ARM: VDSO

Here's another round of the VDSO patch set for ARM.

I would like to specifically thank Andy Lutomirski for his careful review of v7. This version has a fair number of improvements as a result.

If this is looking acceptable for 3.18, I would appreciate guidance on how to route the various patches upstream. I am thinking that at least the following patches:

[03/10] clocksource: arm_arch_timer: change clocksource name if CP15 unavailable [04/10] clocksource: arm_arch_timer: enable counter access for 32-bit ARM

would go through Daniel's tree. The next two, which just remove dead code:

[05/10] ARM: arch_timer: remove unused functions [06/10] arm64: arch_timer: remove unused functions

could go via Daniel since they depend on the previous two patches, or their respective arch maintainers could pick them up. Opinions?

Everything else would go through Russell, but the patches that add the VDSO code (#7-10) depend on patch #4 for the VDSO to work.

===

Provide fast userspace implementations of gettimeofday and clock_gettime on systems that implement the generic timers extension defined in ARMv7. This follows the example of arm64 in conception but significantly differs in some aspects of the implementation (C vs assembly, mainly).

Clocks supported:
- CLOCK_REALTIME
- CLOCK_MONOTONIC
- CLOCK_REALTIME_COARSE
- CLOCK_MONOTONIC_COARSE

This also provides clock_getres (as arm64 does).

getcpu support is planned but not included at this time.

For applications to transparently benefit from this change, ARM-specific support code needs to be added to glibc. I have such a patch, and have verified that glibc's self tests do not detect any regressions. I hope to have that code added to glibc for the 2.21 release.

The VDSO symbols are available for lookup via dlsym even with an unpatched glibc.

Note that while the high-precision realtime and monotonic clock support depends on the generic timers extension, support for clock_getres and coarse clocks is independent of the timer implementation and is provided unconditionally.

Tested on OMAP5 and i.MX6, verifying that results obtained with the vdso are consistent with those obtained from the kernel. On OMAP5 I observe a 3- to 4-fold speedup for gettimeofday / CLOCK_REALTIME, with even better (if less interesting) speedups for the coarse clock ids and clock_getres.

I've been testing and benchmarking this with some custom test code which I have hosted here:

https://github.com/nlynch-mentor/vdsotest

Miscellaneous notes and dependencies:
- This will not build against 3.16-rc; the layout of the timekeeper structure has changed in -next.
- Unpatched FSF GDB may complain "warning: Could not load shared library symbols for linux-vdso.so.1." This is not an ARM-specific issue. Current Fedora and Debian patch their GDB packages to prevent this warning.

Changes since v7:
- Update to next-20140801.
- In arch_setup_additional_pages, fix call to get_unmapped_area - use bytes (not pages) for length argument.
- As x86 does, separate data and text into two VMAs, [vvar] and [vdso] respectively. These have different permissions; [vdso] will allow debuggers to set breakpoints, but [vvar] is read-only and cannot be modified even via ptrace.
- Use _install_special_mapping for signal page, vvar, and vdso mappings.
- Add -DDISABLE_BRANCH_PROFILING.
- Add --no-undefined -Bsymbolic to link options to cause linker to error out on unresolved references, making checkundef script unnecessary.
- Specify max-page-size, common-page-size in linker options so the true alignment is reflected in program header; otherwise gdb gets confused.
- Fix incremental build vs. generated auxvec.h.
- Use appropriate unwind directives in __get_datapage.
- Added vdso_install target and help text. Install build-id symlinks as x86 does.
- Adjust update_vsyscall for changes in struct timekeeper.

Changes since v6:
- Update to 3.16-rc1.
- Remove -lgcc from link step - need to support GCC installations without libgcc.
- Force -O2 compilation to prevent GCC from emitting calls to libgcc math routines.
- Use custom post-processing to clear the EF_ARM_ABI_FLOAT_SOFT flag if set in the ELF header to produce a shared object which is architecturally allowed to be used by both soft- and hard-float code.
- Consolidate common arch timer code instead of duplicating it.
- Prevent the VDSO from attempting CP15 access on memory-only architected timer implementations by renaming the clocksource.

Changes since v5:
- Update to 3.15-rc1.
- Place vdso at a randomized offset above the stack along with the sigpage.
- Properly export asm/auxvec.h.
- Split patch into series for ease of review.

Changes since v4:
- Map data page at the beginning of the VMA to prevent orphan sections at the end of output invalidating the calculated offset.
- Move checkundef into cmd_vdsold to avoid spurious rebuilds.
- Change vdso_init message to pr_debug.
- Add -fno-stack-protector to cflags.

Changes since v3:
- Update to 3.14-rc6.
- Record vdso base in mm context before installing mapping (for the sake of perf_mmap_event).
- Use a more seqcount-like API for critical sections. Using seqcount API directly, however, would leak kernel pointers to userspace when lockdep is enabled.
- Trap instead of looping forever in division-by-zero stubs.

