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Subject[PATCH 16/32] x86/vdso: Generate vdso{,32}-timens.lds
Date
As it has been discussed on timens RFC, adding a new conditional branch
`if (inside_time_ns)` on VDSO for all processes is undesirable.
It will add a penalty for everybody as branch predictor may mispredict
the jump. Also there are instruction cache lines wasted on cmp/jmp.

Those effects of introducing time namespace are very much unwanted
having in mind how much work have been spent on micro-optimisation
vdso code.

Addressing those problems, there are two versions of VDSO's .so:
for host tasks (without any penalty) and for processes inside of time
namespace with clk_to_ns() that subtracts offsets from host's time.

Unfortunately, to allow changing VDSO VMA on a running process,
the entry points to VDSO should have the same offsets (addresses).
That's needed as i.e. application that calls setns() may have already
resolved VDSO symbols in GOT/PLT.

Provide two linker scripts:
- *-timens.lds for building VDSO for processes inside time namespace
(it has bigger functions and needs to build firstly)
- *.lds for host processes VDSO
(it has smaller functions and entry addresses should be adjusted
with the linker script magic to fit with entries from timens)

Signed-off-by: Dmitry Safonov <dima@arista.com>
---
arch/x86/entry/vdso/.gitignore | 1 +
arch/x86/entry/vdso/Makefile | 18 ++++++++++++++----
arch/x86/entry/vdso/vdso-timens.lds.S | 7 +++++++
arch/x86/entry/vdso/vdso32/.gitignore | 1 +
arch/x86/entry/vdso/vdso32/vdso32-timens.lds.S | 8 ++++++++
5 files changed, 31 insertions(+), 4 deletions(-)
create mode 100644 arch/x86/entry/vdso/vdso-timens.lds.S
create mode 100644 arch/x86/entry/vdso/vdso32/vdso32-timens.lds.S

diff --git a/arch/x86/entry/vdso/.gitignore b/arch/x86/entry/vdso/.gitignore
index 9ab4fa4c7e7b..aaddf8f2171c 100644
--- a/arch/x86/entry/vdso/.gitignore
+++ b/arch/x86/entry/vdso/.gitignore
@@ -1,4 +1,5 @@
vdso.lds
+vdso-timens.lds
vdsox32.lds
vdso32-syscall-syms.lds
vdso32-sysenter-syms.lds
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index ccb572831ea1..4e1659619e7e 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -44,11 +44,14 @@ vobjs := $(foreach F,$(vobjs-y),$(obj)/$F)
vobjs32 := $(foreach F,$(vobjs32-y),$(obj)/$F)
vobjs-timens := $(foreach F,$(vobjs-timens-y),$(obj)/$F)
vobjs32-timens := $(foreach F,$(vobjs32-timens-y),$(obj)/$F)
+dep-vdso.lds-$(CONFIG_TIME_NS) += $(obj)/vdso-image-64-timens.c
+dep-vdso32.lds-$(CONFIG_TIME_NS) += $(obj)/vdso-image-32-timens.c

$(obj)/vdso.o: $(obj)/vdso.so

targets += vdso.lds $(vobjs-y) $(vobjs-timens-y) vdso64.entries
targets += vdso32/vdso32.lds $(vobjs32-y) $(vobjs32-timens-y) vdso32.entries
+targets += vdso-timens.lds vdso32/vdso32-timens.lds

# Build the vDSO image C files and link them in.
vdso_img_objs := $(vdso_img-y:%=vdso-image-%.o)
@@ -59,11 +62,13 @@ targets += $(vdso_img_cfiles)
targets += $(vdso_img_sodbg) $(vdso_img-y:%=vdso%.so)

CPPFLAGS_vdso.lds += -P -C
+CPPFLAGS_vdso-timens.lds := $(CPPFLAGS_vdso.lds)

VDSO_LDFLAGS_vdso.lds = -m elf_x86_64 -soname linux-vdso.so.1 --no-undefined \
-z max-page-size=4096
+VDSO_LDFLAGS_vdso-timens.lds := $(VDSO_LDFLAGS_vdso.lds)

