On 11/12/19 10:54 AM, Jeff Law wrote:
On 11/12/19 1:15 AM, Richard Biener wrote:
On Tue, Nov 12, 2019 at 6:10 AM Jeff Law <l...@redhat.com> wrote:
On 11/6/19 3:34 PM, Martin Sebor wrote:
On 11/6/19 2:06 PM, Martin Sebor wrote:
On 11/6/19 1:39 PM, Jeff Law wrote:
On 11/6/19 1:27 PM, Martin Sebor wrote:
On 11/6/19 11:55 AM, Jeff Law wrote:
On 11/6/19 11:00 AM, Martin Sebor wrote:
The -Wstringop-overflow warnings for single-byte and multi-byte
stores mention the amount of data being stored and the amount of
space remaining in the destination, such as:
warning: writing 4 bytes into a region of size 0 [-Wstringop-overflow=]
123 | *p = 0;
| ~~~^~~
note: destination object declared here
45 | char b[N];
| ^
A warning like this can take some time to analyze. First, the size
of the destination isn't mentioned and may not be easy to tell from
the sources. In the note above, when N's value is the result of
some non-trivial computation, chasing it down may be a small project
in and of itself. Second, it's also not clear why the region size
is zero. It could be because the offset is exactly N, or because
it's negative, or because it's in some range greater than N.
Mentioning both the size of the destination object and the offset
makes the existing messages clearer, are will become essential when
GCC starts diagnosing overflow into allocated buffers (as my
follow-on patch does).
The attached patch enhances -Wstringop-overflow to do this by
letting compute_objsize return the offset to its caller, doing
something similar in get_stridx, and adding a new function to
the strlen pass to issue this enhanced warning (eventually, I'd
like the function to replace the -Wstringop-overflow handler in
builtins.c). With the change, the note above might read something
like:
note: at offset 11 to object ‘b’ with size 8 declared here
45 | char b[N];
| ^
Tested on x86_64-linux.
Martin
gcc-store-offset.diff
gcc/ChangeLog:
* builtins.c (compute_objsize): Add an argument and set it to
offset
into destination.
* builtins.h (compute_objsize): Add an argument.
* tree-object-size.c (addr_object_size): Add an argument and
set it
to offset into destination.
(compute_builtin_object_size): Same.
* tree-object-size.h (compute_builtin_object_size): Add an
argument.
* tree-ssa-strlen.c (get_addr_stridx): Add an argument and
set it
to offset into destination.
(maybe_warn_overflow): New function.
(handle_store): Call maybe_warn_overflow to issue warnings.
gcc/testsuite/ChangeLog:
* c-c++-common/Wstringop-overflow-2.c: Adjust text of expected
messages.
* g++.dg/warn/Wstringop-overflow-3.C: Same.
* gcc.dg/Wstringop-overflow-17.c: Same.
Index: gcc/tree-ssa-strlen.c
===================================================================
--- gcc/tree-ssa-strlen.c (revision 277886)
+++ gcc/tree-ssa-strlen.c (working copy)
@@ -189,6 +189,52 @@ struct laststmt_struct
static int get_stridx_plus_constant (strinfo *, unsigned
HOST_WIDE_INT, tree);
static void handle_builtin_stxncpy (built_in_function,
gimple_stmt_iterator *);
+/* Sets MINMAX to either the constant value or the range VAL
is in
+ and returns true on success. */
+
+static bool
+get_range (tree val, wide_int minmax[2], const vr_values *rvals =
NULL)
+{
+ if (tree_fits_uhwi_p (val))
+ {
+ minmax[0] = minmax[1] = wi::to_wide (val);
+ return true;
+ }
+
+ if (TREE_CODE (val) != SSA_NAME)
+ return false;
+
+ if (rvals)
+ {
+ gimple *def = SSA_NAME_DEF_STMT (val);
+ if (gimple_assign_single_p (def)
+ && gimple_assign_rhs_code (def) == INTEGER_CST)
+ {
+ /* get_value_range returns [0, N] for constant
assignments. */
+ val = gimple_assign_rhs1 (def);
+ minmax[0] = minmax[1] = wi::to_wide (val);
+ return true;
+ }
Umm, something seems really off with this hunk. If the SSA_NAME is
set
via a simple constant assignment, then the range ought to be a
singleton
ie [CONST,CONST]. Is there are particular test were this is not
true?
