linux-kernel - Re: [PATCH v4] ARM: decompressor: simplify libfdt builds

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Message-ID: <20200422075854.GK25745@shell.armlinux.org.uk>
Date:   Wed, 22 Apr 2020 08:58:54 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] ARM: decompressor: simplify libfdt builds

On Wed, Apr 22, 2020 at 09:44:38AM +0200, Geert Uytterhoeven wrote:
> Hi Yamada-san, Kees, Russell,
> 
> -CC RMK's patch system
> +CC lakml
> 
> On Sun, Apr 19, 2020 at 9:26 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
> > Copying source files during the build time may not end up with
> > as clean code as expected.
> >
> > lib/fdt*.c simply wrap scripts/dtc/libfdt/fdt*.c, and it works
> > nicely. Let's follow this approach for the arm decompressor, too.
> >
> > Add four wrappers, arch/arm/boot/compressed/fdt*.c and remove
> > the Makefile messes. Another nice thing is we no longer need to
> > maintain the own libfdt_env.h because the decompressor can include
> > <linux/libfdt_env.h>.
> >
> > There is a subtle problem when generated files are turned into
> > check-in files.
> >
> > When you are doing a rebuild of an existing object tree with O=
> > option, there exists stale "shipped" copies that the old Makefile
> > implementation created. The build system ends up with compiling the
> > stale generated files because Make searches for prerequisites in the
> > current directory, i.e. $(objtree) first, and then the directory
> > listed in VPATH, i.e. $(srctree).
> >
> > To mend this issue, I added the following code:
> >
> >   ifdef building_out_of_srctree
> >   $(shell rm -f $(addprefix $(obj)/, fdt_rw.c fdt_ro.c fdt_wip.c fdt.c))
> >   endif
> >
> > This will need to stay for a while because "git bisect" crossing this
> > commit, otherwise, would result in a build error.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> 
> This is now commit 365a6327cd643eed ("ARM: 8968/1: decompressor:
> simplify libfdt builds") in arm/for-next.
> 
> In light of reworking "[PATCH v5] ARM: boot: Obtain start of physical
> memory from DTB"[1] on top of this, which would conditionally add
> another source file to libfdt_objs, I have a few questions.
> 
> > --- a/arch/arm/boot/compressed/Makefile
> > +++ b/arch/arm/boot/compressed/Makefile
> > @@ -76,29 +76,31 @@ compress-$(CONFIG_KERNEL_LZMA) = lzma
> >  compress-$(CONFIG_KERNEL_XZ)   = xzkern
> >  compress-$(CONFIG_KERNEL_LZ4)  = lz4
> >
> > -# Borrowed libfdt files for the ATAG compatibility mode
> > -
> > -libfdt         := fdt_rw.c fdt_ro.c fdt_wip.c fdt.c
> > -libfdt_hdrs    := fdt.h libfdt.h libfdt_internal.h
> > -
> > -libfdt_objs    := $(addsuffix .o, $(basename $(libfdt)))
> > +ifeq ($(CONFIG_ARM_ATAG_DTB_COMPAT),y)
> > +libfdt_objs = fdt_rw.o fdt_ro.o fdt_wip.o fdt.o atags_to_fdt.o
> >
> 
> I guess the code below can be moved out of the ifeq block, as it doesn't
> really do anything if CONFIG_ARM_ATAG_DTB_COMPAT=n, and $(libfdt_objs)
> becomes empty?
> If not, I think I'll have to add a new Kconfig symbol ARM_BOOT_LIBFDT,
> to be selected by ARM_ATAG_DTB_COMPAT and USE_OF.
> 
> > -$(addprefix $(obj)/,$(libfdt) $(libfdt_hdrs)): $(obj)/%: $(srctree)/scripts/dtc/libfdt/%
> > -       $(call cmd,shipped)
> > +OBJS   += $(libfdt_objs)
> >
> > -$(addprefix $(obj)/,$(libfdt_objs) atags_to_fdt.o): \
> > -       $(addprefix $(obj)/,$(libfdt_hdrs))
> > +# -fstack-protector-strong triggers protection checks in this code,
> > +# but it is being used too early to link to meaningful stack_chk logic.
> > +nossp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
> > +$(foreach o, $(libfdt_objs), \
> > +       $(eval CFLAGS_$(o) := -I $(srctree)/scripts/dtc/libfdt $(nossp-flags-y)))
> 
> Is there a real reason this is only applied to a subset of the C object
> files, and not to all of them? Or have we been lucky so far, by not
> triggering the issue in decompressed.c, misc.c, and string.c (yet)?

I don't remember the details. See commit 7f66cd3f5420, which came from
Kees which introduced this.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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