Re: [PATCH v2] livepatch: Reorder to use before freeing a pointer

[Tom Rix]

On 3/21/22 6:39 AM, Joe Lawrence wrote:
> On 3/19/22 9:51 PM, trix@xxxxxxxxxx wrote:
> > From: Tom Rix <trix@xxxxxxxxxx>
> >
> > Clang static analysis reports this issue
> > livepatch-shadow-fix1.c:113:2: warning: Use of
> >    memory after it is freed
> >    pr_info("%s: dummy @ %p, prevented leak @ %p\n",
> >    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > The pointer is freed in the previous statement.
> > Reorder the pr_info to report before the free.
> >
> > Similar issue in livepatch-shadow-fix2.c
> >
> > Signed-off-by: Tom Rix <trix@xxxxxxxxxx>
> > ---
> > v2: Fix similar issue in livepatch-shadow-fix2.c
> >
> >   samples/livepatch/livepatch-shadow-fix1.c | 2 +-
> >   samples/livepatch/livepatch-shadow-fix2.c | 2 +-
> >   2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/samples/livepatch/livepatch-shadow-fix1.c b/samples/livepatch/livepatch-shadow-fix1.c
> > index 918ce17b43fda..6701641bf12d4 100644
> > --- a/samples/livepatch/livepatch-shadow-fix1.c
> > +++ b/samples/livepatch/livepatch-shadow-fix1.c
> > @@ -109,9 +109,9 @@ static void livepatch_fix1_dummy_leak_dtor(void *obj, void *shadow_data)
> >   	void *d = obj;
> >   	int **shadow_leak = shadow_data;
> >   
> > -	kfree(*shadow_leak);
> >   	pr_info("%s: dummy @ %p, prevented leak @ %p\n",
> >   			 __func__, d, *shadow_leak);
> > +	kfree(*shadow_leak);
> >   }
> >   
> >   static void livepatch_fix1_dummy_free(struct dummy *d)
> > diff --git a/samples/livepatch/livepatch-shadow-fix2.c b/samples/livepatch/livepatch-shadow-fix2.c
> > index 29fe5cd420472..361046a4f10cf 100644
> > --- a/samples/livepatch/livepatch-shadow-fix2.c
> > +++ b/samples/livepatch/livepatch-shadow-fix2.c
> > @@ -61,9 +61,9 @@ static void livepatch_fix2_dummy_leak_dtor(void *obj, void *shadow_data)
> >   	void *d = obj;
> >   	int **shadow_leak = shadow_data;
> >   
> > -	kfree(*shadow_leak);
> >   	pr_info("%s: dummy @ %p, prevented leak @ %p\n",
> >   			 __func__, d, *shadow_leak);
> > +	kfree(*shadow_leak);
> >   }
> >   
> >   static void livepatch_fix2_dummy_free(struct dummy *d)
> Hi Tom,
>
> Ordering doesn't matter for the example, so let's clean up the static
> analysis.
>
> Acked-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
>
> But for my sanity, isn't this a false positive?  There shouldn't be harm
> in printing the pointer itself, even after what it points to has been
> freed, i.e.
>
> 	int *i = malloc(sizeof(*i));
> 	free(i);
> 	printf("%p\n", i);      << ok
> 	printf("%d\n", *i);     << NOT ok
>
> But I suppose clang doesn't know that the passed pointer isn't getting
> dereferenced by the function, so it throws up a warning?  Just curious
> what your experience has been with respect to these reports.

The analysis it good for static functions, for extern functions it has 
nothing to analyze so a worst case is assumed.

I agree this is likely a false positive.

Tom

> Thanks,