[PATCH v3] sparse: treat function pointers as pointers to const data

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

This code snippet:

static void bar(void const *arg)
{
	int (*foo)(void) = arg;
}

produces the following warning:

test.c:4:28: warning: incorrect type in initializer (different modifiers)
test.c:4:28:    expected int ( *foo )( ... )
test.c:4:28:    got void const *arg

which is caused by the fact that the function pointer 'foo' is not annotated
as being a pointer to const data. However, dereferencing a function pointer
does not produce an lvalue, so a function pointer points to const data by
definition, and we should treat it accordingly.

To avoid producing a warning on the inverse case, i.e.,

static void bar(void)
{
	void *foo = bar;
}

we only address the case where the function pointer is the target of
an assignment.

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
v3: add rationale for restriction to assignments to commit message
    add R-b

v2: only treat function pointers as pointers to const data when they are the
    target of an assignment

 evaluate.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/evaluate.c b/evaluate.c
index 66556150ddac..a5a830978bda 100644
--- a/evaluate.c
+++ b/evaluate.c
@@ -1359,6 +1359,15 @@ static int compatible_assignment_types(struct expression *expr, struct symbol *t
 				typediff = "different address spaces";
 				goto Err;
 			}
+			/*
+			 * If this is a function pointer assignment, it is
+			 * actually fine to assign a pointer to const data to
+			 * it, as a function pointer points to const data
+			 * implicitly, i.e., dereferencing it does not produce
+			 * an lvalue.
+			 */
+			if (b1->type == SYM_FN)
+				mod1 |= MOD_CONST;
 			if (mod2 & ~mod1) {
 				typediff = "different modifiers";
 				goto Err;
-- 
1.8.3.2