On 08/24/2018 09:58 AM, Martin Sebor wrote:
> The warning suppression for -Wstringop-truncation looks for
> the next statement after a truncating strncpy to see if it
> adds a terminating nul. This only works when the next
> statement can be reached using the Gimple statement iterator
> which isn't until after gimplification. As a result, strncpy
> calls that truncate their constant argument that are being
> folded to memcpy this early get diagnosed even if they are
> followed by the nul assignment:
>
> const char s[] = "12345";
> char d[3];
>
> void f (void)
> {
> strncpy (d, s, sizeof d - 1); // -Wstringop-truncation
> d[sizeof d - 1] = 0;
> }
>
> To avoid the warning I propose to defer folding strncpy to
> memcpy until the pointer to the basic block the strnpy call
> is in can be used to try to reach the next statement (this
> happens as early as ccp1). I'm aware of the preference to
> fold things early but in the case of strncpy (a relatively
> rarely used function that is often misused), getting
> the warning right while folding a bit later but still fairly
> early on seems like a reasonable compromise. I fear that
> otherwise, the false positives will drive users to adopt
> other unsafe solutions (like memcpy) where these kinds of
> bugs cannot be as readily detected.
>
> Tested on x86_64-linux.
>
> Martin
>
> PS There still are outstanding cases where the warning can
> be avoided. I xfailed them in the test for now but will
> still try to get them to work for GCC 9.
>
> gcc-87028.diff
>
>
> PR tree-optimization/87028 - false positive -Wstringop-truncation strncpy
> with global variable source string
> gcc/ChangeLog:
>
> PR tree-optimization/87028
> * gimple-fold.c (gimple_fold_builtin_strncpy): Avoid folding when
> statement doesn't belong to a basic block.
> * tree-ssa-strlen.c (maybe_diag_stxncpy_trunc): Handle MEM_REF on
> the left hand side of assignment.
>
> gcc/testsuite/ChangeLog:
>
> PR tree-optimization/87028
> * c-c++-common/Wstringop-truncation.c: Remove xfails.
> * gcc.dg/Wstringop-truncation-5.c: New test.
>
> diff --git a/gcc/gimple-fold.c b/gcc/gimple-fold.c
> index 07341eb..284c2fb 100644
> --- a/gcc/gimple-fold.c
> +++ b/gcc/gimple-fold.c
> @@ -1702,6 +1702,11 @@ gimple_fold_builtin_strncpy (gimple_stmt_iterator *gsi,
> if (tree_int_cst_lt (ssize, len))
> return false;
>
> + /* Defer warning (and folding) until the next statement in the basic
> + block is reachable. */
> + if (!gimple_bb (stmt))
> + return false;
I think you want cfun->cfg as the test here. They should be equivalent
in practice.
> diff --git a/gcc/tree-ssa-strlen.c b/gcc/tree-ssa-strlen.c
> index d0792aa..f1988f6 100644
> --- a/gcc/tree-ssa-strlen.c
> +++ b/gcc/tree-ssa-strlen.c
> @@ -1981,6 +1981,23 @@ maybe_diag_stxncpy_trunc (gimple_stmt_iterator gsi,
> tree src, tree cnt)
> && known_eq (dstoff, lhsoff)
> && operand_equal_p (dstbase, lhsbase, 0))
> return false;
> +
> + if (code == MEM_REF
> + && TREE_CODE (lhsbase) == SSA_NAME
> + && known_eq (dstoff, lhsoff))
> + {
> + /* Extract the referenced variable from something like
> + MEM[(char *)d_3(D) + 3B] = 0; */
> + gimple *def = SSA_NAME_DEF_STMT (lhsbase);
> + if (gimple_nop_p (def))
> + {
> + lhsbase = SSA_NAME_VAR (lhsbase);
> + if (lhsbase
> + && dstbase
> + && operand_equal_p (dstbase, lhsbase, 0))
> + return false;
> + }
> + }
If you find yourself looking at SSA_NAME_VAR, you're usually barking up
the wrong tree. It'd be easier to suggest something here if I could see
the gimple (with virtual operands). BUt at some level what you really
want to do is make sure the base of the MEM_REF is the same as what got
passed as the destination of the strncpy. You'd want to be testing
SSA_NAMEs in that case.
Jeff
Jeff