Changes since v2:
- Update to 3.14-rc4.
- Make vDSO configurable, depending on AEABI and MMU.
- Defer shifting of nanosecond component of timespec: fixes observed 1ns inconsistencies for CLOCK_REALTIME, CLOCK_MONOTONIC (see 45a7905fc48f for arm64 equivalent).
- Force reload of seq_count when spinning: without a memory clobber after the load of vdata->seq_count, GCC can generate code like this:

    2f8:   e59c9020        ldr     r9, [ip, #32]
    2fc:   e3190001        tst     r9, #1
    300:   1a000033        bne     3d4 <do_realtime+0x104>
    304:   f57ff05b        dmb     ish
    308:   e59c3034        ldr     r3, [ip, #52]   ; 0x34
    ...
    3d4:   eafffffe        b       3d4 <do_realtime+0x104>

- Build vdso.so with -lgcc: calls to __lshrdi3, __divsi3 sometimes emitted (especially with -Os). Override certain libgcc functions to prevent undefined symbols.
- Do not clear PG_reserved on vdso pages.
- Remove unnecessary get_page calls.
- Simplify ELF signature check during init.
- Use volatile for asm syscall fallbacks.
- Check whether vdso_pagelist is initialized in arm_install_vdso.
- Record clocksource mask in data page.
- Reduce code duplication in do_realtime, do_monotonic.
- Reduce calculations performed in critical sections.
- Simplify coarse clock handling.
- Move datapage load to its own assembly routine.
- Tune vdso_data layout and tweak field names.
- Check vdso shared object for undefined symbols during build.

Changes since v1:
- update to 3.14-rc1
- ensure cache coherency for data page
- Document the kernel-to-userspace protocol for vdso data page updates, and note that the timekeeping core prevents concurrent updates.
- update wall-to-monotonic fields unconditionally
- move vdso_start, vdso_end declarations to vdso.h
- correctly build and run when CONFIG_ARM_ARCH_TIMER=n
- rearrange linker script to avoid overlapping sections when CONFIG_DEBUGINFO=n
- remove use_syscall checks from coarse clock paths
- crib BUG_INSTR (0xe7f001f2) from asm/bug.h for text fill

Nathan Lynch (10):
ARM: use _install_special_mapping for sigpage
ARM: place sigpage at a random offset above stack
clocksource: arm_arch_timer: change clocksource name if CP15
unavailable
clocksource: arm_arch_timer: enable counter access for 32-bit ARM
ARM: arch_timer: remove unused functions
arm64: arch_timer: remove unused functions
ARM: miscellaneous vdso infrastructure, preparation
ARM: add vdso user-space code
ARM: vdso initialization, mapping, and synchronization
ARM: add CONFIG_VDSO Kconfig and Makefile bits

 arch/arm/Makefile                    |   8 +
 arch/arm/include/asm/Kbuild          |   1 -
 arch/arm/include/asm/arch_timer.h    |  25 ---
 arch/arm/include/asm/auxvec.h        |   1 +
 arch/arm/include/asm/elf.h           |   9 +
 arch/arm/include/asm/mmu.h           |   3 +
 arch/arm/include/asm/vdso.h          |  34 ++++
 arch/arm/include/asm/vdso_datapage.h |  60 +++++++
 arch/arm/include/uapi/asm/Kbuild     |   1 +
 arch/arm/include/uapi/asm/auxvec.h   |   7 +
 arch/arm/kernel/Makefile             |   1 +
 arch/arm/kernel/asm-offsets.c        |   5 +
 arch/arm/kernel/process.c            |  71 +++++++-
 arch/arm/kernel/vdso.c               | 189 +++++++++++++++++++++
 arch/arm/mm/Kconfig                  |  15 ++
 arch/arm/vdso/.gitignore             |   1 +
 arch/arm/vdso/Makefile               |  74 ++++++++
 arch/arm/vdso/datapage.S             |  15 ++
 arch/arm/vdso/vdso.S                 |  35 ++++
 arch/arm/vdso/vdso.lds.S             |  88 ++++++++++
 arch/arm/vdso/vdsomunge.c            | 208 +++++++++++++++++++++++
 arch/arm/vdso/vgettimeofday.c        | 320 +++++++++++++++++++++++++++++++++++
 arch/arm64/include/asm/arch_timer.h  |  31 ----
 drivers/clocksource/arm_arch_timer.c |  48 +++++-
 24 files changed, 1180 insertions(+), 70 deletions(-)
 create mode 100644 arch/arm/include/asm/auxvec.h
 create mode 100644 arch/arm/include/asm/vdso.h
 create mode 100644 arch/arm/include/asm/vdso_datapage.h
 create mode 100644 arch/arm/include/uapi/asm/auxvec.h
 create mode 100644 arch/arm/kernel/vdso.c
 create mode 100644 arch/arm/vdso/.gitignore
 create mode 100644 arch/arm/vdso/Makefile
 create mode 100644 arch/arm/vdso/datapage.S
 create mode 100644 arch/arm/vdso/vdso.S
 create mode 100644 arch/arm/vdso/vdso.lds.S
 create mode 100644 arch/arm/vdso/vdsomunge.c
 create mode 100644 arch/arm/vdso/vgettimeofday.c

