Re: [PATCH] hw_breakpoint: fix single-stepping when using bpf_overflow_handler

[Tomislav Novak <tnovak@meta.com>]

On Fri, Sep 23, 2022 at 01:36:45PM -0700, Tomislav Novak wrote:
> On ARM platforms is_default_overflow_handler() is used to determine if
> hw_breakpoint code should single-step over the watchpoint trigger or
> let the custom handler deal with it.
> 
> Attaching a BPF program to a watchpoint replaces the handler with
> bpf_overflow_handler, which isn't recognized as a default handler so we
> never step over the instruction triggering the data abort exception (the
> watchpoint keeps firing):
> 
>   # bpftrace -e 'watchpoint:0x10000000:4:w { printf("hit\n"); }' ./wp_test
>   Attaching 1 probe...
>   hit
>   hit
>   hit
>   [...]
> 
> (wp_test performs a single 4-byte store to address 0x10000000)

Adding a few more people (per MAINTAINERS), specifically for arch/arm{,64}
(other targets don't have this issue AFAICT).

> This patch replaces the check with uses_default_overflow_handler(), which
> accounts for the bpf_overflow_handler() case by also testing if the handler
> invokes one of the perf_event_output functions via orig_default_handler.
> 
> Signed-off-by: Tomislav Novak <tnovak@fb.com>
> Tested-by: Samuel Gosselin <sgosselin@fb.com> # arm64
> ---
>  arch/arm/kernel/hw_breakpoint.c   |  8 ++++----
>  arch/arm64/kernel/hw_breakpoint.c |  4 ++--
>  include/linux/perf_event.h        | 22 +++++++++++++++++++---
>  3 files changed, 25 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
> index 054e9199f30d..dc0fb7a81371 100644
> --- a/arch/arm/kernel/hw_breakpoint.c
> +++ b/arch/arm/kernel/hw_breakpoint.c
> @@ -626,7 +626,7 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
>  	hw->address &= ~alignment_mask;
>  	hw->ctrl.len <<= offset;
>  
> -	if (is_default_overflow_handler(bp)) {
> +	if (uses_default_overflow_handler(bp)) {
>  		/*
>  		 * Mismatch breakpoints are required for single-stepping
>  		 * breakpoints.
> @@ -798,7 +798,7 @@ static void watchpoint_handler(unsigned long addr, unsigned int fsr,
>  		 * Otherwise, insert a temporary mismatch breakpoint so that
>  		 * we can single-step over the watchpoint trigger.
>  		 */
> -		if (!is_default_overflow_handler(wp))
> +		if (!uses_default_overflow_handler(wp))
>  			continue;
>  step:
>  		enable_single_step(wp, instruction_pointer(regs));
> @@ -811,7 +811,7 @@ static void watchpoint_handler(unsigned long addr, unsigned int fsr,
>  		info->trigger = addr;
>  		pr_debug("watchpoint fired: address = 0x%x\n", info->trigger);
>  		perf_bp_event(wp, regs);
> -		if (is_default_overflow_handler(wp))
> +		if (uses_default_overflow_handler(wp))
>  			enable_single_step(wp, instruction_pointer(regs));
>  	}
>  
> @@ -886,7 +886,7 @@ static void breakpoint_handler(unsigned long unknown, struct pt_regs *regs)
>  			info->trigger = addr;
>  			pr_debug("breakpoint fired: address = 0x%x\n", addr);
>  			perf_bp_event(bp, regs);
> -			if (is_default_overflow_handler(bp))
> +			if (uses_default_overflow_handler(bp))
>  				enable_single_step(bp, addr);
>  			goto unlock;
>  		}
> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> index b29a311bb055..9659a9555c63 100644
> --- a/arch/arm64/kernel/hw_breakpoint.c
> +++ b/arch/arm64/kernel/hw_breakpoint.c
> @@ -654,7 +654,7 @@ static int breakpoint_handler(unsigned long unused, unsigned long esr,
>  		perf_bp_event(bp, regs);
>  
>  		/* Do we need to handle the stepping? */
> -		if (is_default_overflow_handler(bp))
> +		if (uses_default_overflow_handler(bp))
>  			step = 1;
>  unlock:
>  		rcu_read_unlock();
> @@ -733,7 +733,7 @@ static u64 get_distance_from_watchpoint(unsigned long addr, u64 val,
>  static int watchpoint_report(struct perf_event *wp, unsigned long addr,
>  			     struct pt_regs *regs)
>  {
> -	int step = is_default_overflow_handler(wp);
> +	int step = uses_default_overflow_handler(wp);
>  	struct arch_hw_breakpoint *info = counter_arch_bp(wp);
>  
>  	info->trigger = addr;
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index ee8b9ecdc03b..f174b77437f5 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1105,15 +1105,31 @@ extern int perf_event_output(struct perf_event *event,
>  			     struct pt_regs *regs);
>  
>  static inline bool
> -is_default_overflow_handler(struct perf_event *event)
> +__is_default_overflow_handler(perf_overflow_handler_t overflow_handler)
>  {
> -	if (likely(event->overflow_handler == perf_event_output_forward))
> +	if (likely(overflow_handler == perf_event_output_forward))
>  		return true;
> -	if (unlikely(event->overflow_handler == perf_event_output_backward))
> +	if (unlikely(overflow_handler == perf_event_output_backward))
>  		return true;
>  	return false;
>  }
>  
> +#define is_default_overflow_handler(event) \
> +	__is_default_overflow_handler((event)->overflow_handler)
> +
> +#ifdef CONFIG_BPF_SYSCALL
> +static inline bool uses_default_overflow_handler(struct perf_event *event)
> +{
> +	if (likely(is_default_overflow_handler(event)))
> +		return true;
> +
> +	return __is_default_overflow_handler(event->orig_overflow_handler);
> +}
> +#else
> +#define uses_default_overflow_handler(event) \
> +	is_default_overflow_handler(event)
> +#endif
> +
>  extern void
>  perf_event_header__init_id(struct perf_event_header *header,
>  			   struct perf_sample_data *data,
> -- 
> 2.30.2
>