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@xxxxxx>
> Tested-by: Samuel Gosselin <sgosselin@xxxxxx> # 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
>
