c, ubsan: Instrument even shortened divisions [PR109151]

  Hi!

On the following testcase, the C FE decides to shorten the division because
it has a guarantee that INT_MIN / -1 division won't be encountered, the
first operand is widened from narrower unsigned and/or the second operand is
a constant other than all ones (in this case both are true).
The problem is that the narrower type in this case is _Bool and
ubsan_instrument_division only instruments it if op0's type is INTEGER_TYPE
or REAL_TYPE.  Strangely this doesn't happen in C++ FE.
Anyway, we only shorten divisions if the INT_MIN / -1 case is impossible,
so I think we should be fine even with -fstrict-enums in C++ in case it
shortened to ENUMERAL_TYPEs.

The following patch just instruments those on the ubsan_instrument_division
side.  Perhaps only the first hunk and testcase might be needed because
we shouldn't shorten if the other case could be triggered.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk,
or shall I omit the second hunk?

2023-03-17  Jakub Jelinek  <jakub@redhat.com>

	PR c/109151
	* c-ubsan.cc (ubsan_instrument_division): Handle all scalar integral
	types rather than just INTEGER_TYPE.

	* c-c++-common/ubsan/div-by-zero-8.c: New test.


	Jakub
  

On Fri, 17 Mar 2023, Jakub Jelinek wrote:

> Hi!
> 
> On the following testcase, the C FE decides to shorten the division because
> it has a guarantee that INT_MIN / -1 division won't be encountered, the
> first operand is widened from narrower unsigned and/or the second operand is
> a constant other than all ones (in this case both are true).
> The problem is that the narrower type in this case is _Bool and
> ubsan_instrument_division only instruments it if op0's type is INTEGER_TYPE
> or REAL_TYPE.  Strangely this doesn't happen in C++ FE.
> Anyway, we only shorten divisions if the INT_MIN / -1 case is impossible,
> so I think we should be fine even with -fstrict-enums in C++ in case it
> shortened to ENUMERAL_TYPEs.
> 
> The following patch just instruments those on the ubsan_instrument_division
> side.  Perhaps only the first hunk and testcase might be needed because
> we shouldn't shorten if the other case could be triggered.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk,
> or shall I omit the second hunk?

LGTM as-is

Richard.

> 2023-03-17  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c/109151
> 	* c-ubsan.cc (ubsan_instrument_division): Handle all scalar integral
> 	types rather than just INTEGER_TYPE.
> 
> 	* c-c++-common/ubsan/div-by-zero-8.c: New test.
> 
> --- gcc/c-family/c-ubsan.cc.jj	2023-02-28 11:38:28.965868044 +0100
> +++ gcc/c-family/c-ubsan.cc	2023-03-16 09:49:51.651126302 +0100
> @@ -53,7 +53,7 @@ ubsan_instrument_division (location_t lo
>    op0 = unshare_expr (op0);
>    op1 = unshare_expr (op1);
>  
> -  if (TREE_CODE (type) == INTEGER_TYPE
> +  if (INTEGRAL_TYPE_P (type)
>        && sanitize_flags_p (SANITIZE_DIVIDE))
>      t = fold_build2 (EQ_EXPR, boolean_type_node,
>  		     op1, build_int_cst (type, 0));
> @@ -68,7 +68,7 @@ ubsan_instrument_division (location_t lo
>      t = NULL_TREE;
>  
>    /* We check INT_MIN / -1 only for signed types.  */
> -  if (TREE_CODE (type) == INTEGER_TYPE
> +  if (INTEGRAL_TYPE_P (type)
>        && sanitize_flags_p (SANITIZE_SI_OVERFLOW)
>        && !TYPE_UNSIGNED (type))
>      {
> --- gcc/testsuite/c-c++-common/ubsan/div-by-zero-8.c.jj	2023-03-16 10:01:31.626824994 +0100
> +++ gcc/testsuite/c-c++-common/ubsan/div-by-zero-8.c	2023-03-16 10:03:05.510443440 +0100
> @@ -0,0 +1,14 @@
> +/* PR c/109151 */
> +/* { dg-do run } */
> +/* { dg-options "-fsanitize=integer-divide-by-zero -Wno-div-by-zero -fno-sanitize-recover=integer-divide-by-zero" } */
> +/* { dg-shouldfail "ubsan" } */
> +
> +int d;
> +
> +int
> +main ()
> +{
> +  d = ((short) (d == 1 | d > 9)) / 0;
> +}
> +
> +/* { dg-output "division by zero" } */
> 
> 	Jakub
> 
>
  

On Fri, Mar 17, 2023 at 09:14:04AM +0100, Jakub Jelinek wrote:
> Hi!
> 
> On the following testcase, the C FE decides to shorten the division because
> it has a guarantee that INT_MIN / -1 division won't be encountered, the
> first operand is widened from narrower unsigned and/or the second operand is
> a constant other than all ones (in this case both are true).
> The problem is that the narrower type in this case is _Bool and
> ubsan_instrument_division only instruments it if op0's type is INTEGER_TYPE
> or REAL_TYPE.  Strangely this doesn't happen in C++ FE.

I was curious.  The difference is because C++ passes this

(gdb) pge op0
(int) (short int) (VIEW_CONVERT_EXPR<int>(d) == 1 | VIEW_CONVERT_EXPR<int>(d) > 9)

to shorten_binary_op while C passes:

(gdb) pge op0
(int) (<<< Unknown tree: c_maybe_const_expr
  
  d >>> == 1 || <<< Unknown tree: c_maybe_const_expr
  
  d >>> > 9)

so when we remove the '(int)' cast, we have different types underneath,
either short or bool.

In C, the BIT_IOR_EXPR -> TRUTH_OR_EXPR change is because we call c_convert ->
convert_to_integer -> do_narrow.
In C++, we never called do_narrow so shorten_binary_op gets the original tree.

Anyway, thanks for the patch.

Marek