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From:   Tomislav Novak <tnovak@fb.com>
To:     Alexei Starovoitov <ast@kernel.org>, Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>
CC:     <bpf@vger.kernel.org>, <linux-perf-users@vger.kernel.org>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>, Tomislav Novak <tnovak@fb.com>,
        Samuel Gosselin <sgosselin@fb.com>
Subject: [PATCH] hw_breakpoint: fix single-stepping when using bpf_overflow_handler
Date:   Fri, 23 Sep 2022 13:36:45 -0700
Message-ID: <20220923203644.2731604-1-tnovak@fb.com>
MIME-Version: 1.0
Content-Transfer-Encoding: quoted-printable
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Precedence: bulk

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_tes=
t
  Attaching 1 probe...
  hit
  hit
  hit
  [...]

(wp_test performs a single 4-byte store to address 0x10000000)

This patch replaces the check with uses_default_overflow_handler(), which
accounts for the bpf_overflow_handler() case by also testing if the handl=
er
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_breakpo=
int.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 &=3D ~alignment_mask;
 	hw->ctrl.len <<=3D offset;
=20
-	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, un=
signed 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, un=
signed int fsr,
 		info->trigger =3D addr;
 		pr_debug("watchpoint fired: address =3D 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));
 	}
=20
@@ -886,7 +886,7 @@ static void breakpoint_handler(unsigned long unknown,=
 struct pt_regs *regs)
 			info->trigger =3D addr;
 			pr_debug("breakpoint fired: address =3D 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_bre=
akpoint.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, u=
nsigned long esr,
 		perf_bp_event(bp, regs);
=20
 		/* Do we need to handle the stepping? */
-		if (is_default_overflow_handler(bp))
+		if (uses_default_overflow_handler(bp))
 			step =3D 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 =3D is_default_overflow_handler(wp);
+	int step =3D uses_default_overflow_handler(wp);
 	struct arch_hw_breakpoint *info =3D counter_arch_bp(wp);
=20
 	info->trigger =3D 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 *e=
vent,
 			     struct pt_regs *regs);
=20
 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 =3D=3D perf_event_output_forward))
+	if (likely(overflow_handler =3D=3D perf_event_output_forward))
 		return true;
-	if (unlikely(event->overflow_handler =3D=3D perf_event_output_backward)=
)
+	if (unlikely(overflow_handler =3D=3D perf_event_output_backward))
 		return true;
 	return false;
 }
=20
+#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 *even=
t)
+{
+	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,
--=20
2.30.2