linux-kernel - [PATCH v2 3/3] ARM: ptrace: minimize default bp

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Message-Id: <20250827014113.5852-4-b10902118@ntu.edu.tw>
Date: Wed, 27 Aug 2025 09:41:13 +0800
From: Bill Tsui <b10902118@....edu.tw>
To: oleg@...hat.com,
	linux@...linux.org.uk,
	catalin.marinas@....com,
	will@...nel.org
Cc: linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Bill Tsui <b10902118@....edu.tw>
Subject: [PATCH v2 3/3] ARM: ptrace: minimize default bp_len for hw breakpoints to pass check

PTRACE_SETHBPREGS fails when setting a hardware breakpoint on a
non-4-byte aligned address with a valid length to a 32-bit tracee. The
length should be valid as long as the range started from the address
is within one aligned 4 bytes.

The cause is that ptrace_sethbpregs() can only modify either
addr or ctrl of a breakpoint per call, but always checks alignment in
hw_breakpoint_arch_parse(). If a breakpoint has ctrl.len=4 (the
default), then users cannot set the addr to a non-4-byte aligned
address. Likewise, if the addr was previously set unaligned, users
cannot set ctrl.len to 4.

This patch mitigates the issue by minimizing the default bp_len, so
that any possibly valid address can pass the check with it. However, it
does not solve misaligned addr/len in modifying existing breakpoints;
further work may be needed to remove or relax the alignment check.

Signed-off-by: Bill Tsui <b10902118@....edu.tw>
---
 arch/arm/kernel/ptrace.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index 7951b2c06..920cf77d1 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -415,12 +415,26 @@ static u32 ptrace_get_hbp_resource_info(void)
 static struct perf_event *ptrace_hbp_create(struct task_struct *tsk, int type)
 {
 	struct perf_event_attr attr;
+	int min_len;
+
+	/*
+	 * min_len ensures any possibly valid address can pass alignment
+	 * validation with it.
+	 * In PTRACE_SETHBPREGS, addr and ctrl can only be set one after one. If
+	 * addr is set first, ctrl will remain as initial value.
+	 */
+	if (type == HW_BREAKPOINT_X)
+		min_len = (tsk && is_compat_thread(task_thread_info(tsk))) ?
+				  HW_BREAKPOINT_LEN_2 :
+				  HW_BREAKPOINT_LEN_4;
+	else // watchpoint
+		min_len = HW_BREAKPOINT_LEN_1;

 	ptrace_breakpoint_init(&attr);

 	/* Initialise fields to sane defaults. */
 	attr.bp_addr	= 0;
-	attr.bp_len	= HW_BREAKPOINT_LEN_4;
+	attr.bp_len	= min_len;
 	attr.bp_type	= type;
 	attr.disabled	= 1;

-- 
2.50.1