Re: [PATCH] tree-optimization/105726 - adjust array bound heuristic

[Martin Sebor <msebor@redhat.com>]

On 5/25/22 04:49, Richard Biener wrote:
> There's heuristic to detect ptr[1].a[...] out of bound accesses
> reasoning that if ptr points to an array of aggregates a trailing
> incomplete array has to have size zero.  The following more
> thoroughly constrains the cases this applies to avoid false
> positive diagnostics.

The code in the area below does look like it's missing some logic
(like what you're adding).  The whole -Wrestrict pass is messy and
could use some cleanup (sorry).  The -Warray-bounds logic should
probably be removed (-Wstringop-overflow should now catch most of
the valid instances it issues, and if it doesn't(*) it should be
enhanced).  The pass could even be moved into the access warning
pass and probably also simplified quite a bit.

For what it's worth, a simple C test case for the same bug is:

struct A
{
   int i;
   struct B { char a[1]; } b;
};

void f (char *d, struct A *p)
{
   struct B *q = &p->b;
   __builtin_strncpy (d, q->a, 1);   // { dg-bogus "-Warray-bounds" }
}

Martin

[*] For example, no warning is issued for the following overread:

struct A a;

void g (char *d)
{
   struct B *q = &a.b;
   __builtin_strncpy (d, q->a, 123);   // { dg-warning 
"-Wstringop-overread" "pr???????" { xfail *-*-* } }
}

> 
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
> 
> OK?
> 
> Thanks,
> Richard.
> 
> 2022-05-25  Richard Biener  <rguenther@suse.de>
> 
> 	PR tree-optimization/105726
> 	* gimple-ssa-warn-restrict.cc (builtin_memref::set_base_and_offset):
> 	Constrain array-of-flexarray case more.
> 
> 	* g++.dg/warn/Warray-bounds-27.C: New testcase.
> ---
>   gcc/gimple-ssa-warn-restrict.cc              | 22 ++++++++++++--------
>   gcc/testsuite/g++.dg/warn/Warray-bounds-27.C | 16 ++++++++++++++
>   2 files changed, 29 insertions(+), 9 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/warn/Warray-bounds-27.C
> 
> diff --git a/gcc/gimple-ssa-warn-restrict.cc b/gcc/gimple-ssa-warn-restrict.cc
> index b678e806da3..734cdd7f5b4 100644
> --- a/gcc/gimple-ssa-warn-restrict.cc
> +++ b/gcc/gimple-ssa-warn-restrict.cc
> @@ -525,7 +525,6 @@ builtin_memref::set_base_and_offset (tree expr)
>       {
>         tree memrefoff = fold_convert (ptrdiff_type_node, TREE_OPERAND (base, 1));
>         extend_offset_range (memrefoff);
> -      base = TREE_OPERAND (base, 0);
>   
>         if (refoff != HOST_WIDE_INT_MIN
>         	  && TREE_CODE (expr) == COMPONENT_REF)
> @@ -538,14 +537,19 @@ builtin_memref::set_base_and_offset (tree expr)
>   	     REFOFF is set to s[1].b - (char*)s.  */
>   	  offset_int off = tree_to_shwi (memrefoff);
>   	  refoff += off;
> -      	}
> -
> -      if (!integer_zerop (memrefoff))
> -	/* A non-zero offset into an array of struct with flexible array
> -	   members implies that the array is empty because there is no
> -	   way to initialize such a member when it belongs to an array.
> -	   This must be some sort of a bug.  */
> -	refsize = 0;
> +
> +	  if (!integer_zerop (memrefoff)
> +	      && !COMPLETE_TYPE_P (TREE_TYPE (expr))
> +	      && multiple_of_p (sizetype, memrefoff,
> +				TYPE_SIZE_UNIT (TREE_TYPE (base)), true))
> +	    /* A non-zero offset into an array of struct with flexible array
> +	       members implies that the array is empty because there is no
> +	       way to initialize such a member when it belongs to an array.
> +	       This must be some sort of a bug.  */
> +	    refsize = 0;
> +	}
> +
> +      base = TREE_OPERAND (base, 0);
>       }
>   
>     if (TREE_CODE (ref) == COMPONENT_REF)
> diff --git a/gcc/testsuite/g++.dg/warn/Warray-bounds-27.C b/gcc/testsuite/g++.dg/warn/Warray-bounds-27.C
> new file mode 100644
> index 00000000000..06ce089c4b0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Warray-bounds-27.C
> @@ -0,0 +1,16 @@
> +// PR105726
> +// { dg-do compile }
> +// { dg-require-effective-target c++11 }
> +// { dg-options "-O2 -Warray-bounds" }
> +
> +#include <array>
> +#include <cstring>
> +
> +struct X {
> +    char pad[4];
> +    std::array<char, 1> mField;
> +};
> +
> +void encode(char* aBuffer, const X& aMessage) {
> +    strncpy(aBuffer, aMessage.mField.data(), 1); // { dg-bogus "bounds" }
> +}