On Mon, Feb 28, 2022 at 04:45:11PM -0800, Linus Torvalds wrote:

Really. The "-Wshadow doesn't work on the kernel" is not some new
issue, because you have to do completely insane things to the source
code to enable it.

The first big glitch with -Wshadow was with shadowed global variables.
GCC 4.8 fixed that, but it still yells about shadowed functions. What
_almost_ works is -Wshadow=local. At first glace, all the warnings
look solvable, but then one will eventually discover __wait_event()
and associated macros that mix when and how deeply it intentionally
shadows variables. :)

Another way to try to catch misused shadow variables is
-Wunused-but-set-varible, but it, too, has tons of false positives.

I tried to capture some of the rationale and research here:
https://github.com/KSPP/linux/issues/152

-- 
Kees Cook