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

From: Jessica Yu
Date: Thu Feb 06 2020 - 10:57:00 EST


+++ 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!