Re: [PATCH] kbuild: allow symbol whitelisting with TRIM_UNUSED_KSYMS

Nicolas Pitre
2020-01-29T17:11:21Z

On Wed, 29 Jan 2020, Quentin Perret wrote:

> CONFIG_TRIM_UNUSED_KSYMS currently removes all unused exported symbols
> from ksymtab. This works really well when using in-tree drivers, but
> cannot be used in its current form if some of them are out-of-tree.
>
> Indeed, even if the list of symbols required by out-of-tree drivers is
> known at compile time, the only solution today to guarantee these don't
> get trimmed is to set CONFIG_TRIM_UNUSED_KSYMS=n. This not only wastes
> space, but also makes it difficult to control the ABI usable by vendor
> modules in distribution kernels such as Android. Being able to control
> the kernel ABI surface is particularly useful to ship a unique Generic
> Kernel Image (GKI) for all vendors.
>
> As such, attempt to improve the situation by enabling users to specify a
> symbol 'whitelist' at compile time. Any symbol specified in this
> whitelist will be kept exported when CONFIG_TRIM_UNUSED_KSYMS is set,
> even if it has no in-tree user. The whitelist is defined as a simple
> text file, listing symbols, one per line.

The idea is sound to me. But...

> diff --git a/scripts/adjust_autoksyms.sh b/scripts/adjust_autoksyms.sh
> index a904bf1f5e67..1a6f7f377230 100755
> --- a/scripts/adjust_autoksyms.sh
> +++ b/scripts/adjust_autoksyms.sh
> @@ -48,6 +48,7 @@ cat > "$new_ksyms_file" << EOT
> EOT
> sed 's/ko$/mod/' modules.order |
> xargs -n1 sed -n -e '2{s/ /\n/g;/^$/!p;}' -- |
> +cat - $CONFIG_UNUSED_KSYMS_WHITELIST |

This is a nice trick, however it'll fail if the file path contains
spaces or other shell special characters. You could try something like
this:

[ -z "$CONFIG_UNUSED_KSYMS_WHITELIST" ] \
&& whitelist= \
|| whitelist="\"$CONFIG_UNUSED_KSYMS_WHITELIST\""

And then...

eval cat - $whitelist | ...

This way, if $CONFIG_UNUSED_KSYMS_WHITELIST is non empty, it'll get
quoted.

Nicolas