Re: [bug report] livepatch: Initialize shadow variables safely by a custom callback

[Joe Lawrence <joe.lawrence@redhat.com>]

On 1/7/20 8:29 AM, Dan Carpenter wrote:
> Hello Petr Mladek,
> 
> The patch e91c2518a5d2: "livepatch: Initialize shadow variables
> safely by a custom callback" from Apr 16, 2018, leads to the
> following static checker warning:
> 
> 	samples/livepatch/livepatch-shadow-fix1.c:86 livepatch_fix1_dummy_alloc()
> 	error: 'klp_shadow_alloc()' 'leak' too small (4 vs 8)
> 
> samples/livepatch/livepatch-shadow-fix1.c
>      53  static int shadow_leak_ctor(void *obj, void *shadow_data, void *ctor_data)
>      54  {
>      55          void **shadow_leak = shadow_data;
>      56          void *leak = ctor_data;
>      57
>      58          *shadow_leak = leak;
>      59          return 0;
>      60  }
>      61
>      62  static struct dummy *livepatch_fix1_dummy_alloc(void)
>      63  {
>      64          struct dummy *d;
>      65          void *leak;
>      66
>      67          d = kzalloc(sizeof(*d), GFP_KERNEL);
>      68          if (!d)
>      69                  return NULL;
>      70
>      71          d->jiffies_expire = jiffies +
>      72                  msecs_to_jiffies(1000 * EXPIRE_PERIOD);
>      73
>      74          /*
>      75           * Patch: save the extra memory location into a SV_LEAK shadow
>      76           * variable.  A patched dummy_free routine can later fetch this
>      77           * pointer to handle resource release.
>      78           */
>      79          leak = kzalloc(sizeof(int), GFP_KERNEL);
>      80          if (!leak) {
>      81                  kfree(d);
>      82                  return NULL;
>      83          }
>      84
>      85          klp_shadow_alloc(d, SV_LEAK, sizeof(leak), GFP_KERNEL,
>                                               ^^^^^^^^^^^^
> This doesn't seem right at all?  Leak is a pointer.  Why is leak a void
> pointer instead of an int pointer?
> 

Hi Dan,

If I remember this code correctly, the shadow variable is tracking the 
pointer value itself and not its contents, so sizeof(leak) should be 
correct for the shadow variable data size.

(For kernel/livepatch/shadow.c :: __klp_shadow_get_or_alloc() creates 
new struct klp_shadow with .data[size] to accommodate its meta-data plus 
the desired data).

Why isn't leak an int pointer?  I don't remember why, according to git 
history it's been that way since the beginning.  I think it was coded to 
say, "Give me some storage, any size an int will do.  I'm not going to 
touch it, but I want to demonstrate a memory leak".

Would modifying the pointer type satisfy the static code complaint?

Since the warning is about a size mismatch, what are the parameters that 
it is keying on?  Does it expect to see the typical allocation pattern like:

   int *foo = alloc(sizeof(*foo))

and not:

   int *foo = alloc(sizeof(foo))


Thanks,

-- Joe

>      86                           shadow_leak_ctor, leak);
>      87
>      88          pr_info("%s: dummy @ %p, expires @ %lx\n",
>      89                  __func__, d, d->jiffies_expire);
>      90
>      91          return d;
>      92  }
> 
> regards,
> dan carpenter
>