linux-kernel - [PATCH v2 1/3] um/kconfig: introduce CC_CAN_LINK_STATIC_NO_RUNTIME

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Message-Id: <20200704085213.444645-2-ignat@cloudflare.com>
Date:   Sat,  4 Jul 2020 09:52:11 +0100
From:   Ignat Korchagin <ignat@...udflare.com>
To:     jdike@...toit.com, richard@....at, anton.ivanov@...bridgegreys.com,
        brendanhiggins@...gle.com, linux-um@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Cc:     Ignat Korchagin <ignat@...udflare.com>, kernel-team@...udflare.com
Subject: [PATCH v2 1/3] um/kconfig: introduce CC_CAN_LINK_STATIC_NO_RUNTIME_DEPS

For statically linked UML build it is important to take into account the
standard C-library implementation. Some implementations, notably glibc have
caveats: even when linked statically, the final program might require some
runtime dependencies, if certain functions are used within the code.

Consider the following program:
int main(void)
{
	getpwent();
	return 0;
}

Compiling this program and linking statically with glibc produces the following
warning from the linker:
/usr/sbin/ld: /tmp/ccuthw1o.o: in function `main':
test.c:(.text+0x5): warning: Using 'getpwent' in statically linked
applications requires at runtime the shared libraries from the glibc version
used for linking

We will use the flag to detect such C-library implementation build time and
possibly disable static linking for UML to avoid producing a binary with
unexpected behaviour and dependencies.

Signed-off-by: Ignat Korchagin <ignat@...udflare.com>
---
 init/Kconfig           | 6 ++++++
 scripts/cc-can-link.sh | 5 +++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index a46aa8f3174d..717a3ada765e 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -57,6 +57,12 @@ config CC_CAN_LINK_STATIC
 	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) -static $(m64-flag)) if 64BIT
 	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) -static $(m32-flag))
 
+config CC_CAN_LINK_STATIC_NO_RUNTIME_DEPS
+	bool
+	depends on UML && CC_CAN_LINK_STATIC
+	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) -static -Xlinker --fatal-warnings $(m64-flag)) if 64BIT
+	default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) -static -Xlinker --fatal-warnings $(m32-flag))
+
 config CC_HAS_ASM_GOTO
 	def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
 
diff --git a/scripts/cc-can-link.sh b/scripts/cc-can-link.sh
index 6efcead31989..e5011a46103e 100755
--- a/scripts/cc-can-link.sh
+++ b/scripts/cc-can-link.sh
@@ -2,10 +2,11 @@
 # SPDX-License-Identifier: GPL-2.0
 
 cat << "END" | $@ -x c - -o /dev/null >/dev/null 2>&1
-#include <stdio.h>
+#include <sys/types.h>
+#include <pwd.h>
 int main(void)
 {
-	printf("");
+	getpwent();
 	return 0;
 }
 END
-- 
2.20.1