GCC 8.1 warns for unbounded (and some bounded) string comparisons
involving arrays declared attribute nonstring (i.e., char arrays
that need not be nul-terminated). For instance:
extern __attribute__((nonstring)) char a[4];
int f (void)
{
return strncmp (a, "123", sizeof a);
}
warning: ‘strcmp’ argument 1 declared attribute ‘nonstring’
Note that the warning refers to strcmp even though the call in
the source is to strncmp, because prior passes transform one to
the other.
The warning above is unnecessary (for strcmp) and incorrect for
strncmp because the call reads exactly four bytes from the non-
string array a regardless of the bound and so there is no risk
that it will read past the end of the array.
The attached change enhances the warning to use the length of
the string argument to suppress some of these needless warnings
for both bounded and unbounded string comparison functions.
When the length of the string is unknown, the warning uses its
size (when possible) as the upper bound on the number of accessed
bytes. The change adds no new warnings.
I'm looking for approval to commit it to both trunk and 8-branch.
Martin
PR c/85623 - strncmp() warns about attribute 'nonstring' incorrectly in -Wstringop-overflow
gcc/ChangeLog:
PR c/85623
* calls.c (maybe_warn_nonstring_arg): Use string length to set
or ajust the presumed bound on an operation to avoid unnecessary
warnings.
gcc/testsuite/ChangeLog:
PR c/85623
* c-c++-common/attr-nonstring-3.c: Adjust.
* c-c++-common/attr-nonstring-4.c: Adjust.
* c-c++-common/attr-nonstring-6.c: New test.
diff --git a/gcc/calls.c b/gcc/calls.c
index 9eb0467..f5c8ad4 100644
--- a/gcc/calls.c
+++ b/gcc/calls.c
@@ -55,6 +55,7 @@ along with GCC; see the file COPYING3. If not see
#include "stringpool.h"
#include "attribs.h"
#include "builtins.h"
+#include "gimple-fold.h"
/* Like PREFERRED_STACK_BOUNDARY but in units of bytes, not bits. */
#define STACK_BYTES (PREFERRED_STACK_BOUNDARY / BITS_PER_UNIT)
@@ -1612,15 +1613,36 @@ maybe_warn_nonstring_arg (tree fndecl, tree exp)
/* The bound argument to a bounded string function like strncpy. */
tree bound = NULL_TREE;
+ /* The range of lengths of a string argument to one of the comparison
+ functions. If the length is less than the bound it is used instead. */
+ tree lenrng[2] = { NULL_TREE, NULL_TREE };
+
/* It's safe to call "bounded" string functions with a non-string
argument since the functions provide an explicit bound for this
purpose. */
switch (DECL_FUNCTION_CODE (fndecl))
{
- case BUILT_IN_STPNCPY:
- case BUILT_IN_STPNCPY_CHK:
+ case BUILT_IN_STRCMP:
case BUILT_IN_STRNCMP:
case BUILT_IN_STRNCASECMP:
+ {
+ /* For these, if one argument refers to one or more of a set
+ of string constants or arrays of known size, determine
+ the range of their known or possible lengths and use it
+ conservatively as the bound for the unbounded function,
+ and to adjust the range of the bound of the bounded ones. */
+ unsigned stride = with_bounds ? 2 : 1;
+ for (unsigned argno = 0; argno < nargs && !*lenrng; argno += stride)
+ {
+ tree arg = CALL_EXPR_ARG (exp, argno);
+ if (!get_attr_nonstring_decl (arg))
+ get_range_strlen (arg, lenrng);
+ }
+ }
+ /* Fall through. */
+
+ case BUILT_IN_STPNCPY:
+ case BUILT_IN_STPNCPY_CHK:
case BUILT_IN_STRNCPY:
case BUILT_IN_STRNCPY_CHK:
{
@@ -1647,6 +1669,33 @@ maybe_warn_nonstring_arg (tree fndecl, tree exp)
if (bound)
get_size_range (bound, bndrng);
