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[Frama-c-discuss] WP and memcpy question

  • Subject: [Frama-c-discuss] WP and memcpy question
  • From: loic.correnson at cea.fr (Loïc Correnson)
  • Date: Tue, 19 Jun 2018 11:04:49 +0200
  • In-reply-to: <CAH5O5POv0W5wPH9zYS=s4mKGL-2dYv_Gz2TADxDziJaDFLz0XQ@mail.gmail.com>
  • References: <CAH5O5POv0W5wPH9zYS=s4mKGL-2dYv_Gz2TADxDziJaDFLz0XQ@mail.gmail.com>
Hi Alwyn,

I think the problem is on the granularity of the assigns clause.
WP is using an over approximation of the assigns condition to prove assigns clauses.
Each side effect must be **fully** included in one of the component of the assigns clauses to pass the check.

Here, the memcpy effect on s1_buff will assigns a **range** of s1_buff, namely s1_buff[s1_idx][0..2] if my reading is correct.
But the assigns contract of the function under proof is : assigns s1_buff[s1_idx][0], s1_buff[s1_idx][1], etc.

For the other memcpy, I think an assigns clause regarding exerr_cpy is missing.

With these two additions, it only remains one unproved goal on s0_gen_ensures.

	L.

> Le 19 juin 2018 à 00:44, Alwyn Goodloe <agoodloe at gmail.com> a écrit :
> 
> 
> The annotated C file below The last function  is given as follows
> 
> /*@ requires 0 <= s0_idx < 1;
>     requires 0 <= s1_idx < 2;
>     requires  \separated(exarr_cpy + (0..2), exarr + (0..2));
>     requires \separated(s1, exarr_cpy+(0..2));
>     requires \separated(s1, exarr+(0..2));
>     requires \separated(&s0, exarr_cpy+(0..2));
>     requires \separated(&s0, exarr+(0..2));
>     assigns  s0;
>     assigns  s1;
>     assigns  s0_buff[s0_idx];
>     assigns  s1_buff[s1_idx][0];
>     assigns  s1_buff[s1_idx][1];
>     assigns  s1_buff[s1_idx][2];
>     assigns  s0_idx;
>     assigns  s1_idx;
>     ensures  0 <= s0_idx < 1;
>     ensures  0 <= s1_idx < 2;
>     ensures  \forall int i; 0 <= i < 1&& i != \old(s0_idx) ==>
>         s0_buff[i] == \old(s0_buff[i]);
>     ensures  \forall int i; 0 <= i < 2&& i != \old(s1_idx) ==>
>         s1_buff[i] == \old(s1_buff[i]);
> */
> static void step () {
>   memcpy(exarr_cpy, exarr, sizeof(exarr_cpy));
>   if (farray_guard()) {farray(farray_arg0(),
>                        farray_arg1(),
>                        farray_arg2());
>                      }
>   s0 = s0_gen();
>   s1 = s1_gen();
>   int8_t s1_tmp[3];
>   memcpy(s1_tmp, s1, sizeof(s1_tmp));
>   /*@ assert 0 <= s0_idx < 1; */
>   s0_buff[s0_idx] = s0;
>   memcpy(s1_buff[s1_idx], s1_tmp, sizeof(s1_tmp));
>   ++(s0_idx);
>   ++(s1_idx);
>   s0_idx = s0_idx % 1;
>   s1_idx = s1_idx % 2;
> }
> 
>   
> When I run wp with both alt-ergo and Z3 provers all but four VCs are discharged Each unproven VC seems related to memcpy and the goal is always the assigns s0 in line 117.  Any suggestions on what I might need to do in order to would be appreciated.  Here is one of messages  produced for one of the unproved VCs. 
> 
> Goal Assigns (file NewArray.c, line 117) in 'step' (2/14):
> Call Effect at line 133
> Assume {
>   Type: is_uint32(s0_idx_0) /\ is_uint32(s1_idx_0) /\ is_sint32(status_0).
>   (* Heap *)
>   Have: (region(s1_0.base) <= 0) /\ linked(Malloc_0) /\ sconst(Mchar_0).
>   (* Pre-condition *)
>   Have: (s0_idx_0 <= 0) /\ (0 <= s0_idx_0).
>   (* Pre-condition *)
>   Have: (0 <= s1_idx_0) /\ (s1_idx_0 <= 1).
>   (* Pre-condition *)
>   Have: separated(s1_0, 1, shift_sint8(global(G_exarr_cpy_1569), 0), 3).
>   (* Pre-condition *)
>   Have: separated(s1_0, 1, shift_sint8(global(G_exarr_1191), 0), 3).
>   (* Merge *)
>   Either {
>     Case:
>       Let a = shift_sint8(global(G_exarr_cpy_1569), 0).
>       (* Call 'memcpy' *)
>       Have: L_memcmp(Mchar_0, Mchar_1, a,
>               shift_sint8(global(G_exarr_1191), 0), 3) = 0.
>       (* Call Effects *)
>       Have: havoc(Mchar_0, Mchar_1, a, 3).
>     Case: (* Exit Effects *) Have: havoc(Mchar_0, Mchar_2, a, 3).
>   }
> }
> Prove: false.
> Prover Z3 returns Failed
> Why3 exits with status 1.
> Prover Alt-Ergo returns Unknown (Qed:5ms) (271ms)
> 
>  
> 
> -- 
> Alwyn E. Goodloe, Ph.D.
> agoodloe at gmail.com <mailto:agoodloe at gmail.com>
> 
> Research Computer Engineer
> NASA Langley Research Center
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