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Date:   Sun, 9 Sep 2018 05:11:23 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Theodore Ts'o <tytso@mit.edu>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 06/11] compat_ioctl: remove /dev/random commands
Message-ID: <20180909041114.GD19965@ZenIV.linux.org.uk>
References: <20180908142837.2819693-1-arnd@arndb.de>
 <20180908142837.2819693-6-arnd@arndb.de>
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On Sat, Sep 08, 2018 at 04:28:12PM +0200, Arnd Bergmann wrote:
> These are all handled by the random driver, so instead of listing
> each ioctl, we can just use the same function to deal with both
> native and compat commands.

Umm...  I don't think it's right -

>  	.unlocked_ioctl = random_ioctl,
> +	.compat_ioctl = random_ioctl,


->compat_ioctl() gets called in
                        error = f.file->f_op->compat_ioctl(f.file, cmd, arg);
so you do *NOT* get compat_ptr() for those - they have to do it on their
own.  It's not hard to provide a proper compat_ioctl() instance for that
one, but this is not it.  What you need in drivers/char/random.c part of
that one is something like

diff --git a/drivers/char/random.c b/drivers/char/random.c
index bf5f99fc36f1..1de75c784cf6 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1954,10 +1954,9 @@ static ssize_t random_write(struct file *file, const char __user *buffer,
 	return (ssize_t)count;
 }
 
-static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
+static long __random_ioctl(struct file *f, unsigned int cmd, int __user *p)
 {
 	int size, ent_count;
-	int __user *p = (int __user *)arg;
 	int retval;
 
 	switch (cmd) {
@@ -2011,6 +2010,18 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
 	}
 }
 
+static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
+{
+	return __random_ioctl(f, cmd, (int __user *)arg);
+}
+
+#ifdef CONFIG_COMPAT
+static long compat_random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
+{
+	return __random_ioctl(f, cmd, compat_ptr(arg));
+}
+#endif
+
 static int random_fasync(int fd, struct file *filp, int on)
 {
 	return fasync_helper(fd, filp, on, &fasync);
@@ -2021,6 +2032,9 @@ const struct file_operations random_fops = {
 	.write = random_write,
 	.poll  = random_poll,
 	.unlocked_ioctl = random_ioctl,
+#ifdef CONFIG_COMPAT
+	.compat_ioctl = compat_random_ioctl,
+#endif
 	.fasync = random_fasync,
 	.llseek = noop_llseek,
 };