fortify: Fix __compiletime_strlen() under UBSAN_BOUNDS_LOCAL

In the Linux kernel, the following vulnerability has been resolved:

fortify: Fix __compiletime_strlen() under UBSAN_BOUNDS_LOCAL

With CONFIG_FORTIFY=y and CONFIG_UBSAN_LOCAL_BOUNDS=y enabled, we observe

a runtime panic while running Android's Compatibility Test Suite's (CTS)

android.hardware.input.cts.tests. This is stemming from a strlen()

call in hidinput_allocate().

__compiletime_strlen() is implemented in terms of __builtin_object_size(),

then does an array access to check for NUL-termination. A quirk of

__builtin_object_size() is that for strings whose values are runtime

dependent, __builtin_object_size(str, 1 or 0) returns the maximum size

of possible values when those sizes are determinable at compile time.

Example:

static const char *v = "FOO BAR";

static const char *y = "FOO BA";

unsigned long x (int z) {

// Returns 8, which is:

// max(__builtin_object_size(v, 1), __builtin_object_size(y, 1))

return __builtin_object_size(z ? v : y, 1);

}

So when FORTIFY_SOURCE is enabled, the current implementation of

__compiletime_strlen() will try to access beyond the end of y at runtime

using the size of v. Mixed with UBSAN_LOCAL_BOUNDS we get a fault.

hidinput_allocate() has a local C string whose value is control flow

dependent on a switch statement, so __builtin_object_size(str, 1)

evaluates to the maximum string length, making all other cases fault on

the last character check. hidinput_allocate() could be cleaned up to

avoid runtime calls to strlen() since the local variable can only have

literal values, so there's no benefit to trying to fortify the strlen

call site there.

Perform a __builtin_constant_p() check against index 0 earlier in the

macro to filter out the control-flow-dependant case. Add a KUnit test

for checking the expected behavioral characteristics of FORTIFY_SOURCE

internals.