usercopy_warn in __copy_to_user

From: rishabhb
Date: Wed Dec 19 2018 - 14:51:23 EST

In the 4.19 kernel, we are seeing a USERCOPY_WARN in __copy_to_user during bootup.
The code-flow is something like this:

struct sigset_t *set;
__copy_to_user(&sf->uc.uc_sigmask, set, sizeof(*set))

__copy_to_user(void __user *to, const void *from, unsigned long n)
kasan_check_read(from, n);
check_object_size(from, n, true);
return raw_copy_to_user(to, from, n);

static __always_inline void check_object_size(const void *ptr,
unsigned long n, bool to_user)
if (!__builtin_constant_p(n))
__check_object_size(ptr, n, to_user);

Since sizeof(*set) is constant, __builtin_constant_p(n) should return true.
But we are seeing that its returning the value as false. Because of which
the code goes on to __check_object_size and generates a USERCOPY_WARN
("usercopy: WARN() on slab cache usercopy
region violations").

We are using LLVM clang version 6.0 to compile the kernel and not gcc.
In clang, __builtin_constant_p is evaluated immediately, before inlining
or other optimizations run, gcc evaluates it later.
We believe that maybe causing __builtin_constant_p(n) to return false.
Thereâs upstream work to change LLVM, so __builtin_constant_p works more
like gcc when optimization is enabled, but its still in progress.

For this scenario is there a way to avoid the warning? Should the code be
written in a different to avoid dependency on compiler?