Re: [PATCH] ubsan: missed -fsanitize=bounds for compound ops [PR108060]

[Richard Biener <rguenther@suse.de>]

On Wed, 8 Mar 2023, Marek Polacek wrote:

> In this PR we are dealing with a missing .UBSAN_BOUNDS, so the
> out-of-bounds access in the test makes the program crash before
> a UBSan diagnostic was emitted.  In C and C++, c_genericize gets
> 
>   a[b] = a[b] | c;
> 
> but in C, both a[b] are one identical shared tree (not in C++ because
> cp_fold/ARRAY_REF created two same but not identical trees).  Since
> ubsan_walk_array_refs_r keeps a pset, in C we produce
> 
>   a[.UBSAN_BOUNDS (0B, SAVE_EXPR <b>, 8);, SAVE_EXPR <b>;] = a[b] | c;
> 
> because the LHS is walked before the RHS.
> 
> Since r7-1900, we gimplify the RHS before the LHS.  So the statement above
> gets gimplified into
> 
>     _1 = a[b];
>     c.0_2 = c;
>     b.1 = b;
>     .UBSAN_BOUNDS (0B, b.1, 8);
> 
> With this patch we produce:
> 
>   a[b] = a[.UBSAN_BOUNDS (0B, SAVE_EXPR <b>, 8);, SAVE_EXPR <b>;] | c;
> 
> which gets gimplified into:
> 
>     b.0 = b;
>     .UBSAN_BOUNDS (0B, b.0, 8);
>     _1 = a[b.0];
> 
> therefore we emit a runtime error before making the bad array access.
> 
> I think it's OK that only the RHS gets a .UBSAN_BOUNDS, as in few lines
> above: the instrumented array access dominates the array access on the
> LHS, and I've verified that
> 
>   b = 0;
>   a[b] = (a[b], b = -32768, a[0] | c);
> 
> works as expected: the inner a[b] is OK but we do emit an error for the
> a[b] on the LHS.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/12?

I think this is a reasonable way to address the regression, so OK.

Thanks,
Richard.

