Re: [PATCH v4 1/3] kbuild: allow symbol whitelisting with TRIM_UNUSED_KSYMS

[Matthias Maennich]

On Wed, Feb 12, 2020 at 08:21:38PM +0000, 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, which is a first step in the
> direction of getting all vendors to contribute their code upstream.
>
> 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.
>
> Acked-by: Jessica Yu <jeyu@xxxxxxxxxx>
> Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
> ---
> init/Kconfig                | 13 +++++++++++++
> scripts/adjust_autoksyms.sh |  5 +++++
> 2 files changed, 18 insertions(+)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index cfee56c151f1..58b672afceb2 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -2210,6 +2210,19 @@ config TRIM_UNUSED_KSYMS
>
> 	  If unsure, or if you need to build out-of-tree modules, say N.
>
> +config UNUSED_KSYMS_WHITELIST
> +	string "Whitelist of symbols to keep in ksymtab"
> +	depends on TRIM_UNUSED_KSYMS
> +	help
> +	  By default, all unused exported symbols will be un-exported from the
> +	  build when TRIM_UNUSED_KSYMS is selected.
> +
> +	  UNUSED_KSYMS_WHITELIST allows to whitelist symbols that must be kept
> +	  exported at all times, even in absence of in-tree users. The value to
> +	  set here is the path to a text file containing the list of symbols,
> +	  one per line. The path can be absolute, or relative to the kernel
> +	  source tree.
> +
> endif # MODULES
>
> config MODULES_TREE_LOOKUP
> diff --git a/scripts/adjust_autoksyms.sh b/scripts/adjust_autoksyms.sh
> index a904bf1f5e67..93f4d10e66e6 100755
> --- a/scripts/adjust_autoksyms.sh
> +++ b/scripts/adjust_autoksyms.sh
> @@ -38,6 +38,10 @@ esac
> # We need access to CONFIG_ symbols
> . include/config/auto.conf
>
> +# Use 'eval' to expand the whitelist path and check if it is relative
> +eval ksym_wl="${CONFIG_UNUSED_KSYMS_WHITELIST:-/dev/null}"
> +[ "${ksym_wl:0:1}" = "/" ] || ksym_wl="$abs_srctree/$ksym_wl"
> +
> # Generate a new ksym list file with symbols needed by the current
> # set of modules.
> cat > "$new_ksyms_file" << EOT
> @@ -48,6 +52,7 @@ cat > "$new_ksyms_file" << EOT
> EOT
> sed 's/ko$/mod/' modules.order |
> xargs -n1 sed -n -e '2{s/ /\n/g;/^$/!p;}' -- |
> +cat - "$ksym_wl" |

In case the whitelist file can't be found, the error message is

 cat: path/to/file: file not found

I wonder whether we can make this error message a bit more specific by
telling the user that the KSYMS_WHITELIST is missing.

With the above addressed (and your amend for the absolute path test),
please feel free to add

Tested-by: Matthias Maennich <maennich@xxxxxxxxxx>
Reviewed-by: Matthias Maennich <maennich@xxxxxxxxxx>

Cheers,
Matthias

> sort -u |
> sed -e 's/\(.*\)/#define __KSYM_\1 1/' >> "$new_ksyms_file"
>
> --
> 2.25.0.225.g125e21ebc7-goog