On 11/20/20 12:29 PM, Marek Polacek wrote:
On Fri, Nov 20, 2020 at 12:00:58PM -0700, Martin Sebor via Gcc-patches wrote:
To detect a subset of VLA misuses, the C front associates the bounds
of VLAs in function argument lists with the corresponding variables
by implicitly adding an instance of attribute access to each function
declared to take VLAs with the bound expressions chained on the list
of attribute arguments.
Some of these expressions end up modified by the middle end, which
results in references to nonlocal variables (and perhaps other nodes)
used in these expression getting garbage collected. A simple example
of this is described in pr97172.
By unsharing the bound expressions the patch below prevents this from
happening (it's not a fix for pr97172).
My understanding of the details of node sharing and garbage collection
in GCC is very limited (I didn't expect a tree to be garbage-collected
if it's still referenced by something). Is this the right approach
to solving this problem?
ISTM that a more natural thing would be to use build_distinct_type_copy
to copy the type you're about to modify.
The get_parm_array_spec function doesn't modify a type. It's called
from push_parm_decl() to build an "arg spec" attribute with the VLA
bounds as arguments. push_parm_decl() then adds the attribute to
the function's PARM_DECL by calling decl_attributes(). When all of
the function's parameters have been processed the "arg specs" are
then extracted and added as an attribute access specification with
the VLA bounds added to the function declaration.
Martin
diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index d348e39c27a..4aea4dcafb9 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -58,7 +58,7 @@ along with GCC; see the file COPYING3. If not see
#include "c-family/name-hint.h"
#include "c-family/known-headers.h"
#include "c-family/c-spellcheck.h"
-
+#include "gimplify.h"
#include "tree-pretty-print.h"
/* In grokdeclarator, distinguish syntactic contexts of declarators. */
@@ -5780,6 +5780,7 @@ get_parm_array_spec (const struct c_parm *parm, tree
attrs)
/* Each variable VLA bound is represented by the dollar
sign. */
spec += "$";
+ nelts = unshare_expr (nelts);
tpbnds = tree_cons (NULL_TREE, nelts, tpbnds);
}
}
@@ -5834,6 +5835,7 @@ get_parm_array_spec (const struct c_parm *parm, tree
attrs)
/* Each variable VLA bound is represented by a dollar sign. */
spec += "$";
+ nelts = unshare_expr (nelts);
vbchain = tree_cons (NULL_TREE, nelts, vbchain);
}
Marek
