Re: [PATCH v2 1/4] stacktrace: add interface based on shadow call stack

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Mar 23, 2022 at 04:32:52PM +0100, andrey.konovalov@linux.dev wrote:
> From: Andrey Konovalov <andreyknvl@google.com>
> 
> Add a new interface stack_trace_save_shadow() for collecting stack traces
> by copying frames from the Shadow Call Stack.
> 
> Collecting stack traces this way is significantly faster: boot time
> of a defconfig build with KASAN enabled gets descreased by ~30%.

Hmm... just to check, do ou know if that's just because of hte linear copy, or
because we're skipping other work we have to do in the regular stacktrace?

> The few patches following this one add an implementation of
> stack_trace_save_shadow() for arm64.
> 
> The implementation of the added interface is not meant to use
> stack_trace_consume_fn to avoid making a function call for each
> collected frame to further improve performance.

... because we could easily provide an inline-optimized stack copy *without*
having to write a distinct unwinder, and I'd *really* like to avoid having a
bunch of distinct unwinders for arm64, as it really hinders maintenance. We're
working on fixing/improving the arm64 unwinder for things like
RELIABLE_STACKTRACE, and I know that some of that work is non-trivial to make
work with an SCS-based unwind rather than an FP-based unwind, and/or will
undermine the saving anyway.

> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  arch/Kconfig               |  6 ++++++
>  include/linux/stacktrace.h | 15 +++++++++++++++
>  kernel/stacktrace.c        | 21 +++++++++++++++++++++
>  3 files changed, 42 insertions(+)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index e12a4268c01d..207c1679c53a 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -1041,6 +1041,12 @@ config HAVE_RELIABLE_STACKTRACE
>  	  arch_stack_walk_reliable() function which only returns a stack trace
>  	  if it can guarantee the trace is reliable.
>  
> +config HAVE_SHADOW_STACKTRACE
> +	bool
> +	help
> +	  If this is set, the architecture provides the arch_stack_walk_shadow()
> +	  function, which collects the stack trace from the shadow call stack.
> +
>  config HAVE_ARCH_HASH
>  	bool
>  	default n
> diff --git a/include/linux/stacktrace.h b/include/linux/stacktrace.h
> index 97455880ac41..b74d1e42e157 100644
> --- a/include/linux/stacktrace.h
> +++ b/include/linux/stacktrace.h
> @@ -60,6 +60,9 @@ int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry, void *cookie,
>  
>  void arch_stack_walk_user(stack_trace_consume_fn consume_entry, void *cookie,
>  			  const struct pt_regs *regs);
> +
> +int arch_stack_walk_shadow(unsigned long *store, unsigned int size,
> +			   unsigned int skipnr);
>  #endif /* CONFIG_ARCH_STACKWALK */
>  
>  #ifdef CONFIG_STACKTRACE
> @@ -108,4 +111,16 @@ static inline int stack_trace_save_tsk_reliable(struct task_struct *tsk,
>  }
>  #endif
>  
> +#if defined(CONFIG_STACKTRACE) && defined(CONFIG_HAVE_SHADOW_STACKTRACE)
> +int stack_trace_save_shadow(unsigned long *store, unsigned int size,
> +			    unsigned int skipnr);
> +#else
> +static inline int stack_trace_save_shadow(unsigned long *store,
> +					  unsigned int size,
> +					  unsigned int skipnr)
> +{
> +	return -ENOSYS;
> +}
> +#endif
> +
>  #endif /* __LINUX_STACKTRACE_H */
> diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c
> index 9ed5ce989415..fe305861fd55 100644
> --- a/kernel/stacktrace.c
> +++ b/kernel/stacktrace.c
> @@ -237,6 +237,27 @@ unsigned int stack_trace_save_user(unsigned long *store, unsigned int size)
>  }
>  #endif
>  
> +#ifdef CONFIG_HAVE_SHADOW_STACKTRACE
> +/**
> + * stack_trace_save_shadow - Save a stack trace based on shadow call stack
> + * @store:	Pointer to the storage array
> + * @size:	Size of the storage array
> + * @skipnr:	Number of entries to skip at the start of the stack trace
> + *
> + * Return: Number of trace entries stored.
> + */
> +int stack_trace_save_shadow(unsigned long *store, unsigned int size,
> +			    unsigned int skipnr)
> +{
> +	/*
> +	 * Do not use stack_trace_consume_fn to avoid making a function
> +	 * call for each collected frame to improve performance.
> +	 * Skip + 1 frame to skip stack_trace_save_shadow.
> +	 */
> +	return arch_stack_walk_shadow(store, size, skipnr + 1);
> +}
> +#endif

If we really need this, can we make it an __always_inline in a header so that
we can avoid the skip? Generally the skipping is problematic due to
inlining/outlining and LTO, and I'd like to avoid adding more of it
unnecessarily.

Thanks,
Mark.

> +
>  #else /* CONFIG_ARCH_STACKWALK */
>  
>  /*
> -- 
> 2.25.1
>