linux-kernel - [PATCH v2] arm64: hw_breakpoint: warn on invalid breakpoint length

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Message-ID: <20251213112336.26503-1-osama.abdelkader@gmail.com>
Date: Sat, 13 Dec 2025 12:23:35 +0100
From: Osama Abdelkader <osama.abdelkader@...il.com>
To: Will Deacon <will@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Osama Abdelkader <osama.abdelkader@...il.com>
Subject: [PATCH v2] arm64: hw_breakpoint: warn on invalid breakpoint length

Some tools (e.g. perf) incorrectly assume that breakpoints should be
sizeof(long), but this is wrong for AArch64 where breakpoints must be
4 bytes. Add a warning when we silently fix up the parameter so tool
developers can get notified.

This addresses the FIXME comment by adding diagnostic output rather
than breaking existing tools by returning -EINVAL.

Signed-off-by: Osama Abdelkader <osama.abdelkader@...il.com>
---
v2: fix warning messageline splitting
---
 arch/arm64/kernel/hw_breakpoint.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
index ab76b36dce82..cce306145d78 100644
--- a/arch/arm64/kernel/hw_breakpoint.c
+++ b/arch/arm64/kernel/hw_breakpoint.c
@@ -475,11 +475,13 @@ static int arch_build_bp_info(struct perf_event *bp,
 				return -EINVAL;
 		} else if (hw->ctrl.len != ARM_BREAKPOINT_LEN_4) {
 			/*
-			 * FIXME: Some tools (I'm looking at you perf) assume
-			 *	  that breakpoints should be sizeof(long). This
-			 *	  is nonsense. For now, we fix up the parameter
-			 *	  but we should probably return -EINVAL instead.
+			 * Some tools (e.g. perf) incorrectly assume that
+			 * breakpoints should be sizeof(long). This is wrong
+			 * for AArch64 where breakpoints must be 4 bytes.
+			 * Warn the user and fix up the parameter.
 			 */
+			pr_warn_once("hw_breakpoint: invalid AArch64 breakpoint length %d, fixing to 4 bytes\n",
+				     hw->ctrl.len);
 			hw->ctrl.len = ARM_BREAKPOINT_LEN_4;
 		}
 	}
-- 
2.43.0