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From:   Masahiro Yamada <masahiroy@kernel.org>
To:     linux-kbuild@vger.kernel.org
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nicolas Schier <nicolas@fjasle.eu>,
        linux-kernel@vger.kernel.org
Subject: [PATCH] kbuild: resolve symlinks for O= properly
Date:   Thu, 14 Dec 2023 23:03:05 +0900
Message-Id: <20231214140305.531963-1-masahiroy@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

Currently, Kbuild follows the logical chain of directories for the O=
option, just like 'cd' (or 'realpath --logical') does.

Example:

    $ mkdir -p /tmp/a /tmp/x/y
    $ ln -s /tmp/x/y /tmp/a/b
    $ realpath /tmp/a/b/..
    /tmp/x
    $ realpath --logical /tmp/a/b/..
    /tmp/a
    $ make O=/tmp/a/b/.. defconfig
    make[1]: Entering directory '/tmp/a'
      [snip]
    make[1]: Leaving directory '/tmp/a'

'make O=/tmp/a/b/.. defconfig' creates the kernel configuration in
/tmp/a instead of /tmp/x despite the directory path /tmp/a/b/..
resolves to the physical directory path /tmp/x.

This is because Kbuild internally uses the 'cd ... && pwd' for the
path resolution, but this behavior is not predictable for users.
Additionally, it is not consistent with how the Kbuild handles the
M= option or GNU Make works with 'make -C /tmp/a/b/..'.

Using the physical directory structure for the O= option seems more
reasonable.

The comment says "expand a shell special character '~'", but it has
already been expanded to the home directory in the command line.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 Makefile | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/Makefile b/Makefile
index 24fac1889997..a05f0f7c99e0 100644
--- a/Makefile
+++ b/Makefile
@@ -190,14 +190,11 @@ ifeq ("$(origin O)", "command line")
 endif
 
 ifneq ($(KBUILD_OUTPUT),)
-# Make's built-in functions such as $(abspath ...), $(realpath ...) cannot
-# expand a shell special character '~'. We use a somewhat tedious way here.
-abs_objtree := $(shell mkdir -p $(KBUILD_OUTPUT) && cd $(KBUILD_OUTPUT) && pwd)
-$(if $(abs_objtree),, \
-     $(error failed to create output directory "$(KBUILD_OUTPUT)"))
-
+# $(realpath ...) gets empty if the path does not exist. Run 'mkdir -p' first.
+$(shell mkdir -p $(KBUILD_OUTPUT))
 # $(realpath ...) resolves symlinks
-abs_objtree := $(realpath $(abs_objtree))
+abs_objtree := $(realpath $(KBUILD_OUTPUT))
+$(if $(abs_objtree),,$(error failed to create output directory "$(KBUILD_OUTPUT)"))
 endif # ifneq ($(KBUILD_OUTPUT),)
 
 ifneq ($(words $(subst :, ,$(abs_srctree))), 1)
-- 
2.40.1