/arm64/kernel/time.c | 9 +++--
>> 6 files changed, 64 insertions(+), 35 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
>> index eb29b1fe8255..6fcd58553fb1 100644
>> --- a/arch/arm64/include/asm/stacktrace.h
>> +++ b/arch/arm64/include/asm/stacktrace.h
>> @@ -30,6 +30,12 @@ struct stack_info {
>> enum stack_type type;
>> };
>>
>> +enum unwind_rc {
>> + UNWIND_CONTINUE, /* No errors encountered */
>> + UNWIND_ABORT, /* Fatal errors encountered */
>> + UNWIND_FINISH, /* End of stack reached successfully */
>> +};
>
> Generally, there are a bunch of properties we might need to check for an
> unwind step relating to reliabiltiy (e.g. as you add
> UNWIND_CONTINUE_WITH_RISK in the next patch), and I'd prefer that we
> check those properties on the struct stackframe, and simplify
> unwind_frame() to return a bool.
>
> Something akin to the x86 unwinders, where the main loop looks like:
>
> for (unwind_start(&state, ...);
> !unwind_done(&state) && !unwind_error(&state);
> unwind_next_frame(&state) {
> ...
> }
>
> That way we don't have to grow the enum to handle every variation that
> we can think of, and it's simple enough for users to check the
> properties with the helpers.
>
I can do that.
>> +
>> /*
>> * A snapshot of a frame record or fp/lr register values, along with some
>> * accounting information necessary for robust unwinding.
>> @@ -61,7 +67,8 @@ struct stackframe {
>> #endif
>> };
>>
>> -extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
>> +extern enum unwind_rc unwind_frame(struct task_struct *tsk,
>> + struct stackframe *frame);
>> extern void walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
>> bool (*fn)(void *, unsigned long), void *data);
>> extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
>> @@ -148,8 +155,8 @@ static inline bool on_accessible_stack(const struct task_struct *tsk,
>> return false;
>> }
>>
>> -static inline void start_backtrace(struct stackframe *frame,
>> - unsigned long fp, unsigned long pc)
>> +static inline enum unwind_rc start_backtrace(struct stackframe *frame,
>> + unsigned long fp, unsigned long pc)
>> {
>> frame->fp = fp;
>> frame->pc = pc;
>> @@ -169,6 +176,7 @@ static inline void start_backtrace(struct stackframe *frame,
>> bitmap_zero(frame->stacks_done, __NR_STACK_TYPES);
>> frame->prev_fp = 0;
>> frame->prev_type = STACK_TYPE_UNKNOWN;
>> + return UNWIND_CONTINUE;
>> }
>>
>> #endif /* __ASM_STACKTRACE_H */
>> diff --git a/arch/arm64/kernel/perf_callchain.c b/arch/arm64/kernel/perf_callchain.c
>> index 88ff471b0bce..f459208149ae 100644
>> --- a/arch/arm64/kernel/perf_callchain.c
>> +++ b/arch/arm64/kernel/perf_callchain.c
>> @@ -148,13 +148,16 @@ void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
>> struct pt_regs *regs)
>> {
>> struct stackframe frame;
>> + enum unwind_rc rc;
>>
>> if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
>> /* We don't support guest os callchain now */
>> return;
>> }
>>
>> - start_backtrace(&frame, regs->regs[29], regs->pc);
>> + rc = start_backtrace(&frame, regs->regs[29], regs->pc);
>> + if (rc == UNWIND_FINISH || rc == UNWIND_ABORT)
>> + return;
>> walk_stackframe(current, &frame, callchain_trace, entry);
>
> As a first step, could we convert this over to arch_stack_walk()?
>
OK.
>> }
>>
>> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
>> index 6e60aa3b5ea9..e9c763b44fd4 100644
>> --- a/arch/arm64/kernel/process.c
>> +++ b/arch/arm64/kernel/process.c
>> @@ -573,6 +573,7 @@ unsigned long get_wchan(struct task_struct *p)
>> struct stackframe frame;
>> unsigned long stack_page, ret = 0;
>> int count = 0;
>> + enum unwind_rc rc;
>> if (!p || p == current || p->state == TASK_RUNNING)
>> return 0;
>>
>> @@ -580,10 +581,13 @@ unsigned long get_wchan(struct task_struct *p)
>> if (!stack_page)
>> return 0;
>>
>> - start_backtrace(&frame, thread_saved_fp(p), thread_saved_pc(p));
>> + rc = start_backtrace(&frame, thread_saved_fp(p), thread_saved_pc(p));
>> + if (rc == UNWIND_FINISH || rc == UNWIND_ABORT)
>> + return 0;
>>
>> do {
>> - if (unwind_frame(p, &frame))
>> + rc = unwind_frame(p, &frame);
>> + if (rc == UNWIND_FINISH || rc == UNWIND_ABORT)
>> goto out;
>> if (!in_sched_functions(frame.pc)) {
>> ret = frame.pc;
>
> Likewise, can we convert this to use arch_stack_walk()?
>
OK.
>> diff --git a/arch/arm64/kernel/return_address.c b/arch/arm64/kernel/return_address.c
>> index a6d18755652f..1224e043e98f 100644
>> --- a/arch/arm64/kernel/return_address.c
>> +++ b/arch/arm64/kernel/return_address.c
>> @@ -36,13 +36,17 @@ void *return_address(unsigned int level)
>> {
>> struct return_address_data data;
>> struct stackframe frame;
>> + enum unwind_rc rc;
>>
>> data.level = level + 2;
>> data.addr = NULL;
>>
>> - start_backtrace(&frame,
>> - (unsigned long)__builtin_frame_address(0),
>> - (unsigned long)return_address);
>> + rc = start_backtrace(&frame,
>> + (unsigned long)__builtin_frame_address(0),
>> + (unsigned long)return_address);
>> + if (rc == UNWIND_FINISH || rc == UNWIND_ABORT)
>> + return NULL;
>> +
>> walk_stackframe(current, &frame, save_return_addr, &data);
>
> Likewise, can we convert this to use arch_stack_walk()?
>
OK.
Thanks.
Madhavan
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