[PATCH 4/9] Simplify bound checks in the MTRR code

[Arjan van de Ven]

From: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Subject: [PATCH 4/9] Simplify bound checks in the MTRR code
CC: mingo@xxxxxxx
CC: hpa@xxxxxxxxx
CC: tglx@xxxxxxx

The current bound checks for copy_from_user in the MTRR driver
are not as obvious as they could be, and gcc agrees with that.

This patch simplifies the boundary checks to the point that gcc
can now prove to itself that the copy_from_user() is never going
past its bounds.

Signed-off-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>

diff --git a/arch/x86/kernel/cpu/mtrr/if.c b/arch/x86/kernel/cpu/mtrr/if.c
index f04e725..3c1b12d 100644
--- a/arch/x86/kernel/cpu/mtrr/if.c
+++ b/arch/x86/kernel/cpu/mtrr/if.c
@@ -96,17 +96,24 @@ mtrr_write(struct file *file, const char __user *buf, size_t len, loff_t * ppos)
 	unsigned long long base, size;
 	char *ptr;
 	char line[LINE_SIZE];
+	int length;
 	size_t linelen;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
-	if (!len)
-		return -EINVAL;
 
 	memset(line, 0, LINE_SIZE);
-	if (len > LINE_SIZE)
-		len = LINE_SIZE;
-	if (copy_from_user(line, buf, len - 1))
+
+	length = len;
+	length--;
+
+	if (length > LINE_SIZE - 1)
+		length = LINE_SIZE - 1;
+
+	if (length < 0)
+		return -EINVAL;
+
+	if (copy_from_user(line, buf, length))
 		return -EFAULT;
 
 	linelen = strlen(line);



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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