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From:   Max Filippov <jcmvbkbc@gmail.com>
To:     linux-xtensa@linux-xtensa.org
Cc:     Chris Zankel <chris@zankel.net>, linux-kernel@vger.kernel.org,
        Max Filippov <jcmvbkbc@gmail.com>
Subject: [PATCH v3 2/3] xtensa: support artificial division by 0 exception
Date:   Tue, 17 May 2022 03:44:57 -0700
Message-Id: <20220517104458.257799-3-jcmvbkbc@gmail.com>
In-Reply-To: <20220517104458.257799-1-jcmvbkbc@gmail.com>
References: <20220517104458.257799-1-jcmvbkbc@gmail.com>
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On xtensa cores wihout hardware division option division support
functions from libgcc react to division by 0 attempt by executing
illegal instruction followed by the characters 'DIV0'. Recognize this
pattern in illegal instruction exception handler and convert it to
division by 0.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
---
Changes v2->v3:

- simplify check_div0 by dropping check for the specific illegal
  instruction and only verifying the 'DIV0' signature

Changes v1->v2:

- split ABI probing improvement from the artificial division by 0
- don't limit artificial division by 0 pattern detection to
  configurations without HW division opcodes, do it always

 arch/xtensa/kernel/traps.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/arch/xtensa/kernel/traps.c b/arch/xtensa/kernel/traps.c
index 24d11b44fa57..2b75b252b626 100644
--- a/arch/xtensa/kernel/traps.c
+++ b/arch/xtensa/kernel/traps.c
@@ -293,12 +293,35 @@ static void do_interrupt(struct pt_regs *regs)
 	set_irq_regs(old_regs);
 }
 
+static bool check_div0(struct pt_regs *regs)
+{
+	static const u8 pattern[] = {'D', 'I', 'V', '0'};
+	const u8 *p;
+	u8 buf[5];
+
+	if (user_mode(regs)) {
+		if (copy_from_user(buf, (void __user *)regs->pc + 2, 5))
+			return 0;
+		p = buf;
+	} else {
+		p = (const u8 *)regs->pc + 2;
+	}
+
+	return memcmp(p, pattern, sizeof(pattern)) == 0 ||
+		memcmp(p + 1, pattern, sizeof(pattern)) == 0;
+}
+
 /*
  * Illegal instruction. Fatal if in kernel space.
  */
 
 static void do_illegal_instruction(struct pt_regs *regs)
 {
+	if (check_div0(regs)) {
+		do_div0(regs);
+		return;
+	}
+
 	__die_if_kernel("Illegal instruction in kernel", regs, SIGKILL);
 
 	/* If in user mode, send SIGILL signal to current process. */
-- 
2.30.2