linux-kernel - [PATCH v6 16/27] compiler: Option to add PROVIDE

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Date:   Thu, 31 Jan 2019 11:24:23 -0800
From:   Thomas Garnier <thgarnie@...omium.org>
To:     kernel-hardening@...ts.openwall.com
Cc:     kristen@...ux.intel.com, Thomas Garnier <thgarnie@...omium.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Michael Forney <forney@...gle.com>,
        Cao jin <caoj.fnst@...fujitsu.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v6 16/27] compiler: Option to add PROVIDE_HIDDEN replacement for weak symbols

Provide an option to have a PROVIDE_HIDDEN (linker script) entry for
each weak symbol. This option solves an error in x86_64 where the linker
optimizes PIE generated code to be non-PIE because --emit-relocs was used
instead of -pie (to reduce dynamic relocations).

Signed-off-by: Thomas Garnier <thgarnie@...omium.org>
---
 init/Kconfig            |  7 +++++++
 scripts/link-vmlinux.sh | 14 ++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index 116e0de4817f..2261c9891631 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2104,6 +2104,13 @@ config ASN1
 	  inform it as to what tags are to be expected in a stream and what
 	  functions to call on what tags.
 
+config WEAK_PROVIDE_HIDDEN
+	bool
+	help
+	  Generate linker script PROVIDE_HIDDEN entries for all weak symbols. It
+	  allows to prevent non-PIE code being replaced by the linker if the
+	  emit-relocs option is used instead of PIE (useful for x86_64 PIE).
+
 source "kernel/Kconfig.locks"
 
 config ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index bc7f1fc1f55b..abf44a804c79 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -119,6 +119,17 @@ kallsyms()
 	${CC} ${aflags} -c -o ${2} ${afile}
 }
 
+gen_weak_provide_hidden()
+{
+        if [ -n "${CONFIG_WEAK_PROVIDE_HIDDEN}" ]; then
+                local pattern="s/^\s\+ w \(\w\+\)$/PROVIDE_HIDDEN(\1 = .);/gp"
+                echo -e "SECTIONS {\n. = _end;" > .tmp_vmlinux_hiddenld
+                ${NM} ${1} | sed -n "${pattern}" >> .tmp_vmlinux_hiddenld
+                echo "}" >> .tmp_vmlinux_hiddenld
+                LDFLAGS_vmlinux="${LDFLAGS_vmlinux} -T .tmp_vmlinux_hiddenld"
+        fi
+}
+
 # Create map file with all symbols from ${1}
 # See mksymap for additional details
 mksysmap()
@@ -200,6 +211,9 @@ modpost_link vmlinux.o
 # modpost vmlinux.o to check for section mismatches
 ${MAKE} -f "${srctree}/scripts/Makefile.modpost" vmlinux.o
 
+# Generate weak linker script
+gen_weak_provide_hidden vmlinux.o
+
 kallsymso=""
 kallsyms_vmlinux=""
 if [ -n "${CONFIG_KALLSYMS}" ]; then
-- 
2.20.1.495.gaa96b0ce6b-goog