linux-kernel - [PATCH] kbuild: use $(obj)/ instead of $(src)/ for COPY

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Message-ID: <20251006193839.1350626-1-elsk@google.com>
Date: Mon,  6 Oct 2025 19:38:38 +0000
From: HONG Yifan <elsk@...gle.com>
To: Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas.schier@...ux.dev>
Cc: Masahiro Yamada <masahiroy@...nel.org>, HONG Yifan <elsk@...gle.com>, kernel-team@...roid.com, 
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] kbuild: use $(obj)/ instead of $(src)/ for COPY

Similar to
commit 9a0ebe5011f4 ("kbuild: use $(obj)/ instead of $(src)/ for common pattern rules")

This change updates the COPY rule to use $(obj) instead of $(src). This
makes Kbuild rules like

    always-y += libfoo/.foo.o.cmd

work when the user provides libfoo/.foo.o.cmd_shipped, even when obj and
src is different and src is an absolute path. This is useful when foo.o
and .foo.o.cmd are checked-in as prebuilts.

(Admittedly, `always-y += libfoo/.foo.o.cmd` is not recommended in
kbuild/modules.rst, "Several Subdirectories".)

For example, if

    obj=.
    src=/some/path

then the original rule

    $(obj)/%: $(src)/%_shipped

expands to

    ./%: /some/path/%_shipped

And when matching against the above example, the stem is just `bar.o`
[^1] so the following is looked up:

    libfoo/.foo.o.cmd: libfoo//some/path/.foo.o.cmd_shipped

... and it cannot be matched.

With this change, the rule expands to

    ./%: ./%_shipped

... and it should work, at least for files that does not have a more
specific pattern rule.

NOTE: that after this change, code like

    bar-y += libfoo/foo.o

... with libfoo/foo.o_shipped provided still DOES NOT work, because
the pattern rule $(obj)/%.o takes priority. For .o_shipped files,
the user still needs an explicit `$(obj)/%.o: $(obj)/%.o_shipped` rule
in its own Kbuild file.

[^1]: https://www.gnu.org/software/make/manual/html_node/Pattern-Match.html

Signed-off-by: HONG Yifan <elsk@...gle.com>
---
 scripts/Makefile.lib | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 1d581ba5df66..e066b7b00bcc 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -272,7 +272,7 @@ endef
 quiet_cmd_copy = COPY    $@
       cmd_copy = cat $< > $@

-$(obj)/%: $(src)/%_shipped
+$(obj)/%: $(obj)/%_shipped
 	$(call cmd,copy)

 # Touch a file
-- 
2.51.0.618.g983fd99d29-goog