linux-kernel - [PATCH] bpfilter: allow to build bpfilter_umh as a module without static library

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Message-Id: <20200701092644.762234-1-masahiroy@kernel.org>
Date:   Wed,  1 Jul 2020 18:26:44 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     linux-kbuild@...r.kernel.org, Michal Kubecek <mkubecek@...e.cz>,
        linux-kernel@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Andrii Nakryiko <andriin@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Martin KaFai Lau <kafai@...com>,
        Sam Ravnborg <sam@...nborg.org>,
        Song Liu <songliubraving@...com>,
        Valdis Kl ē tnieks <valdis.kletnieks@...edu>,
        Yonghong Song <yhs@...com>
Subject: [PATCH] bpfilter: allow to build bpfilter_umh as a module without static library

Originally, bpfilter_umh was linked with -static only when
CONFIG_BPFILTER_UMH=y.

Commit 8a2cc0505cc4 ("bpfilter: use 'userprogs' syntax to build
bpfilter_umh") silently, accidentally dropped the CONFIG_BPFILTER_UMH=y
test in the Makefile. Revive it in order to link it dynamically when
CONFIG_BPFILTER_UMH=m.

Since commit b1183b6dca3e ("bpfilter: check if $(CC) can link static
libc in Kconfig"), the compiler must be capable of static linking to
enable CONFIG_BPFILTER_UMH, but it requires more than needed.

To loosen the compiler requirement, I changed the dependency as follows:

    depends on CC_CAN_LINK
    depends on m || CC_CAN_LINK_STATIC

If CONFIG_CC_CAN_LINK_STATIC in unset, CONFIG_BPFILTER_UMH is restricted
to 'm' or 'n'.

In theory, CONFIG_CC_CAN_LINK is not required for CONFIG_BPFILTER_UMH=y,
but I did not come up with a good way to describe it.

Fixes: 8a2cc0505cc4 ("bpfilter: use 'userprogs' syntax to build bpfilter_umh")
Reported-by: Michal Kubecek <mkubecek@...e.cz>
Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
---

 net/bpfilter/Kconfig  | 10 ++++++----
 net/bpfilter/Makefile |  2 ++
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/bpfilter/Kconfig b/net/bpfilter/Kconfig
index 84015ef3ee27..73d0b12789f1 100644
--- a/net/bpfilter/Kconfig
+++ b/net/bpfilter/Kconfig
@@ -9,12 +9,14 @@ menuconfig BPFILTER
 if BPFILTER
 config BPFILTER_UMH
 	tristate "bpfilter kernel module with user mode helper"
-	depends on CC_CAN_LINK_STATIC
+	depends on CC_CAN_LINK
+	depends on m || CC_CAN_LINK_STATIC
 	default m
 	help
 	  This builds bpfilter kernel module with embedded user mode helper
 
-	  Note: your toolchain must support building static binaries, since
-	  rootfs isn't mounted at the time when __init functions are called
-	  and do_execv won't be able to find the elf interpreter.
+	  Note: To compile this as built-in, your toolchain must support
+	  building static binaries, since rootfs isn't mounted at the time
+	  when __init functions are called and do_execv won't be able to find
+	  the elf interpreter.
 endif
diff --git a/net/bpfilter/Makefile b/net/bpfilter/Makefile
index f23b53294fba..cdac82b8c53a 100644
--- a/net/bpfilter/Makefile
+++ b/net/bpfilter/Makefile
@@ -7,10 +7,12 @@ userprogs := bpfilter_umh
 bpfilter_umh-objs := main.o
 userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi
 
+ifeq ($(CONFIG_BPFILTER_UMH), y)
 # builtin bpfilter_umh should be linked with -static
 # since rootfs isn't mounted at the time of __init
 # function is called and do_execv won't find elf interpreter
 userldflags += -static
+endif
 
 $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
 
-- 
2.25.1