1.9.3

_install_special_mapping allows the VMA to be identifed in /proc/pid/maps without the use of arch_vma_name, providing a slight net reduction in object size:

  text    data     bss     dec     hex filename
  2996      96     144    3236     ca4 arch/arm/kernel/process.o (before)
  2956     104     144    3204     c84 arch/arm/kernel/process.o (after)

---
 arch/arm/kernel/process.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

index 81ef686a91ca..46fbbb3701a0 100644
@@ -472,19 +472,23 @@ int in_gate_area_no_mm(unsigned long addr)
 
 const char *arch_vma_name(struct vm_area_struct *vma)
 {
-	return is_gate_vma(vma) ? "[vectors]" :
-		(vma->vm_mm && vma->vm_start == vma->vm_mm->context.sigpage) ?
-		 "[sigpage]" : NULL;
+	return is_gate_vma(vma) ? "[vectors]" : NULL;
 }
 
 static struct page *signal_page;
 extern struct page *get_signal_page(void);
 
+static const struct vm_special_mapping sigpage_mapping = {
+	.name = "[sigpage]",
+	.pages = &signal_page,
+};
+
 int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 {
 	struct mm_struct *mm = current->mm;
+	struct vm_area_struct *vma;
 	unsigned long addr;
-	int ret;
+	int ret = 0;
 
 	if (!signal_page)
 		signal_page = get_signal_page();

@@ -498,12 +502,16 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 		goto up_fail;
 	}
 
-	ret = install_special_mapping(mm, addr, PAGE_SIZE,
+	vma = _install_special_mapping(mm, addr, PAGE_SIZE,
 		VM_READ | VM_EXEC | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC,
-		&signal_page);
+		&sigpage_mapping);
+
+	if (IS_ERR(vma)) {
+		ret = PTR_ERR(vma);
+		goto up_fail;
+	}
 
-	if (ret == 0)
-		mm->context.sigpage = addr;
+	mm->context.sigpage = addr;
 
  up_fail:
 	up_write(&mm->mmap_sem);

1.9.3

I think arch_vma_name is still needed for arm as long as the vectors page is not installed using _install_special_mapping. I'm honestly not sure whether moving the vectors page to that API would be appropriate, but it would be a larger undertaking.

Actually... if we give arm's gate_vma a vm_ops with a ->name() routine that returns "[vectors]" we could get rid of arch_vma_name, I think. This seems to be what x86_64 does.

To be honest, I forgot about the vectors page, but giving it a name sounds like a sensible idea.

Will

The sigpage is currently placed alongside shared libraries etc in the address space. Similar to what x86_64 does for its VDSO, place the sigpage at a randomized offset above the stack so that learning the base address of the sigpage doesn't help expose where shared libraries are loaded in the address space (and vice versa).

---
 arch/arm/kernel/process.c | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

index 46fbbb3701a0..9e0d931dd475 100644
@@ -475,6 +475,38 @@ const char *arch_vma_name(struct vm_area_struct *vma)
 	return is_gate_vma(vma) ? "[vectors]" : NULL;
 }
 
+/* If possible, provide a placement hint at a random offset from the
+ * stack for the signal page.
+ */
+static unsigned long sigpage_addr(const struct mm_struct *mm, unsigned int npages)
+{
+	unsigned long offset;
+	unsigned long first;
+	unsigned long last;
+	unsigned long addr;
+	unsigned int slots;
+
+	first = PAGE_ALIGN(mm->start_stack);
+
+	last = TASK_SIZE - (npages << PAGE_SHIFT);
+
+	/* No room after stack? */
+	if (first > last)
+		return 0;
+
+	/* Just enough room? */
+	if (first == last)
+		return first;
+
+	slots = ((last - first) >> PAGE_SHIFT) + 1;
+
+	offset = get_random_int() % slots;
+
+	addr = first + (offset << PAGE_SHIFT);
+
+	return addr;
+}
+
 static struct page *signal_page;
 extern struct page *get_signal_page(void);
 

@@ -488,6 +520,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
 	unsigned long addr;
+	unsigned long hint;
 	int ret = 0;
 
 	if (!signal_page)

@@ -496,7 +529,8 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 		return -ENOMEM;
 
 	down_write(&mm->mmap_sem);
-	addr = get_unmapped_area(NULL, 0, PAGE_SIZE, 0, 0);
+	hint = sigpage_addr(mm, 1);
+	addr = get_unmapped_area(NULL, hint, PAGE_SIZE, 0, 0);
 	if (IS_ERR_VALUE(addr)) {
 		ret = addr;
 		goto up_fail;

1.9.3

The arm and arm64 VDSOs need CP15 access to the architected counter. If this is unavailable (which is allowed by ARM v7), indicate this by changing the clocksource name to "arch_mem_counter" before registering the clocksource.