> 	PR sanitizer/108060
> 	PR sanitizer/109050
> 
> gcc/c-family/ChangeLog:
> 
> 	* c-gimplify.cc (ubsan_walk_array_refs_r): For a MODIFY_EXPR, instrument
> 	the RHS before the LHS.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* c-c++-common/ubsan/bounds-17.c: New test.
> 	* c-c++-common/ubsan/bounds-18.c: New test.
> 	* c-c++-common/ubsan/bounds-19.c: New test.
> 	* c-c++-common/ubsan/bounds-20.c: New test.
> ---
>  gcc/c-family/c-gimplify.cc                   | 12 ++++++++++++
>  gcc/testsuite/c-c++-common/ubsan/bounds-17.c | 17 +++++++++++++++++
>  gcc/testsuite/c-c++-common/ubsan/bounds-18.c | 17 +++++++++++++++++
>  gcc/testsuite/c-c++-common/ubsan/bounds-19.c | 20 ++++++++++++++++++++
>  gcc/testsuite/c-c++-common/ubsan/bounds-20.c | 16 ++++++++++++++++
>  5 files changed, 82 insertions(+)
>  create mode 100644 gcc/testsuite/c-c++-common/ubsan/bounds-17.c
>  create mode 100644 gcc/testsuite/c-c++-common/ubsan/bounds-18.c
>  create mode 100644 gcc/testsuite/c-c++-common/ubsan/bounds-19.c
>  create mode 100644 gcc/testsuite/c-c++-common/ubsan/bounds-20.c
> 
> diff --git a/gcc/c-family/c-gimplify.cc b/gcc/c-family/c-gimplify.cc
> index 74b276b2b26..ef5c7d919fc 100644
> --- a/gcc/c-family/c-gimplify.cc
> +++ b/gcc/c-family/c-gimplify.cc
> @@ -106,6 +106,18 @@ ubsan_walk_array_refs_r (tree *tp, int *walk_subtrees, void *data)
>      }
>    else if (TREE_CODE (*tp) == ARRAY_REF)
>      ubsan_maybe_instrument_array_ref (tp, false);
> +  else if (TREE_CODE (*tp) == MODIFY_EXPR)
> +    {
> +      /* Since r7-1900, we gimplify RHS before LHS.  Consider
> +	   a[b] |= c;
> +	 wherein we can have a single shared tree a[b] in both LHS and RHS.
> +	 If we only instrument the LHS and the access is invalid, the program
> +	 could crash before emitting a UBSan error.  So instrument the RHS
> +	 first.  */
> +      *walk_subtrees = 0;
> +      walk_tree (&TREE_OPERAND (*tp, 1), ubsan_walk_array_refs_r, pset, pset);
> +      walk_tree (&TREE_OPERAND (*tp, 0), ubsan_walk_array_refs_r, pset, pset);
> +    }
>    return NULL_TREE;
>  }
>  
> diff --git a/gcc/testsuite/c-c++-common/ubsan/bounds-17.c b/gcc/testsuite/c-c++-common/ubsan/bounds-17.c
> new file mode 100644
> index 00000000000..b727e3235b8
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/ubsan/bounds-17.c
> @@ -0,0 +1,17 @@
> +/* PR sanitizer/108060 */
> +/* { dg-do run } */
> +/* { dg-options "-fsanitize=bounds" } */
> +/* { dg-skip-if "" { *-*-* } "-flto" } */
> +/* { dg-shouldfail "ubsan" } */
> +
> +int a[8];
> +int c;
> +
> +int
> +main ()
> +{
> +  int b = -32768;
> +  a[b] |= c;
> +}
> +
> +/* { dg-output "index -32768 out of bounds for type 'int \\\[8\\\]'" } */
> diff --git a/gcc/testsuite/c-c++-common/ubsan/bounds-18.c b/gcc/testsuite/c-c++-common/ubsan/bounds-18.c
> new file mode 100644
> index 00000000000..556abc0e1c0
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/ubsan/bounds-18.c
> @@ -0,0 +1,17 @@
> +/* PR sanitizer/108060 */
> +/* { dg-do run } */
> +/* { dg-options "-fsanitize=bounds" } */
> +/* { dg-skip-if "" { *-*-* } "-flto" } */
> +/* { dg-shouldfail "ubsan" } */
> +
> +int a[8];
> +int c;
> +
> +int
> +main ()
> +{
> +  int b = -32768;
> +  a[b] = a[b] | c;
> +}
> +
> +/* { dg-output "index -32768 out of bounds for type 'int \\\[8\\\]'" } */
> diff --git a/gcc/testsuite/c-c++-common/ubsan/bounds-19.c b/gcc/testsuite/c-c++-common/ubsan/bounds-19.c
> new file mode 100644
> index 00000000000..54217ae399f
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/ubsan/bounds-19.c
> @@ -0,0 +1,20 @@
> +/* PR sanitizer/108060 */
> +/* { dg-do run } */
> +/* { dg-options "-fsanitize=bounds" } */
> +/* { dg-skip-if "" { *-*-* } "-flto" } */
> +/* { dg-shouldfail "ubsan" } */
> +
> +int a[8];
> +int a2[18];
> +int c;
> +
> +int
> +main ()
> +{
> +  int b = 0;
> +  a[0] = (a2[b], b = -32768, a[0] | c);
> +  b = 0;
> +  a[b] = (a[b], b = -32768, a[0] | c);
> +}
> +
> +/* { dg-output "index -32768 out of bounds for type 'int \\\[8\\\]'" } */
> diff --git a/gcc/testsuite/c-c++-common/ubsan/bounds-20.c b/gcc/testsuite/c-c++-common/ubsan/bounds-20.c
> new file mode 100644
> index 00000000000..a78c67129e0
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/ubsan/bounds-20.c
> @@ -0,0 +1,16 @@
> +/* PR sanitizer/109050 */
> +/* { dg-do run } */
> +/* { dg-options "-fsanitize=bounds -fno-sanitize-recover=all" } */
> +/* { dg-shouldfail "ubsan" } */
> +
> +long a;
> +int b;
> +int
> +main ()
> +{
> +  int c[4] = {0, 1, 2, 3};
> +  a = 0;
> +  c[a - 9806816] |= b;
> +}
> +
> +/* { dg-output "index -9806816 out of bounds for type 'int \\\[4\\\]'" } */
> 
> base-commit: 2e3dd14dd287ca94b72c36ed28a1ae30887f77ce
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)