issue with _Static_assert and __builtin()s
Ben Dooks <[email protected]> Wed, 7 Jan 2026 19:41:45 +0000
| Newsgroups | org.kernel.vger.linux-sparse |
|---|---|
| Organization | Codethink Limited. |
| Message-ID | <[email protected]> |
So I am looking at why the Linux kernel's check for nul characters
in strings is causing errors out of sparse.
EG:
drivers/md/dm.c:3813:1: error: bad constant expression
drivers/md/dm.c:3814:1: error: bad constant expression
drivers/md/dm.c:3816:1: error: bad constant expression
drivers/md/dm.c:3817:1: error: bad constant expression
I've tracked it down to the sizeof(str) - 1 == __builtin_strlen(str)
failing to be a good constant expression...
This is an example of the assert which isn't working:
_Static_assert(sizeof("moo") - 1 == __builtin_strlen("moo"), "nul!");
This does at least get past w/o warnings
_Static_assert(__builtin_types_compatible_p(int, int), "doh!");
I've had a go at updating builtin.c to deal with __builtin_strlen()
for a string constant (attached) but that's just changing the
output to "error: bad integer constant expression" so not sure
what I've missed here.
--
Ben Dooks http://www.codethink.co.uk/
Senior Engineer Codethink - Providing Genius
https://www.codethink.co.uk/privacy.html
initial-strlen.patch
(text/x-patch, 1.7 KB)
diff --git a/builtin.c b/builtin.c
index e4751445..d7e56b34 100644
--- a/builtin.c
+++ b/builtin.c
@@ -596,6 +596,49 @@ static struct symbol_op object_size_op = {
.expand = expand_object_size,
};
+#include <string.h>
+static int expand_strlen(struct expression *expr, int cost)
+{
+ struct expression *init, *arg = first_expression(expr->args);
+ unsigned long val = 0;
+
+ if (!arg)
+ return UNSAFE; // ? ok
+
+ switch (arg->type) {
+ case EXPR_STRING:
+ //todo//
+ break;
+ case EXPR_SYMBOL:
+ if (arg->symbol->ident)
+ goto not_literal;
+
+ init = arg->symbol->initializer;
+ if (!init || init->type != EXPR_STRING)
+ goto not_literal;
+
+ val = strlen(init->string->data);
+ break;
+ default:
+ goto not_literal;
+ break;
+ }
+
+ expr->type = EXPR_VALUE;
+ expr->flags |= CEF_SET_ICE;
+ expr->value = val;
+ expr->taint = 0;
+ return 0;
+
+not_literal:
+ return UNSAFE;
+}
+
+
+static struct symbol_op strlen_op = {
+ .expand = expand_strlen,
+};
+
/*
* Builtin functions
*/
@@ -775,7 +818,7 @@ static const struct builtin_fn builtins_common[] = {
{ "__builtin_strcpy", &string_ctype, 0, { &string_ctype, &const_string_ctype }},
{ "__builtin_strcspn", size_t_ctype, 0, { &const_string_ctype, &const_string_ctype }},
{ "__builtin_strdup", &string_ctype, 0, { &const_string_ctype }},
- { "__builtin_strlen", size_t_ctype, 0, { &const_string_ctype }},
+ { "__builtin_strlen", size_t_ctype, 1, { &string_ctype }, .op = &strlen_op },
{ "__builtin_strncasecmp", &int_ctype, 0, { &const_string_ctype, &const_string_ctype, size_t_ctype }},
{ "__builtin_strncat", &string_ctype, 0, { &string_ctype, &const_string_ctype, size_t_ctype }},
{ "__builtin_strncmp", &int_ctype, 0, { &const_string_ctype, &const_string_ctype, size_t_ctype }},