Suggested by Stephen Boyd.

---
 drivers/clocksource/arm_arch_timer.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

index 5163ec13429d..c99afdf12e98 100644
@@ -429,11 +429,19 @@ static void __init arch_counter_register(unsigned type)
 	u64 start_count;
 
 	/* Register the CP15 based counter if we have one */
-	if (type & ARCH_CP15_TIMER)
+	if (type & ARCH_CP15_TIMER) {
 		arch_timer_read_counter = arch_counter_get_cntvct;
-	else
+	} else {
 		arch_timer_read_counter = arch_counter_get_cntvct_mem;
 
+		/* If the clocksource name is "arch_sys_counter" the
+		 * VDSO will attempt to read the CP15-based counter.
+		 * Ensure this does not happen when CP15-based
+		 * counter is not available.
+		 */
+		clocksource_counter.name = "arch_mem_counter";
+	}
+
 	start_count = arch_timer_read_counter();
 	clocksource_register_hz(&clocksource_counter, arch_timer_rate);
 	cyclecounter.mult = clocksource_counter.mult;

1.9.3

FWIW, x86 does this using the archdata field in the clocksource, but I see nothing wrong with doing it this way instead.

--Andy

The only difference between arm and arm64's implementations of arch_counter_set_user_access is that 32-bit ARM does not enable user access to the virtual counter. We want to enable this access for the 32-bit ARM VDSO, so copy the arm64 version to the driver itself, giving it a slightly different name (arch_counter_set_access) to avoid redefinition.

The arch_timer_evtstrm_enable hooks are also substantially similar, the only difference being a CONFIG_COMPAT-conditional section which is relevant only for arm64. Copy the arm64 version to the driver, again slightly renaming (arch_timer_evtstrm_enable => arch_timer_enable_evtstrm).

With this change, the arch_counter_set_user_access and arch_timer_evtstrm_enable hooks in both arm and arm64 become unused, to be removed in followup patches.

---
 drivers/clocksource/arm_arch_timer.c | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

index c99afdf12e98..0188332ad24b 100644
@@ -23,6 +23,7 @@
 #include <linux/sched_clock.h>
 
 #include <asm/arch_timer.h>
+#include <asm/hwcap.h>
 #include <asm/virt.h>
 
 #include <clocksource/arm_arch_timer.h>

@@ -299,6 +300,20 @@ static void __arch_timer_setup(unsigned type,
 	clockevents_config_and_register(clk, arch_timer_rate, 0xf, 0x7fffffff);
 }
 
+static void arch_timer_enable_evtstrm(int divider)
+{
+	u32 cntkctl = arch_timer_get_cntkctl();
+	cntkctl &= ~ARCH_TIMER_EVT_TRIGGER_MASK;
+	/* Set the divider and enable virtual event stream */
+	cntkctl |= (divider << ARCH_TIMER_EVT_TRIGGER_SHIFT)
+			| ARCH_TIMER_VIRT_EVT_EN;
+	arch_timer_set_cntkctl(cntkctl);
+	elf_hwcap |= HWCAP_EVTSTRM;
+#ifdef CONFIG_COMPAT
+	compat_elf_hwcap |= COMPAT_HWCAP_EVTSTRM;
+#endif
+}
+
 static void arch_timer_configure_evtstream(void)
 {
 	int evt_stream_div, pos;

@@ -309,7 +324,24 @@ static void arch_timer_configure_evtstream(void)
 	if (pos > 1 && !(evt_stream_div & (1 << (pos - 2))))
 		pos--;
 	/* enable event stream */
-	arch_timer_evtstrm_enable(min(pos, 15));
+	arch_timer_enable_evtstrm(min(pos, 15));
+}
+
+static void arch_counter_set_access(void)
+{
+	u32 cntkctl = arch_timer_get_cntkctl();
+
+	/* Disable user access to the timers and the physical counter */
+	/* Also disable virtual event stream */
+	cntkctl &= ~(ARCH_TIMER_USR_PT_ACCESS_EN
+			| ARCH_TIMER_USR_VT_ACCESS_EN
+			| ARCH_TIMER_VIRT_EVT_EN
+			| ARCH_TIMER_USR_PCT_ACCESS_EN);
+
+	/* Enable user access to the virtual counter */
+	cntkctl |= ARCH_TIMER_USR_VCT_ACCESS_EN;
+
+	arch_timer_set_cntkctl(cntkctl);
 }
 
 static int arch_timer_setup(struct clock_event_device *clk)

@@ -324,7 +356,7 @@ static int arch_timer_setup(struct clock_event_device *clk)
 			enable_percpu_irq(arch_timer_ppi[PHYS_NONSECURE_PPI], 0);
 	}
 
-	arch_counter_set_user_access();
+	arch_counter_set_access();
 	if (IS_ENABLED(CONFIG_ARM_ARCH_TIMER_EVTSTREAM))
 		arch_timer_configure_evtstream();
 

1.9.3

The counter access and evtstream configuration are now done in common code, so remove the now-unused arch_counter_set_user_access and arch_timer_evtstrm_enable.

