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Date:   Tue, 5 Dec 2023 22:32:05 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Andy Shevchenko <andy@kernel.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>, tanzirh@google.com,
        Kees Cook <keescook@chromium.org>,
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        Nick DeSaulniers <nnn@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, llvm@lists.linux.dev
Subject: Re: [PATCH] lib/string: shrink lib/string.i via IWYU
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On Tue, Dec 05, 2023 at 10:15:21PM +0000, Al Viro wrote:

> It would certainly be nice to have such information in the tree;
> "where should I pick $SYMBOL from?" is something one needs to
> find out often enough.  To a large extent it's covered by "where
> in include/*.h do we have it defined?", but that's not all there
> is to it.  E.g. "get_user() => use linux/uaccess.h".
> 
> There's also stuff like "$SYMBOL should not be used outside of arch/*
> and include/*, better use $OTHER_SYMBOL", etc.

Speaking of...

arch/arm64/include/asm/asm-prototypes.h:18:#include <asm/uaccess.h>
arch/powerpc/include/asm/inst.h:8:#include <asm/uaccess.h>
arch/powerpc/kvm/book3s_xive_native.c:16:#include <asm/uaccess.h>
arch/powerpc/mm/book3s64/radix_pgtable.c:31:#include <asm/uaccess.h>
arch/riscv/kernel/sys_riscv.c:15:#include <asm/uaccess.h>
arch/xtensa/include/asm/asm-prototypes.h:10:#include <asm/uaccess.h>
include/linux/uaccess.h:11:#include <asm/uaccess.h>
tools/testing/crypto/chacha20-s390/test-cipher.c:8:#include <asm/uaccess.h>

Only one of those is legitimate (linux/uaccess.h); asm-prototypes.h
ones are borderline, but probably should be pulling linux/uaccess.h,
everything in *.c is really wrong.  Powerpc asm/inst.h is the really
unpleasant one here - it's pulled by quite a few other places, including
several asm/*.h.  Hell knows; might be worth splitting the
{__,}{get_user_instr,copy_inst_from_kernel_nofault}() off to
a separate header (asm/inst-uaccess.h?), to be included only by the
files that use any of those (that would be
arch/powerpc/kernel/{align,hw_breakpoint_constraints,module_32,traps,vecemu}.c
and arch/powerpc/kernel/trace/{ftrace,ftrace_64_pg}.c - not a single header
among them).  That header would pull linux/uaccess.h and asm/inst.h wouldn't
need any uaccess at all; might spill into explicit includes of linux/uaccess.h
in some of the places in arch/powerpc that pull asm/inst.h, directly or
not...