Messages in this thread | | | Date | Wed, 19 Dec 2018 11:51:19 -0800 | From | rishabhb@codeauro ... | Subject | usercopy_warn in __copy_to_user |
| |
In the 4.19 kernel, we are seeing a USERCOPY_WARN in __copy_to_user during bootup. The code-flow is something like this:
(arch/arm64/kernel/signal.c) struct sigset_t *set; __copy_to_user(&sf->uc.uc_sigmask, set, sizeof(*set))
(include/linux/uaccess.h) __copy_to_user(void __user *to, const void *from, unsigned long n) { might_fault(); kasan_check_read(from, n); check_object_size(from, n, true); return raw_copy_to_user(to, from, n); }
(include/linux/thread_info.h) 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. There’s 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?
Thanks, Rishabh
| |