---
 arch/arm/include/asm/arch_timer.h | 25 -------------------------
 1 file changed, 25 deletions(-)

index 0704e0cf5571..92793ba69c40 100644
@@ -99,31 +99,6 @@ static inline void arch_timer_set_cntkctl(u32 cntkctl)
 	asm volatile("mcr p15, 0, %0, c14, c1, 0" : : "r" (cntkctl));
 }
 
-static inline void arch_counter_set_user_access(void)
-{
-	u32 cntkctl = arch_timer_get_cntkctl();
-
-	/* Disable user access to both physical/virtual counters/timers */
-	/* Also disable virtual event stream */
-	cntkctl &= ~(ARCH_TIMER_USR_PT_ACCESS_EN
-			| ARCH_TIMER_USR_VT_ACCESS_EN
-			| ARCH_TIMER_VIRT_EVT_EN
-			| ARCH_TIMER_USR_VCT_ACCESS_EN
-			| ARCH_TIMER_USR_PCT_ACCESS_EN);
-	arch_timer_set_cntkctl(cntkctl);
-}
-
-static inline void arch_timer_evtstrm_enable(int divider)
-{
-	u32 cntkctl = arch_timer_get_cntkctl();
-	cntkctl &= ~ARCH_TIMER_EVT_TRIGGER_MASK;
-	/* Set the divider and enable virtual event stream */
-	cntkctl |= (divider << ARCH_TIMER_EVT_TRIGGER_SHIFT)
-			| ARCH_TIMER_VIRT_EVT_EN;
-	arch_timer_set_cntkctl(cntkctl);
-	elf_hwcap |= HWCAP_EVTSTRM;
-}
-
 #endif
 
 #endif

1.9.3

It should. All call sites of these are fixed up in the previous patch.
Anything in particular you're concerned about?

The entire series is intended to be bisectable; let me know if I've missed something.

Aha, I didn't realise you'd renamed the functions. In which case, this looks find annd sorry for the noise.

Will

The counter access and evtstream configuration are now done in common code, so remove the now-unused arch_counter_set_user_access and arch_timer_evtstrm_enable.

---
 arch/arm64/include/asm/arch_timer.h | 31 -------------------------------
 1 file changed, 31 deletions(-)

index 9400596a0f39..f19097134b02 100644
@@ -104,37 +104,6 @@ static inline void arch_timer_set_cntkctl(u32 cntkctl)
 	asm volatile("msr	cntkctl_el1, %0" : : "r" (cntkctl));
 }
 
-static inline void arch_counter_set_user_access(void)
-{
-	u32 cntkctl = arch_timer_get_cntkctl();
-
-	/* Disable user access to the timers and the physical counter */
-	/* Also disable virtual event stream */
-	cntkctl &= ~(ARCH_TIMER_USR_PT_ACCESS_EN
-			| ARCH_TIMER_USR_VT_ACCESS_EN
-			| ARCH_TIMER_VIRT_EVT_EN
-			| ARCH_TIMER_USR_PCT_ACCESS_EN);
-
-	/* Enable user access to the virtual counter */
-	cntkctl |= ARCH_TIMER_USR_VCT_ACCESS_EN;
-
-	arch_timer_set_cntkctl(cntkctl);
-}
-
-static inline void arch_timer_evtstrm_enable(int divider)
-{
-	u32 cntkctl = arch_timer_get_cntkctl();
-	cntkctl &= ~ARCH_TIMER_EVT_TRIGGER_MASK;
-	/* Set the divider and enable virtual event stream */
-	cntkctl |= (divider << ARCH_TIMER_EVT_TRIGGER_SHIFT)
-			| ARCH_TIMER_VIRT_EVT_EN;
-	arch_timer_set_cntkctl(cntkctl);
-	elf_hwcap |= HWCAP_EVTSTRM;
-#ifdef CONFIG_COMPAT
-	compat_elf_hwcap |= COMPAT_HWCAP_EVTSTRM;
-#endif
-}
-
 static inline u64 arch_counter_get_cntvct(void)
 {
 	u64 cval;

1.9.3

Define the layout of the data structure shared between kernel and userspace.

Track the vdso address in the mm_context; needed for communicating AT_SYSINFO_EHDR to the ELF loader.

Add declarations for arm_install_vdso; implementation is in a following patch.