+ if (*lenrng)
+ {
+ /* Add one for the nul. */
+ lenrng[0] = const_binop (PLUS_EXPR, TREE_TYPE (lenrng[0]),
+ lenrng[0], size_one_node);
+ lenrng[1] = const_binop (PLUS_EXPR, TREE_TYPE (lenrng[1]),
+ lenrng[1], size_one_node);
+
+ if (!bndrng[0])
+ {
+ /* Conservatively use the upper bound of the lengths for
+ both the lower and the upper bound of the operation. */
+ bndrng[0] = lenrng[1];
+ bndrng[1] = lenrng[1];
+ bound = void_type_node;
+ }
+ else
+ {
+ /* Replace the bound on the oparation with the upper bound
+ of the length of the string if the latter is smaller. */
+ if (tree_int_cst_lt (lenrng[1], bndrng[0]))
+ bndrng[0] = lenrng[1];
+ else if (tree_int_cst_lt (lenrng[1], bndrng[1]))
+ bndrng[1] = lenrng[1];
+ }
+ }
+
/* Iterate over the built-in function's formal arguments and check
each const char* against the actual argument. If the actual
argument is declared attribute non-string issue a warning unless
@@ -1689,18 +1738,28 @@ maybe_warn_nonstring_arg (tree fndecl, tree exp)
tree type = TREE_TYPE (decl);
+ /* The maximum number of array elements accessed. */
offset_int wibnd = 0;
if (bndrng[0])
wibnd = wi::to_offset (bndrng[0]);
+ /* Size of the array. */
offset_int asize = wibnd;
+ /* Determine the array size. For arrays of unknown bound and
+ pointers reset BOUND to trigger the appropriate warning. */
if (TREE_CODE (type) == ARRAY_TYPE)
- if (tree arrbnd = TYPE_DOMAIN (type))
- {
- if ((arrbnd = TYPE_MAX_VALUE (arrbnd)))
- asize = wi::to_offset (arrbnd) + 1;
- }
+ {
+ if (tree arrbnd = TYPE_DOMAIN (type))
+ {
+ if ((arrbnd = TYPE_MAX_VALUE (arrbnd)))
+ asize = wi::to_offset (arrbnd) + 1;
+ }
+ else if (bound == void_type_node)
+ bound = NULL_TREE;
+ }
+ else if (bound == void_type_node)
+ bound = NULL_TREE;
location_t loc = EXPR_LOCATION (exp);
diff --git a/gcc/testsuite/c-c++-common/attr-nonstring-3.c b/gcc/testsuite/c-c++-common/attr-nonstring-3.c
index 1c50e0d..a54e73a 100644
--- a/gcc/testsuite/c-c++-common/attr-nonstring-3.c
+++ b/gcc/testsuite/c-c++-common/attr-nonstring-3.c
@@ -47,7 +47,10 @@ char* strndup (const char*, size_t);
#define NONSTRING __attribute__ ((nonstring))
-char str[4];
+/* STR needs to be bigger than ARR to trigger warnings, otherwise
+ since STR must be a string, using both in a string function
+ can be assumed to be safe even if ARR isn't nul-terminated. */
+char str[5];
char arr[4] NONSTRING;
char *ptr;
@@ -55,7 +58,7 @@ char *parr NONSTRING;
struct MemArrays
{
- char str[4];
+ char str[5];
char arr[4] NONSTRING;
char *parr NONSTRING;
};
diff --git a/gcc/testsuite/c-c++-common/attr-nonstring-4.c b/gcc/testsuite/c-c++-common/attr-nonstring-4.c
index 0571e46..597bbb3 100644
--- a/gcc/testsuite/c-c++-common/attr-nonstring-4.c
+++ b/gcc/testsuite/c-c++-common/attr-nonstring-4.c
@@ -37,23 +37,23 @@ int warn_strcmp_cst_2 (void)
int warn_strncmp_cst_1 (void)
{
- return strncmp ("bar", ar5, X); /* { dg-warning "argument 2 declared attribute .nonstring." } */
+ return strncmp ("12345", ar5, X); /* { dg-warning "argument 2 declared attribute .nonstring." } */
}
int warn_strncmp_cst_2 (void)
{
- return strncmp (ar5, "foo", X); /* { dg-warning "argument 1 declared attribute .nonstring." } */
+ return strncmp (ar5, "12345", X); /* { dg-warning "argument 1 declared attribute .nonstring." } */
}
int nowarn_strncmp_cst_1 (void)
{
- return strncmp ("bar", ar5, N);
+ return strncmp ("12345", ar5, N);
}
int nowarn_strncmp_cst_2 (void)
