Re: [poky] [PATCH v3] poky-tiny: enable section removal

["Paul Barker" <pbarker@konsulko.com>]

On Mon, 9 Nov 2020 at 22:38, Sinan Kaya <okaya@kernel.org> wrote:
>
> Use GCC sections flags so that unused sections can be garbage
> collected at link time.
>
> Signed-off-by: Sinan Kaya <okaya@kernel.org>
> ---
>  meta-poky/conf/distro/poky-tiny.conf | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/meta-poky/conf/distro/poky-tiny.conf b/meta-poky/conf/distro/poky-tiny.conf
> index c6d4b88f83d..1e37394875d 100644
> --- a/meta-poky/conf/distro/poky-tiny.conf
> +++ b/meta-poky/conf/distro/poky-tiny.conf
> @@ -122,3 +122,21 @@ PNBLACKLIST[core-image-weston] = "not buildable with poky-tiny"
>
>  # Disable python usage in opkg-utils since it won't build with tiny config
>  PACKAGECONFIG_remove_pn-opkg-utils = "python"
> +
> +# set default for all targets
> +CFLAGS_append = " -ffunction-sections -fdata-sections"
> +LDFLAGS_append = " -Wl,--gc-sections"
> +
> +# gc-sections requires either an entry or an undefined symbol
> +CFLAGS_remove_pn-glibc = "-ffunction-sections -fdata-sections"
> +LDFLAGS_remove_pn-glibc = "-Wl,--gc-sections"
> +
> +# perl won't cross compile with sections
> +CFLAGS_remove_pn-perl = "-ffunction-sections -fdata-sections"
> +LDFLAGS_remove_pn-perl = "-Wl,--gc-sections"
> +
> +# Unknown float word ordering. You need to manually preset
> +CFLAGS_remove_pn-cairo = "-ffunction-sections -fdata-sections"
> +LDFLAGS_remove_pn-cairo = "-Wl,--gc-sections"
> +
> +

Using _remove in a common layer like this is discouraged as it's very
difficult to override anywhere else.

I'd recommend defining new variables with default values which enable
the flags you want then append them to CFLAGS and LDFLAGS
respectively. This allows the new variables to be overridden per
package very easily. Also consider that you may want to apply these
only for the target and not for native/nativesdk packages.

Here's a quick untested example:

CFLAGS_SECTION_REMOVAL = "-ffunction-sections -fdata-sections"
LDFLAGS_SECTION_REMOVAL = "-Wl,--gc-sections"

CFLAGS_SECTION_REMOVAL_pn-glibc = ""
LDFLAGS_SECTION_REMOVAL_pn-glibc = ""
CFLAGS_SECTION_REMOVAL_pn-perl = ""
LDFLAGS_SECTION_REMOVAL_pn-perl = ""
CFLAGS_SECTION_REMOVAL_pn-cairo = ""
LDFLAGS_SECTION_REMOVAL_pn-cairo = ""

CFLAGS_append_class-target = " ${CFLAGS_SECTION_REMOVAL}"
LDFLAGS_append_class-target = " ${LDFLAGS_SECTION_REMOVAL}"

You could also look at `meta/conf/distro/include/security_flags.inc`
in oe-core as an example.

And lastly it looks like you have a couple of extra newlines added at
the end of the file, these can be dropped.

Thanks,

-- 
Paul Barker
Konsulko Group