Define AT_SYSINFO_EHDR, and, if CONFIG_VDSO=y, report the vdso shared object address via the ELF auxiliary vector.

Note - this adds the AT_SYSINFO_EHDR in a new user-visible header asm/auxvec.h; this is consistent with other architectures.

---
 arch/arm/include/asm/Kbuild          |  1 -
 arch/arm/include/asm/auxvec.h        |  1 +
 arch/arm/include/asm/elf.h           |  9 ++++++
 arch/arm/include/asm/mmu.h           |  3 ++
 arch/arm/include/asm/vdso.h          | 34 ++++++++++++++++++++
 arch/arm/include/asm/vdso_datapage.h | 60 ++++++++++++++++++++++++++++++++++++
 arch/arm/include/uapi/asm/Kbuild     |  1 +
 arch/arm/include/uapi/asm/auxvec.h   |  7 +++++
 8 files changed, 115 insertions(+), 1 deletion(-)
 create mode 100644 arch/arm/include/asm/auxvec.h
 create mode 100644 arch/arm/include/asm/vdso.h
 create mode 100644 arch/arm/include/asm/vdso_datapage.h
 create mode 100644 arch/arm/include/uapi/asm/auxvec.h

index 70cd84eb7fda..9fb902425606 100644
@@ -1,6 +1,5 @@
 
 
-generic-y += auxvec.h
 generic-y += bitsperlong.h
 generic-y += cputime.h
 generic-y += current.h

new file mode 100644
index 000000000000..fbd388c46299
@@ -0,0 +1 @@
+#include <uapi/asm/auxvec.h>

index f4b46d39b9cf..e9822ac77efe 100644
@@ -1,7 +1,9 @@
 #ifndef __ASMARM_ELF_H
 #define __ASMARM_ELF_H
 
+#include <asm/auxvec.h>
 #include <asm/hwcap.h>
+#include <asm/vdso_datapage.h>
 
 /*
  * ELF register definitions..

@@ -129,6 +131,13 @@ extern unsigned long arch_randomize_brk(struct mm_struct *mm);
 #define arch_randomize_brk arch_randomize_brk
 
 #ifdef CONFIG_MMU
+#ifdef CONFIG_VDSO
+#define ARCH_DLINFO						\
+do {								\
+	NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
+		    (elf_addr_t)current->mm->context.vdso);	\
+} while (0)
+#endif
 #define ARCH_HAS_SETUP_ADDITIONAL_PAGES 1
 struct linux_binprm;
 int arch_setup_additional_pages(struct linux_binprm *, int);

index 64fd15159b7d..a5b47421059d 100644
@@ -11,6 +11,9 @@ typedef struct {
 #endif
 	unsigned int	vmalloc_seq;
 	unsigned long	sigpage;
+#ifdef CONFIG_VDSO
+	unsigned long	vdso;
+#endif
 } mm_context_t;
 
 #ifdef CONFIG_CPU_HAS_ASID

new file mode 100644
index 000000000000..08d06e73ccf2
@@ -0,0 +1,34 @@
+#ifndef __ASM_VDSO_H
+#define __ASM_VDSO_H
+
+#ifdef __KERNEL__
+
+#ifndef __ASSEMBLY__
+
+struct mm_struct;
+
+#ifdef CONFIG_VDSO
+
+void arm_install_vdso(struct mm_struct *mm, unsigned long addr);
+
+extern char vdso_start, vdso_end;
+
+extern unsigned int vdso_total_pages;
+
+#else /* CONFIG_VDSO */
+
+static inline void arm_install_vdso(struct mm_struct *mm, unsigned long addr)
+{
+}
+
+static const unsigned int vdso_total_pages = 0;
+
+#endif /* CONFIG_VDSO */
+
+#endif /* __ASSEMBLY__ */
+
+#define VDSO_LBASE	0x0
+
+#endif /* __KERNEL__ */
+
+#endif /* __ASM_VDSO_H */

index 70a1c9da30ca..a1c05f93d920 100644
@@ -1,6 +1,7 @@
 # UAPI Header export list
 include include/uapi/asm-generic/Kbuild.asm
 
+header-y += auxvec.h
 header-y += byteorder.h
 header-y += fcntl.h
 header-y += hwcap.h

new file mode 100644
index 000000000000..f56936b97ec2
@@ -0,0 +1,7 @@
+#ifndef __ASM_AUXVEC_H
+#define __ASM_AUXVEC_H
+
+/* vDSO location */
+#define AT_SYSINFO_EHDR	33
+
+#endif

1.9.3

Place vdso-related user-space code in arch/arm/kernel/vdso/.

It is almost completely written in C with some assembly helpers to load the data page address, sample the counter, and fall back to system calls when necessary.

If CONFIG_ARM_ARCH_TIMER is not enabled, the vdso cannot service high-resolution clocks and falls back to syscalls. Low-resolution clocks e.g. CLOCK_REALTIME_COARSE can be serviced regardless.

