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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 v2 2/3] xtensa: support artificial division by 0 exception
Date:   Fri, 13 May 2022 20:34:36 -0700
Message-Id: <20220514033437.4182899-3-jcmvbkbc@gmail.com>
In-Reply-To: <20220514033437.4182899-1-jcmvbkbc@gmail.com>
References: <20220514033437.4182899-1-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 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 | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/arch/xtensa/kernel/traps.c b/arch/xtensa/kernel/traps.c
index 24d11b44fa57..47445b2d4217 100644
--- a/arch/xtensa/kernel/traps.c
+++ b/arch/xtensa/kernel/traps.c
@@ -293,12 +293,42 @@ static void do_interrupt(struct pt_regs *regs)
 	set_irq_regs(old_regs);
 }
 
+static int check_div0(struct pt_regs *regs)
+{
+	u8 buf[7];
+	void *p;
+	static const u8 pattern1[] = {0, 0, 0, 'D', 'I', 'V', '0'};
+#if defined(__XTENSA_EB__)
+	static const u8 pattern2[] = {0xd6, 0x0f, 'D', 'I', 'V', '0'};
+#elif defined(__XTENSA_EL__)
+	static const u8 pattern2[] = {0x6d, 0xf0, 'D', 'I', 'V', '0'};
+#else
+#error Unsupported Xtensa endianness
+#endif
+
+	if (user_mode(regs)) {
+		if (copy_from_user(buf, (void __user *)regs->pc, 7))
+			return 0;
+		p = buf;
+	} else {
+		p = (void *)regs->pc;
+	}
+
+	return memcmp(p, pattern1, sizeof(pattern1)) == 0 ||
+		memcmp(p, pattern2, sizeof(pattern2)) == 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