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).
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.
Martin
This is only done when we record the allocation statements;
this patch doesn't do that yet. It's meant be just the simple
infrastructure bits for the follow-up work.
It's mostly a reminder that there may be room for improvement
here. Maybe not for ranges of sizes but possibly for ranges
of offsets (e.g., if an offset's range is the union of
[-4, -1] and [7, 9] and the destination array is 4 byes big
the access is invalid).
I'd be surprised if this happens in practice, but maybe I'm missing
something.
I've been thinking about how to handle multiple ranges when
the new range info makes them available. I'm not sure I see
how it will be possible to retrofit the existing code to make
use of them. It seems that even code that tries to put anti-
ranges to use today will need to change. It will be a fun
exercise.
It's certainly going to be interesting. As you may have heard in the
meeting yesterday, Aldy and Andrew are looking at expanding how many
subranges are in the representation because we're losing data with the
current representation.
To more than three? Fun!
Martin
