Re: [PATCH v2] kbuild: allow symbol whitelisting with TRIM_UNUSED_KSYMS

[Jessica Yu]

+++ Quentin Perret [29/01/20 18:15 +0000]:
> 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.
>
> Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
>
> ---
> v2: make sure to quote the whitelist path properly (Nicolas)
> ---
> init/Kconfig                | 12 ++++++++++++
> scripts/adjust_autoksyms.sh |  1 +
> 2 files changed, 13 insertions(+)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index a34064a031a5..d9c977ef7de5 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -2180,6 +2180,18 @@ 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 trimmed from the
> +	  build when TRIM_UNUSED_KSYMS is selected.

Hm, I thought TRIM_UNUSED_KSYMS just *unexports* unused symbols, no?
"Trimmed from the build" sounds like the symbols are not compiled in
or dropped completely. Please correct me if I misunderstood.

> +	  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.
> +
> endif # MODULES
>
> config MODULES_TREE_LOOKUP
> diff --git a/scripts/adjust_autoksyms.sh b/scripts/adjust_autoksyms.sh
> index a904bf1f5e67..8e1b7f70e800 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:-/dev/null}" |
> sort -u |
> sed -e 's/\(.*\)/#define __KSYM_\1 1/' >> "$new_ksyms_file"

In general, I agree with the motivation behind this patch, even though
we try not to provide too much support for out-of-tree modules.
However, in this particular case, I think it's fair to provide some
mechanism to keep some exported symbols around that we know will have
users, despite having no in-tree users for a particular
configuration/build. For example, livepatch exports symbols that have
no in-tree users (except for the sample livepatch module, but you'd
have to enable SAMPLES), and all livepatch users will always be out of
tree. 

I also agree with Matthias' feedback, so assuming that gets
incorporated into v3:

Acked-by: Jessica Yu <jeyu@xxxxxxxxxx>

Thanks!