[PATCH v2] xtensa/simdisk: fix proc_read_simdisk()

[Max Filippov <jcmvbkbc@gmail.com>]

From: Yi Yang <yiyang13@huawei.com>

The commit a69755b18774 ("xtensa simdisk: switch to proc_create_data()")
split read operation into two parts, first retrieving the path when it's
non-null and second retrieving the trailing '\n'. However when the path
is non-null the first simple_read_from_buffer updates ppos, and the
second simple_read_from_buffer returns 0 if ppos is greater than 1 (i.e.
almost always). As a result reading from that proc file is almost always
empty.

Fix it by making a temporary copy of the path with the trailing '\n' and
using simple_read_from_buffer on that copy.

Fixes: a69755b18774 ("xtensa simdisk: switch to proc_create_data()")
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
---
Changes v1 -> v2:

- change title
- expand commit message
- rearrange changes, fix build warning

 arch/xtensa/platforms/iss/simdisk.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/arch/xtensa/platforms/iss/simdisk.c b/arch/xtensa/platforms/iss/simdisk.c
index 0f0e0724397f..4255b92fa3eb 100644
--- a/arch/xtensa/platforms/iss/simdisk.c
+++ b/arch/xtensa/platforms/iss/simdisk.c
@@ -211,12 +211,18 @@ static ssize_t proc_read_simdisk(struct file *file, char __user *buf,
 	struct simdisk *dev = pde_data(file_inode(file));
 	const char *s = dev->filename;
 	if (s) {
-		ssize_t n = simple_read_from_buffer(buf, size, ppos,
-							s, strlen(s));
-		if (n < 0)
-			return n;
-		buf += n;
-		size -= n;
+		ssize_t len = strlen(s);
+		char *temp = kmalloc(len + 2, GFP_KERNEL);
+
+		if (!temp)
+			return -ENOMEM;
+
+		len = scnprintf(temp, len + 2, "%s\n", s);
+		len = simple_read_from_buffer(buf, size, ppos,
+					      temp, len);
+
+		kfree(temp);
+		return len;
 	}
 	return simple_read_from_buffer(buf, size, ppos, "\n", 1);
 }
-- 
2.30.2