{
- return strncmp (ar5, "foo", N);
+ return strncmp (ar5, "12345", N);
}
diff --git a/gcc/testsuite/c-c++-common/attr-nonstring-6.c b/gcc/testsuite/c-c++-common/attr-nonstring-6.c
new file mode 100644
index 0000000..19ceaac
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/attr-nonstring-6.c
@@ -0,0 +1,185 @@
+/* PR 85623 - strncmp() warns about attribute 'nonstring' incorrectly
+ in -Wstringop-overflow
+ { dg-do compile }
+ { dg-options "-O2 -Wstringop-overflow -ftrack-macro-expansion=0" } */
+
+#include "../gcc.dg/range.h"
+
+#if __cplusplus
+extern "C" {
+#endif
+
+extern int strcmp (const char*, const char*);
+extern int strncmp (const char*, const char*, size_t);
+extern int strncasecmp (const char*, const char*, size_t);
+
+extern size_t strspn (const char*, const char*);
+extern size_t strcspn (const char*, const char*);
+
+#if __cplusplus
+}
+#endif
+
+#define S26 "0123456789abcdefghijklmnopqrstuvwxyz"
+#define S(n) (S26 + sizeof S26 - 1 - (n))
+
+char __attribute__ ((nonstring)) a3[3];
+char __attribute__ ((nonstring)) a5[5];
+
+void sink (int);
+
+#define T(call) sink (call)
+
+void test_strcmp_cst (void)
+{
+ /* Verify that no warning is issued for strcmp() calls with a non-string
+ array argument when the other argument is a string whose length is
+ less than the size of the array. Because the function stops reading
+ at the first nul character there is no chance that it will read past
+ the end of the array. */
+ T (strcmp (S (0), a3));
+ T (strcmp (S (1), a3));
+ T (strcmp (S (2), a3));
+ /* The following reads a3[3]. */
+ T (strcmp (S (3), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ /* The following also reads past the end of a3. */
+ T (strcmp (S (9), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ T (strcmp (a3, S (0)));
+ T (strcmp (a3, S (1)));
+ T (strcmp (a3, S (2)));
+ T (strcmp (a3, S (3))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strcmp (a3, S (9))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+}
+
+
+void test_strcmp_range (const char *s)
+{
+ s = signed_value () < 0 ? S (0) : S (1);
+ T (strcmp (a3, s));
+
+ s = signed_value () < 0 ? S (0) : S (2);
+ T (strcmp (a3, s));
+
+ s = signed_value () < 0 ? S (0) : S (3);
+ T (strcmp (a3, s)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ s = signed_value () < 0 ? S (1) : S (2);
+ T (strcmp (a3, s));
+
+ s = signed_value () < 0 ? S (1) : S (3);
+ T (strcmp (a3, s)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ s = signed_value () < 0 ? S (3) : S (4);
+ T (strcmp (a3, s)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+}
+
+
+void test_strncmp_cst (void)
+{
+ T (strncmp (S (0), a3, 1));
+ T (strncmp (S (1), a3, 2));
+ T (strncmp (S (2), a3, 3));
+ T (strncmp (S (3), a3, 3));
+ T (strncmp (S (3), a3, 4)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ T (strncmp (S (9), a3, 3));
+ T (strncmp (S (9), a3, 4)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strncmp (S (9), a3, 5)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ T (strncmp (a3, S (0), 1));
+ T (strncmp (a3, S (1), 2));
+ T (strncmp (a3, S (2), 3));
+ T (strncmp (a3, S (3), 3));
+ T (strncmp (a3, S (3), 4)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ T (strncmp (a3, S (9), 3));
+ T (strncmp (a3, S (9), 4)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strncmp (a3, S (9), 5)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+}
+