-$(obj)/vdso64-timens.so.dbg: $(obj)/vdso.lds $(vobjs-timens) FORCE
+$(obj)/vdso64-timens.so.dbg: $(obj)/vdso-timens.lds $(vobjs-timens) FORCE
$(call if_changed,vdso)

$(obj)/vdso64.so.dbg: $(obj)/vdso.lds $(vobjs) FORCE
@@ -79,6 +84,9 @@ quiet_cmd_vdso2c = VDSO2C $@
$(obj)/vdso-image-%.c: $(obj)/vdso%.so.dbg $(obj)/vdso%.so $(obj)/vdso2c FORCE
$(call if_changed,vdso2c)

+$(obj)/vdso.lds: $(dep-vdso.lds-y) $(obj)/vdso2c FORCE
+$(obj)/vdso32/vdso32.lds: $(dep-vdso32.lds-y) $(obj)/vdso2c FORCE
+
#
# Don't omit frame pointers for ease of userspace debugging, but do
# optimize sibling calls.
@@ -142,8 +150,10 @@ $(obj)/%.so: $(obj)/%.so.dbg
$(obj)/vdsox32.so.dbg: $(obj)/vdsox32.lds $(vobjx32s) FORCE
$(call if_changed,vdso)

-CPPFLAGS_vdso32.lds = $(CPPFLAGS_vdso.lds)
-VDSO_LDFLAGS_vdso32.lds = -m elf_i386 -soname linux-gate.so.1
+CPPFLAGS_vdso32.lds := $(CPPFLAGS_vdso.lds)
+CPPFLAGS_vdso32-timens.lds := $(CPPFLAGS_vdso32.lds)
+VDSO_LDFLAGS_vdso32.lds := -m elf_i386 -soname linux-gate.so.1
+VDSO_LDFLAGS_vdso32-timens.lds := $(VDSO_LDFLAGS_vdso32.lds)

KBUILD_AFLAGS_32 := $(filter-out -m64,$(KBUILD_AFLAGS)) -DBUILD_VDSO
$(obj)/vdso32.so.dbg: KBUILD_AFLAGS = $(KBUILD_AFLAGS_32)
@@ -172,7 +182,7 @@ endif
$(obj)/vdso32.so.dbg: KBUILD_CFLAGS = $(KBUILD_CFLAGS_32)
$(obj)/vdso32-timens.so.dbg: KBUILD_CFLAGS = $(KBUILD_CFLAGS_32)

-$(obj)/vdso32-timens.so.dbg: $(obj)/vdso32/vdso32.lds $(vobjs32-timens) FORCE
+$(obj)/vdso32-timens.so.dbg: $(obj)/vdso32/vdso32-timens.lds $(vobjs32-timens) FORCE
$(call if_changed,vdso)

$(obj)/vdso32.so.dbg: $(obj)/vdso32/vdso32.lds $(vobjs32) FORCE
diff --git a/arch/x86/entry/vdso/vdso-timens.lds.S b/arch/x86/entry/vdso/vdso-timens.lds.S
new file mode 100644
index 000000000000..687aba3bc5f0
--- /dev/null
+++ b/arch/x86/entry/vdso/vdso-timens.lds.S
@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Linker script for 64-bit timens vDSO.
+ */
+
+#define UNALIGNED_ENTRIES
+#include "vdso.lds.S"
diff --git a/arch/x86/entry/vdso/vdso32/.gitignore b/arch/x86/entry/vdso/vdso32/.gitignore
index e45fba9d0ced..ce4afb6ffb62 100644
--- a/arch/x86/entry/vdso/vdso32/.gitignore
+++ b/arch/x86/entry/vdso/vdso32/.gitignore
@@ -1 +1,2 @@
vdso32.lds
+vdso32-timens.lds
diff --git a/arch/x86/entry/vdso/vdso32/vdso32-timens.lds.S b/arch/x86/entry/vdso/vdso32/vdso32-timens.lds.S
new file mode 100644
index 000000000000..1a3b3b1f0517
--- /dev/null
+++ b/arch/x86/entry/vdso/vdso32/vdso32-timens.lds.S
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Linker script for 32-bit timens vDSO.
+ */
+
+#define UNALIGNED_ENTRIES
+
+#include "vdso32.lds.S"
--
2.20.1
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