[PATCH v3] libfs: fix error cast of negative value in simple_attr_write()

[Yicong Yang]

The attr->set() receive a value of u64, but simple_strtoll() is used
for doing the conversion. It will lead to the error cast if user inputs
a negative value.

Use kstrtoull() instead of simple_strtoll() to convert a string got
from the user to an unsigned value. The former will return '-EINVAL' if
it gets a negetive value, but the latter can't handle the situation
correctly. Make 'val' unsigned long long as what kstrtoull() takes, this
will eliminate the compile warning on no 64-bit architectures.

Fixes: f7b88631a897 ("fs/libfs.c: fix simple_attr_write() on 32bit machines")
Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
---
Change since v1:
- address the compile warning for non-64 bit platform.
Change since v2:
Link: https://lore.kernel.org/linux-fsdevel/1605000324-7428-1-git-send-email-yangyicong@xxxxxxxxxxxxx/
- make 'val' unsigned long long and mentioned in the commit
Link: https://lore.kernel.org/linux-fsdevel/1605261369-551-1-git-send-email-yangyicong@xxxxxxxxxxxxx/

 fs/libfs.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index fc34361..7124c2e 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -959,7 +959,7 @@ ssize_t simple_attr_write(struct file *file, const char __user *buf,
 			  size_t len, loff_t *ppos)
 {
 	struct simple_attr *attr;
-	u64 val;
+	unsigned long long val;
 	size_t size;
 	ssize_t ret;
 
@@ -977,7 +977,9 @@ ssize_t simple_attr_write(struct file *file, const char __user *buf,
 		goto out;
 
 	attr->set_buf[size] = '\0';
-	val = simple_strtoll(attr->set_buf, NULL, 0);
+	ret = kstrtoull(attr->set_buf, 0, &val);
+	if (ret)
+		goto out;
 	ret = attr->set(attr->data, val);
 	if (ret == 0)
 		ret = len; /* on success, claim we got the whole input */
-- 
2.8.1