Of particular note is that a post-processing step ("vdsomunge") is necessary to produce a shared object which is architecturally allowed to be used by both soft- and hard-float EABI programs.

The 2012 edition of the ARM ABI defines Tag_ABI_VFP_args = 3 "Code is compatible with both the base and VFP variants; the user did not permit non-variadic functions to pass FP parameters/results." Unfortunately current toolchains do not support this tag, which is ideally what we would use.

The best available option is to ensure that both EF_ARM_ABI_FLOAT_SOFT and EF_ARM_ABI_FLOAT_HARD are unset in the ELF header's e_flags, indicating that the shared object is "old" and should be accepted for backward compatibility's sake. While binutils < 2.24 appear to produce a vdso.so with both flags clear, 2.24 always sets EF_ARM_ABI_FLOAT_SOFT, with no way to inhibit this behavior. So we have to fix things up with a custom post-processing step.

In fact, the VDSO code in glibc does much less validation (including checking these flags) than the code for handling conventional file-backed shared libraries, so this is a bit moot unless glibc's VDSO code becomes more strict.

---
 arch/arm/kernel/asm-offsets.c |   5 +
 arch/arm/vdso/.gitignore      |   1 +
 arch/arm/vdso/Makefile        |  74 ++++++++++
 arch/arm/vdso/datapage.S      |  15 ++
 arch/arm/vdso/vdso.S          |  35 +++++
 arch/arm/vdso/vdso.lds.S      |  88 ++++++++++++
 arch/arm/vdso/vdsomunge.c     | 208 +++++++++++++++++++++++++++
 arch/arm/vdso/vgettimeofday.c | 320 ++++++++++++++++++++++++++++++++++++++++++
 8 files changed, 746 insertions(+)
 create mode 100644 arch/arm/vdso/.gitignore
 create mode 100644 arch/arm/vdso/Makefile
 create mode 100644 arch/arm/vdso/datapage.S
 create mode 100644 arch/arm/vdso/vdso.S
 create mode 100644 arch/arm/vdso/vdso.lds.S
 create mode 100644 arch/arm/vdso/vdsomunge.c
 create mode 100644 arch/arm/vdso/vgettimeofday.c

index 713e807621d2..72b4c3873dc8 100644
@@ -24,6 +24,7 @@
 #include <asm/memory.h>
 #include <asm/procinfo.h>
 #include <asm/suspend.h>
+#include <asm/vdso_datapage.h>
 #include <asm/hardware/cache-l2x0.h>
 #include <linux/kbuild.h>
 

@@ -200,5 +201,9 @@ int main(void)
 #endif
   DEFINE(KVM_VTTBR,		offsetof(struct kvm, arch.vttbr));
 #endif
+  BLANK();
+#ifdef CONFIG_VDSO
+  DEFINE(VDSO_DATA_SIZE,	sizeof(union vdso_data_store));
+#endif
   return 0; 
 }

new file mode 100644
index 000000000000..f8b69d84238e
@@ -0,0 +1 @@
+vdso.lds

new file mode 100644
index 000000000000..a2e60367931b
@@ -0,0 +1,15 @@
+#include <linux/linkage.h>
+#include <asm/asm-offsets.h>
+
+	.align 2
+.L_vdso_data_ptr:
+	.long	_start - . - VDSO_DATA_SIZE
+
+ENTRY(__get_datapage)
+	.fnstart
+	adr	r0, .L_vdso_data_ptr
+	ldr	r1, [r0]
+	add	r0, r0, r1
+	bx	lr
+	.fnend
+ENDPROC(__get_datapage)

new file mode 100644
index 000000000000..b2b97e3e7bab
@@ -0,0 +1,35 @@
+/*
+ * Adapted from arm64 version.
+ *
+ * Copyright (C) 2012 ARM Limited
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Will Deacon <[email protected]>
+ */
+
+#include <linux/init.h>
+#include <linux/linkage.h>
+#include <linux/const.h>
+#include <asm/page.h>
+
+	__PAGE_ALIGNED_DATA
+
+	.globl vdso_start, vdso_end
+	.balign PAGE_SIZE
+vdso_start:
+	.incbin "arch/arm/vdso/vdso.so"
+	.balign PAGE_SIZE
+vdso_end:
+
+	.previous

1.9.3

Initialize the vdso page list at boot, install the vdso mapping at exec time, and update the data page during timer ticks. This code is not built if CONFIG_VDSO is not enabled.

Account for the vdso length when randomizing the offset from the stack. The vdso is placed immediately following the sigpage with a separate install_special_mapping call in arm_install_vdso.

