linux-kernel - Re: [PATCH v1 1/4] lib: introduce copy_struct_from

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Date:   Wed, 25 Sep 2019 19:20:49 +0200
From:   Aleksa Sarai <cyphar@...har.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        GNU C Library <libc-alpha@...rceware.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/4] lib: introduce copy_struct_from_user() helper

On 2019-09-25, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Wed, Sep 25, 2019 at 10:00 AM Aleksa Sarai <cyphar@...har.com> wrote:
> >
> > +int is_zeroed_user(const void __user *from, size_t size)
> 
> I like how you've done this, but it's buggy and only works on 64-bit.
> 
> All the "u64" and "8" cases need to be "unsigned long" and
> "sizeof(unsigned long)".
> 
> Part of that requirement is:
> 
> > +               unsafe_get_user(val, (u64 __user *) from, err_fault);
> 
> This part works fine - although 64-bit accesses migth be much more
> expensive and the win of unrolling might not be sufficient - but:
> 
> > +               if (align) {
> > +                       /* @from is unaligned. */
> > +                       val &= ~aligned_byte_mask(align);
> > +                       align = 0;
> > +               }
> 
> This part fundamentally only works on 'unsigned long'.

Just to make sure I understand, the following diff would this solve the
problem? If so, I'll apply it, and re-send in a few hours.

--8<--------------------------------------------------------------------------

 int is_zeroed_user(const void __user *from, size_t size)
 {
-       u64 val;
-       uintptr_t align = (uintptr_t) from % 8;
+       unsigned long val;
+       uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
 
        if (unlikely(!size))
                return true;
@@ -150,8 +150,8 @@ int is_zeroed_user(const void __user *from, size_t size)
        if (!user_access_begin(from, size))
                return -EFAULT;
 
-       while (size >= 8) {
-               unsafe_get_user(val, (u64 __user *) from, err_fault);
+       while (size >= sizeof(unsigned long)) {
+               unsafe_get_user(val, (unsigned long __user *) from, err_fault);
                if (align) {
                        /* @from is unaligned. */
                        val &= ~aligned_byte_mask(align);
@@ -159,12 +159,12 @@ int is_zeroed_user(const void __user *from, size_t size)
                }
                if (val)
                        goto done;
-               from += 8;
-               size -= 8;
+               from += sizeof(unsigned long);
+               size -= sizeof(unsigned long);
        }
        if (size) {
                /* (@from + @size) is unaligned. */
-               unsafe_get_user(val, (u64 __user *) from, err_fault);
+               unsafe_get_user(val, (unsigned long __user *) from, err_fault);
                val &= aligned_byte_mask(size);
        }

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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