linux-kernel - Re: [PATCH] coccinelle: api: add kzfree script

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Message-ID: <65dee3e0-7df9-5b38-fe4c-5de3f70380a0@linux.com>
Date:   Thu, 4 Jun 2020 20:22:13 +0300
From:   Denis Efremov <efremov@...ux.com>
To:     Julia Lawall <julia.lawall@...ia.fr>
Cc:     cocci@...teme.lip6.fr, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] coccinelle: api: add kzfree script

> Could you send an example of some C code on which the result is not
> suitable?

I've updated the pattern to handle false positives:

@ifok@
position p;
expression *E;
@@

(
  if (...) {
    ...
    memset(E, 0, ...)@p;
    ...
  }
|
  if (...) {
    ...
  } else {
    ...
    memset(E, 0, ...)@p;
    ...
  }
)

// Ignore kzfree definition
// Ignore kasan test
@r depends on !patch && !(file in "lib/test_kasan.c") && !(file in "mm/slab_common.c")@
expression *E;
position p != ifok.p;
@@

* memset(E, 0, ...)@p;
  ... when != E
      when != if (...) { ... E ... }
      when != for (...;...;...) { ... E ... }
      when != while (...) { ... E ... }
      when strict
* kfree(E);


Example of false positives:

void test_memset_under_if(void)
{
   char *p = malloc(10, GFP_KERNEL);
   if (p % 5) {
      p[5] = 1;
   } else {
      memset(p, 0, 10);
   }
   kfree(p);
}

void test_memset_under_if(void)
{
   int i;
   char *p = malloc(10, GFP_KERNEL);
   for (i = 0; i < 10; ++i) {
      memset(p, 0, 10);
   }
   kfree(p);
}

void test_E_in_if(void)
{
   char *p = malloc(10, GFP_KERNEL);
   memset(p, 0, 10); // when != E is not enough
   if (10) {        // when != if (...) { ... E ... } is required
      p[5] = 1;
   }
   kfree(p);
}

void test_E_in_for(void)
{
   char *p = malloc(10, GFP_KERNEL);
   memset(p, 0, 10);
   for(;;) {
      p[5] = 1;
   }
   kfree(p);
}

void test_E_in_while(void)
{
   char *p = malloc(10, GFP_KERNEL);
   memset(p, 0, 10);
   while(1) {
      p[6] = 2;
   }
   kfree(p);
}

void test_E_in_struct(void)
{
   struct t { int a[3]; };
   struct t *p = malloc(10 * sizeof(struct(struct t)), GFP_KERNEL);
   memset(p, 0, 10);
   for(;;) {
      if (1) {
        p->a[2] = 1; // I give up on this
        p->a[0] = 10;
      }
   }
   kfree(p);
}

After all it seems reasonable to me to add forall and memset_explicit rather
than handle all these false positives. Something like this for v2?

@r depends on !patch && !(file in "lib/test_kasan.c") && !(file in "mm/slab_common.c") forall@
expression *E;
position p;
@@

* \(memset\|memset_explicit\)(E, 0, ...);
  ... when != E
* kfree(E)@p;

Do I need to add "when strict" with forall or it's already enabled in this case?
Do I need to enable forall for pathing "-/+"?

Thanks,
Denis