linux-kernel - [PATCH] x86/math64: handle #DE in mul_u64_u64_div

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Message-ID: <20250721130422.GA31640@redhat.com>
Date: Mon, 21 Jul 2025 15:04:22 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: David Laight <david.laight.linux@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Li,Rongqing" <lirongqing@...du.com>,
	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: [PATCH] x86/math64: handle #DE in mul_u64_u64_div_u64()

Change mul_u64_u64_div_u64() to return ULONG_MAX if the result doesn't
fit u64, this matches the generic implementation in lib/math/div64.c.

Reported-by: "Li,Rongqing" <lirongqing@...du.com>
Link: https://lore.kernel.org/all/78a0d7bb20504c0884d474868eccd858@baidu.com/
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 arch/x86/include/asm/div64.h | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
index 9931e4c7d73f..dfd25cfd1c33 100644
--- a/arch/x86/include/asm/div64.h
+++ b/arch/x86/include/asm/div64.h
@@ -79,20 +79,27 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
 
 #else
 # include <asm-generic/div64.h>
+# include <asm/asm.h>
+# include <asm/bug.h>
 
 /*
- * Will generate an #DE when the result doesn't fit u64, could fix with an
- * __ex_table[] entry when it becomes an issue.
+ * Returns ULONG_MAX when the result doesn't fit u64.
  */
 static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
 {
+	int ok = 0;
 	u64 q;
 
-	asm ("mulq %2; divq %3" : "=a" (q)
-				: "a" (a), "rm" (mul), "rm" (div)
-				: "rdx");
+	asm ("mulq %3; 1: divq %4; movl $1,%1; 2:\n"
+		_ASM_EXTABLE(1b, 2b)
+		: "=a" (q), "+r" (ok)
+		: "a" (a), "rm" (mul), "rm" (div)
+		: "rdx");
 
-	return q;
+	if (ok)
+		return q;
+	WARN_ON_ONCE(!div);
+	return ~(u64)0;
 }
 #define mul_u64_u64_div_u64 mul_u64_u64_div_u64
 
-- 
2.25.1.362.g51ebf55