Re: [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree -v1: Eliminate the Linux kernel's "Dependency Hell"

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Jan 4, 2022 at 2:47 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Nathan Chancellor <nathan@kernel.org> wrote:
>
> > Hi Ingo,
> >
> > On Sun, Jan 02, 2022 at 10:57:35PM +0100, Ingo Molnar wrote:
> > I took the series for a spin with clang and GCC on arm64 and x86_64 and
> > I found a few warnings/errors.
>
> Thank you!
>
> > 1. Position of certain attributes
> >
> > In some commits, you move the cacheline_aligned attributes from after
> > the closing brace on structures to before the struct keyword, which
> > causes clang to warn (and error with CONFIG_WERROR):
> >
> > In file included from arch/arm64/kernel/asm-offsets.c:9:
> > In file included from arch/arm64/kernel/../../../kernel/sched/per_task_area_struct.h:33:
> > In file included from ./include/linux/perf_event_api.h:17:
> > In file included from ./include/linux/perf_event_types.h:41:
> > In file included from ./include/linux/ftrace.h:18:
> > In file included from ./arch/arm64/include/asm/ftrace.h:53:
> > In file included from ./include/linux/compat.h:11:
> > ./include/linux/fs_types.h:997:1: error: attribute '__aligned__' is ignored, place it after "struct" to apply attribute to type declaration [-Werror,-Wignored-attributes]
> > ____cacheline_aligned
> > ^
> > ./include/linux/cache.h:41:46: note: expanded from macro '____cacheline_aligned'
> > #define ____cacheline_aligned __attribute__((__aligned__(SMP_CACHE_BYTES)))
>
> Yeah, so this is a *really* stupid warning from Clang.
>
> Putting the attribute after 'struct' risks the hard to track down bugs when
> a <linux/cache.h> inclusion is missing, which scenario I pointed out in
> this commit:
>
>     headers/deps: dcache: Move the ____cacheline_aligned attribute to the head of the definition
>
>     When changing <linux/dcache.h> I removed the <linux/spinlock_api.h> header,
>     which caused a couple of hundred of mysterious, somewhat obscure link time errors:
>
>       ld: net/sctp/tsnmap.o:(.bss+0x0): multiple definition of `____cacheline_aligned_in_smp'; init/do_mounts_rd.o:(.bss+0x0): first defined here
>       ld: net/sctp/tsnmap.o:(.bss+0x40): multiple definition of `____cacheline_aligned'; init/do_mounts_rd.o:(.bss+0x40): first defined here
>       ld: net/sctp/debug.o:(.bss+0x0): multiple definition of `____cacheline_aligned_in_smp'; init/do_mounts_rd.o:(.bss+0x0): first defined here
>       ld: net/sctp/debug.o:(.bss+0x40): multiple definition of `____cacheline_aligned'; init/do_mounts_rd.o:(.bss+0x40): first defined here
>
>     After a bit of head-scratching, what happened is that 'struct dentry_operations'
>     has the ____cacheline_aligned attribute at the tail of the type definition -
>     which turned into a local variable definition when <linux/cache.h> was not
>     included - which <linux/spinlock_api.h> includes into <linux/dcache.h> indirectly.
>
>     There were no compile time errors, only link time errors.
>
>     Move the attribute to the head of the definition, in which case
>     a missing <linux/cache.h> inclusion creates an immediate build failure:
>
>       In file included from ./include/linux/fs.h:9,
>                        from ./include/linux/fsverity.h:14,
>                        from fs/verity/fsverity_private.h:18,
>                        from fs/verity/read_metadata.c:8:
>       ./include/linux/dcache.h:132:22: error: expected ‘;’ before ‘struct’
>         132 | ____cacheline_aligned
>             |                      ^
>             |                      ;
>         133 | struct dentry_operations {
>             | ~~~~~~
>
>     No change in functionality.
>
>     Signed-off-by: Ingo Molnar <mingo@kernel.org>
>
> Can this Clang warning be disabled?

Clang is warning that the attribute will be ignored because of that
positioning. If you disable the warning, code will probably stop
working as intended.  This warning has at least been helping us make
the kernel coding style more consistent.

This made me think of d5b421fe02827 ("docs: Explain the desired
position of function attributes"), where we adding some text to
Documentation/process/coding-style.rst about the positioning of
__attribute__'s in function signatures, but I guess this case is data.
We probably should add something to the coding style about attributes
on data, too.

The C standards body is also working on standardizing attributes; at
the least I expect some of these things to be ironed out more soon.

>
> > 2. Error with CONFIG_SHADOW_CALL_STACK
>
> So this feature depends on Clang:
>
>  # Supported by clang >= 7.0
>  config CC_HAVE_SHADOW_CALL_STACK
>          def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
>
> No way to activate it under my GCC cross-build toolchain, right?
>
> But ... I hacked the build mode on with GCC using this patch:

Dan Li is working on a GCC patch. If you're up for building GCC from source:
https://gcc.gnu.org/pipermail/gcc-patches/2021-December/586204.html

--

This is a really cool series Ingo.  I'm sure Arnd has seen it by now,
but Arnd has been thinking about this area a lot, too.  I haven't but
I have played with running "include what you use" on the kernel
sources; Kconfig being the biggest impediment to that approach.

To me, I'm most nervous about "backsliding;" let's say this work
lands, at some point probably years in the future, I assume without
any form of automation that we might find ourselves at a similar point
of header dependencies getting all tangled again.

What are your thoughts on where/how/what we could automate to try to
help developers in the future keep their header dependencies simpler?
(Sorry if this was already answered in the cover letter)

It would be really useful if you were planning a talk at something
like plumbers how you go about making these changes.  I really hope
once others understand your workflow that we might help with some form
of automation.  Nice work!
--
Thanks,
~Nick Desaulniers