Re: kcsan -Wmaybe-uninitialized warning in ntfs3

On Tue, Apr 21, 2026, at 11:33, Marco Elver wrote:
> On Tue, 21 Apr 2026 at 09:54, Arnd Bergmann <[email protected]> wrote:
>> What happens here is that copy_from_user() is used in a very
>> normal way to copy a user space structure into a previously
>> uninitialized on-stack buffer. With KCSAN enabled, this passes
>> the pointer to that buffer into __kcsan_check_access(), which
>> takes a 'const volatile void *' pointer. Gcc correctly notices
>> that there is no way to access the data behind this pointer
>> when it hasn't been initialized yet, as both read and write
>> would cause undefined behavior.
>>
>> I'm not sure what a good solution would be to avoid this,
>> but I assume this should be fixed in the kcsan instrumentation.
>> I tried changing that code to pass non-const pointers for
>> any instrument_write variant, which does avoid the warning,
>> but also adds a bit of complexity, see below.
>>
>> Any other ideas?
>
> Indeed, the below is rather complex, and seems unnecessary to me just
> to suppress this warning which not even the whole kernel enables.
>
> How can I reproduce this? GCC version? .config? I can't seem to with
> an x86 defconfig + CONFIG_NTFS3_FS + KCSAN config + GCC on linux-next
> (20260420).

I ran into this during randconfig testing, I have attached a
reproducer .config here, but have not tried to narrow down the
configuration options that are required for triggering it.

The compiler I used is a recent gcc-16.0.1 snapshot, but
the same config produces the warning using earlier compilers
as well, I see this with gcc-12.5 but not with gcc-11.5 or
earlier.

> There's likely a simpler option (attribute or warning suppression
> around these functions) I want to try.

I tried the 'unused' and 'uninitialized' attributes on the
argument without success.

I'm a bit worried that just using __diag_ignore() around the
call would still leave the problem that gcc warns about, in case
the compiler decides that this is undefined behavior and do
something we don't want.

Changing the type of the 'ptr' argument in the internal
__kcsan_check_access() call to 'unsigned long' or perhaps
a non-const pointer should avoid the warning.

       Arnd