0002390: dubious discharge of postcondition - Frama-C Bug Tracking System

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0002390

Frama-C

Plug-in > wp

public

2018-07-24 17:57

2019-10-17 18:00

Reporter

jens

Assigned To

correnson

Priority

normal

Severity

minor

Reproducibility

always

Status

closed

Resolution

no change required

Product Version

Frama-C 17-Chlorine

Target Version

Fixed in Version

Summary

0002390: dubious discharge of postcondition

Description

Consider the following function

/*@
   assigns \nothing;
   ensures a == 0;
*/
void foo(int a)
{
   a = 0;
}

The postcondition can of course not be verified because the assignment happens to a copy "a" and furthermore nothing is known about the value of "a" in the pre-state.

If I add, however, the admittedly weird looking precondition "requires \valid(&a);" as shown here

/*@
   requires \valid(&a);
   assigns \nothing;
   ensures a == 0;
*/
void bar(int a)
{
   a = 0;
}

then Qed discharges the postcondition.
I am not sure this a bug or rather caused by taking the address of a formal argument in the precondition
but in any case it would be nice if someone could have a look at it.

Steps To Reproduce

Call
frama-c -wp -wp-rte foo.c
on the attached file.

Tags

No tags attached.

Attached Files

foo.c (187 bytes) 2018-07-24 17:57


/*@
   assigns  \nothing;
   ensures  a == 0;
*/
void foo(int a)
{
   a = 0;
}


/*@
   requires \valid(&a);
   assigns  \nothing;
   ensures  a == 0;
*/
void bar(int a)
{
   a = 0;
}

foo.c (187 bytes) 2018-07-24 17:57

Relationships

Relationships

Notes
~0006607 correnson (manager) 2018-07-25 14:20	Indeed, formals are not (yet) allocated in the pre-state, although they are allocated just before the first statement of the body... This is a corner case, of course. Typically, the following program is proven: @ requires \valid(&a); ensures \false; / void f(int a) { return ; } But, then, the following one fails with a WP error: void g(int x) { f(x); } [wp] User Error: Address of logic value (x_0) This is because, when you instantiate the pre-condition of f with the formal parameters, you get an incorrect expression with (&a).

~0006608 correnson (manager) 2018-07-25 14:20	Formals not allocated in the pre-state

~0006609 correnson (manager) 2018-07-25 14:22	Side note : we are also working on running smoke tests to detect infeasible requires (those that would allow to prove false). Second side node : probably it would be a good idea to detect the usage of (&x) on a formal (x) inside pre-conditions.

~0006610 virgile (developer) 2018-07-25 14:33	Third side note: this would also warrant a clarification in the ACSL manual itself, because at the moment there is nothing in it that precludes the usage of \valid(&a) in a requires. By the way what would be the status of \valid(&a) in an ensures clause?

~0006611 jens (reporter) 2018-07-25 14:44	Shall I create a request for clarification in the ACSL manual?

~0006612 virgile (developer) 2018-07-25 14:48	@jens, That'd be great, thanks

~0006614 correnson (manager) 2018-07-25 15:06	For the same reason, `\valid(&a)` is not valid in the ensures clause, also it is still valid just at the end of the body. Although, `\valid(&a)` reveals the same problem of accessing the address of values when used at call-sites. Actually, the Kernel shall reject any access to the address of a formal inside a function specification.

~0006615 virgile (developer) 2018-07-25 15:25	this will be on the todo list as soon as this point of view is endorsed by the ACSL manual 😛

Notes

Date Modified	Username	Field	Change
Issue History
2018-07-24 17:57	jens	New Issue
2018-07-24 17:57	jens	Status	new => assigned
2018-07-24 17:57	jens	Assigned To	=> correnson
2018-07-24 17:57	jens	File Added: foo.c
2018-07-25 14:20	correnson	Note Added: 0006607
2018-07-25 14:20	correnson	Note Added: 0006608
2018-07-25 14:20	correnson	Status	assigned => resolved
2018-07-25 14:20	correnson	Resolution	open => no change required
2018-07-25 14:22	correnson	Note Added: 0006609
2018-07-25 14:33	virgile	Note Added: 0006610
2018-07-25 14:44	jens	Note Added: 0006611
2018-07-25 14:44	jens	Status	resolved => feedback
2018-07-25 14:44	jens	Resolution	no change required => reopened
2018-07-25 14:48	virgile	Note Added: 0006612
2018-07-25 15:06	correnson	Note Added: 0006614
2018-07-25 15:25	virgile	Note Added: 0006615
2019-10-17 17:07	correnson	Status	feedback => resolved
2019-10-17 17:07	correnson	Resolution	reopened => no change required
2019-10-17 18:00	signoles	Status	resolved => closed

Issue History