Re: copy_mount_options()

Andrew Morton
2004-08-20T22:44:11Z

"David S. Miller" <davem@redhat.com> wrote:
>
> > Something like this?
>
> Looks pretty good. Although I would do a verify_area() above
> the loop and then use __get_user().

OK, here's #2, with enhanced comment and changelog!

Untested though.

davem says that copy_mount_options is failing in obscure ways if the
architecture's copy_from_user() doesn't return an exact count of the number of
uncopied bytes.

Fixing that up in each architecture is a pain - it involves falling back to
byte-at-a-time copies.

It's simple to open-code this in namespace.c. If we find other places in the
kernel which care about this we can promote this to a global function.

Signed-off-by: Andrew Morton <akpm@osdl.org>
---

25-akpm/fs/namespace.c | 30 ++++++++++++++++++++++++++++--
1 files changed, 28 insertions(+), 2 deletions(-)

diff -puN fs/namespace.c~copy_mount_options-size-fix fs/namespace.c
--- 25/fs/namespace.c~copy_mount_options-size-fix Fri Aug 20 15:43:26 2004
+++ 25-akpm/fs/namespace.c Fri Aug 20 15:46:05 2004
@@ -930,7 +930,33 @@ void mark_mounts_for_expiry(struct list_

EXPORT_SYMBOL_GPL(mark_mounts_for_expiry);

-int copy_mount_options (const void __user *data, unsigned long *where)
+/*
+ * Some copy_from_user() implementations do not return the exact number of
+ * bytes remaining to copy on a fault. But copy_mount_options() requires that.
+ * Note that this function differs from copy_from_user() in that it will oops
+ * on bad values of `to', rather than returning a short copy.
+ */
+static long
+exact_copy_from_user(void *to, const void __user *from, unsigned long n)
+{
+ char *t = to;
+ const char __user *f = from;
+ char c;
+
+ if (!access_ok(VERIFY_READ, from, n))
+ return n;
+
+ while (n) {
+ if (__get_user(c, f))
+ break;
+ *t++ = c;
+ f++;
+ n--;
+ }
+ return n;
+}
+
+int copy_mount_options(const void __user *data, unsigned long *where)
{
int i;
unsigned long page;
@@ -952,7 +978,7 @@ int copy_mount_options (const void __use
if (size > PAGE_SIZE)
size = PAGE_SIZE;

- i = size - copy_from_user((void *)page, data, size);
+ i = size - exact_copy_from_user((void *)page, data, size);
if (!i) {
free_page(page);
return -EFAULT;
_