The only way offhand I could see this happening is if originally
the RHS
wasn't a constant, but due to optimizations it either simplified
into a
constant or a constant was propagated into an SSA_NAME appearing on
the
RHS. This would have to happen between the last range analysis and
the
point where you're making this query.
Yes, I think that's right. Here's an example where it happens:
void f (void)
{
char s[] = "1234";
unsigned n = strlen (s);
char vla[n]; // or malloc (n)
vla[n] = 0; // n = [4, 4]
...
}
The strlen call is folded to 4 but that's not propagated to
the access until sometime after the strlen pass is done.
Hmm. Are we calling set_range_info in that case? That goes behind the
back of pass instance of vr_values. If so, that might argue we want to
be setting it in vr_values too.
No, set_range_info is only called for ranges. In this case,
handle_builtin_strlen replaces the strlen() call with 4:
s = "1234";
_1 = __builtin_strlen (&s);
n_2 = (unsigned int) _1;
a.1_8 = __builtin_alloca_with_align (_1, 8);
(*a.1_8)[n_2] = 0;
When the access is made, the __builtin_alloca_with_align call
is found as the destination and the _1 SSA_NAME is used to
get its size. We get back the range [4, 4].
By the way, I glossed over one detail. The above doesn't work
exactly as is because the allocation size is the SSA_NAME _1
(with the range [4, 4]) but the index is the SSA_NAME n_2 (with
the range [0, 4]; the range is [0, 4] because it was set based
on the size of the argument to the strlen() call well before
the strlen pass even ran).
Which would tend to argue that we should forward propagate the constant
to the uses of _1. That should expose that the RHS of the assignment to
n_2 is a constant as well.
To make it work across assignments we need to propagate the strlen
results down the CFG somehow. I'm hoping the on-demand VRP will
do this automagically.
It would, but it's probably more heavyweight than we need. We just need
to forward propagate the discovered constant to the use points and pick
up any secondary opportunities that arise.
Yes. And the usual way of doing this is to keep a constant-and-copy
lattice (and for copies you'd need to track availability) and before optimizing
a stmt substitute its operands with the lattice contents.
forwprop has a scheme that can be followed doing a RPO walk, strlen
does a DOM walk, there you can follow what DOM/PRE elimination do
(for tracking copy availability - if you just track constants you can
elide that).
I'm also note sure handling copies is all that interesting here and if
we just handle constants/invariants, then it's pretty easy.
Whenever we replace a strlen call with a const, we put the LHS (assuming
its an SSA_NAME) of the statement on a worklist.
We pull items off the worklist and propagate the value to the use points
and try to simplify the resulting statement. If the RHS of the use
point simplified to a constant, then put the LHS of the use statement
onto the worklist. Iterate until the list is empty.
That would capture everything of interest I suspect and ought to be cheap.
This sounds like a significant project of its own, and well beyond
the scope of the simple infrastructure enhancement I'm making here:
all this does is improve the accuracy of the diagnostics. Is
implementing it a precondition for accepting this patch?
If yes, since I don't think I have the time for it for GCC 10
I need to decide whether to drop just this improvement or all
of the buffer overflow checks that depend on it.
Let me know which you prefer.
Martin
jeff
I think whenever we substitute a constant or SSA_NAME for a strlen call,
we can just replace uses of the LHS of the assignment with the
const/copy. Any statements we propagate into are put on a worklist.
We pull statements off the worklist and try to simplify their RHS. If
the RHS simplifies to a const/copy, then then we repeat the process of
propagating to the use points and
x = <whatever>;
If we find <whatever> collapses to a constant or copy we can just record
it in SSA_NAME_VALUE. As we walk through statements, we can propagate
Richard.
jeff