+void test_strncmp_range (const char *s)
+{
+ T (strncmp (a3, S (2), UR (0, 3)));
+ T (strncmp (a3, S (2), UR (1, 4)));
+ T (strncmp (a3, S (2), UR (2, 5)));
+ T (strncmp (a3, S (2), UR (3, 6)));
+ T (strncmp (a3, S (2), UR (4, 7)));
+
+ T (strncmp (a3, S (5), UR (0, 3)));
+ T (strncmp (a3, S (5), UR (1, 4)));
+ T (strncmp (a3, S (5), UR (2, 5)));
+ T (strncmp (a3, S (5), UR (3, 6)));
+ T (strncmp (a3, S (5), UR (4, 7))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strncmp (a3, S (5), UR (7, 9))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ s = signed_value () < 0 ? S (0) : S (1);
+ T (strncmp (a3, s, UR (1, 3)));
+ T (strncmp (a3, s, UR (2, 5)));
+
+ s = signed_value () < 0 ? S (2) : S (5);
+ T (strncmp (a3, s, UR (1, 3)));
+
+ s = signed_value () < 0 ? S (2) : S (5);
+ T (strncmp (a3, s, UR (1, 4)));
+ T (strncmp (a3, s, UR (2, 5)));
+ T (strncmp (a3, s, UR (3, 6)));
+ T (strncmp (a3, s, UR (4, 7))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+}
+
+void test_strncasecmp (void)
+{
+ T (strncasecmp (S (0), a3, 1));
+ T (strncasecmp (S (1), a3, 2));
+ T (strncasecmp (S (2), a3, 3));
+ T (strncasecmp (S (3), a3, 3));
+ T (strncasecmp (S (3), a3, 4)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ T (strncasecmp (S (9), a3, 3));
+ T (strncasecmp (S (9), a3, 4)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strncasecmp (S (9), a3, 5)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ T (strncasecmp (a3, S (0), 1));
+ T (strncasecmp (a3, S (1), 2));
+ T (strncasecmp (a3, S (2), 3));
+ T (strncasecmp (a3, S (3), 3));
+ T (strncasecmp (a3, S (3), 4)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ T (strncasecmp (a3, S (9), 3));
+ T (strncasecmp (a3, S (9), 4)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strncasecmp (a3, S (9), 5)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+}
+
+void test_strspn (void)
+{
+ /* strspn must traverse all characters in the second argument except
+ when the first string is empty. */
+ T (strspn (S (0), a3));
+ T (strspn (S (1), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strspn (S (2), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strspn (S (3), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strspn (S (9), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ /* Similarly, strspn must traverse all characters in the first argument
+ except when the second string is empty. */
+ T (strspn (a3, S (0)));
+ T (strspn (a3, S (1))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strspn (a3, S (2))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strspn (a3, S (3))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strspn (a3, S (9))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+}
+
+void test_strcspn (void)
+{
+ T (strcspn (S (0), a3));
+ T (strcspn (S (1), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strcspn (S (2), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strcspn (S (3), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strcspn (S (9), a3)); /* { dg-warning "\\\[-Wstringop-overflow" } */
+
+ T (strcspn (a3, S (0))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strcspn (a3, S (1))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strcspn (a3, S (2))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strcspn (a3, S (3))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+ T (strcspn (a3, S (9))); /* { dg-warning "\\\[-Wstringop-overflow" } */
+}
