Re: [PATCH] kbuild: remove duplicate dependencies from .mod files

[Sami Tolvanen]

On Wed, Feb 12, 2020 at 5:23 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
> In which case are undefined symbols duplicated?

When a module consists of multiple compilation units, which depend on
the same external symbols. In Android, we ran into this when adding
hardening features that all depend on an external error handler
function with a rather long name. When CONFIG_TRIM_UNUSED_SYMS was
later enabled, we ran into this:

$ llvm-nm drivers/gpu/drm/nouveau/nouveau.o | sed -n 's/^  *U //p' |
xargs echo | wc
      2    9136  168660

xargs defaults to 128kiB limit for command line size, so the output
was split into two lines, which means some of the dependencies were
dropped and we ran into modpost errors. One method of fixing this is
to increase the limit:

$ llvm-nm drivers/gpu/drm/nouveau/nouveau.o | sed -n 's/^  *U //p' |
xargs -s 262144 echo | wc
      1    9136  168660

But it seems removing duplicates is a better solution as the length of
the dependency list is reduced significantly:

$ llvm-nm drivers/gpu/drm/nouveau/nouveau.o | sed -n 's/^  *U //p' |
sort -u | xargs echo | wc
      1    2716   50461

> Do you have a .config to reproduce it?

I can currently reproduce this on an Android kernel that has
Control-Flow Integrity (CFI) enabled. While this feature is not
upstreamed yet, there's nothing that would prevent us from hitting the
command line limit with sufficiently large modules otherwise as well.

Sami