---
 arch/arm/kernel/process.c |  17 ++++-
 arch/arm/kernel/vdso.c    | 189 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 203 insertions(+), 3 deletions(-)
 create mode 100644 arch/arm/kernel/vdso.c

index 9e0d931dd475..7aa1025aff1f 100644
@@ -41,6 +41,7 @@
 #include <asm/system_misc.h>
 #include <asm/mach/time.h>
 #include <asm/tls.h>
+#include <asm/vdso.h>
 
 #ifdef CONFIG_CC_STACKPROTECTOR
 #include <linux/stackprotector.h>

@@ -476,7 +477,7 @@ const char *arch_vma_name(struct vm_area_struct *vma)
 }
 
 /* If possible, provide a placement hint at a random offset from the
- * stack for the signal page.
+ * stack for the sigpage and vdso pages.
  */
 static unsigned long sigpage_addr(const struct mm_struct *mm, unsigned int npages)
 {

@@ -519,6 +520,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
+	unsigned long npages;
 	unsigned long addr;
 	unsigned long hint;
 	int ret = 0;

@@ -528,9 +530,12 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 	if (!signal_page)
 		return -ENOMEM;
 
+	npages = 1; /* for sigpage */
+	npages += vdso_total_pages;
+
 	down_write(&mm->mmap_sem);
-	hint = sigpage_addr(mm, 1);
-	addr = get_unmapped_area(NULL, hint, PAGE_SIZE, 0, 0);
+	hint = sigpage_addr(mm, npages);
+	addr = get_unmapped_area(NULL, hint, npages << PAGE_SHIFT, 0, 0);
 	if (IS_ERR_VALUE(addr)) {
 		ret = addr;
 		goto up_fail;

@@ -547,6 +552,12 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 
 	mm->context.sigpage = addr;
 
+	/* Unlike the sigpage, failure to install the vdso is unlikely
+	 * to be fatal to the process, so no error check needed
+	 * here.
+	 */
+	arm_install_vdso(mm, addr + PAGE_SIZE);
+
  up_fail:
 	up_write(&mm->mmap_sem);
 	return ret;

1.9.3

Allow users to enable the vdso in Kconfig; include the vdso in the build if CONFIG_VDSO is enabled. Add 'vdso_install' target.

---
 arch/arm/Makefile        |  8 ++++++++
 arch/arm/kernel/Makefile |  1 +
 arch/arm/mm/Kconfig      | 15 +++++++++++++++
 3 files changed, 24 insertions(+)

index 0ce9d0f71f2a..109c011ef4ed 100644
@@ -264,6 +264,7 @@ core-$(CONFIG_FPE_FASTFPE)	+= $(FASTFPE_OBJ)
 core-$(CONFIG_VFP)		+= arch/arm/vfp/
 core-$(CONFIG_XEN)		+= arch/arm/xen/
 core-$(CONFIG_KVM_ARM_HOST) 	+= arch/arm/kvm/
+core-$(CONFIG_VDSO)		+= arch/arm/vdso/
 
 # If we have a machine-specific directory, then include it in the build.
 core-y				+= arch/arm/kernel/ arch/arm/mm/ arch/arm/common/

@@ -316,6 +317,12 @@ PHONY += dtbs dtbs_install
 dtbs dtbs_install: prepare scripts
 	$(Q)$(MAKE) $(build)=$(boot)/dts MACHINE=$(MACHINE) $@
 
+PHONY += vdso_install
+vdso_install:
+ifeq ($(CONFIG_VDSO),y)
+	$(Q)$(MAKE) $(build)=arch/arm/vdso $@
+endif
+
 # We use MRPROPER_FILES and CLEAN_FILES now
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)

@@ -340,4 +347,5 @@ define archhelp
   echo  '                  Install using (your) ~/bin/$(INSTALLKERNEL) or'
   echo  '                  (distribution) /sbin/$(INSTALLKERNEL) or'
   echo  '                  install to $$(INSTALL_PATH) and run lilo'
+  echo  '  vdso_install  - Install unstripped vdso.so to $$(INSTALL_MOD_PATH)/vdso'
 endef

index 38ddd9f83d0e..3e2b80658344 100644
@@ -86,6 +86,7 @@ obj-$(CONFIG_PERF_EVENTS)	+= perf_regs.o
 obj-$(CONFIG_HW_PERF_EVENTS)	+= perf_event.o perf_event_cpu.o
 AFLAGS_iwmmxt.o			:= -Wa,-mcpu=iwmmxt
 obj-$(CONFIG_ARM_CPU_TOPOLOGY)  += topology.o
+obj-$(CONFIG_VDSO)		+= vdso.o
 
 ifneq ($(CONFIG_ARCH_EBSA110),y)
   obj-y		+= io.o

index ae69809a9e47..c256323040d6 100644
@@ -824,6 +824,21 @@ config KUSER_HELPERS
 	  Say N here only if you are absolutely certain that you do not
 	  need these helpers; otherwise, the safe option is to say Y.
 
+config VDSO
+	bool "Enable vDSO for acceleration of some system calls"
+	depends on AEABI && MMU
+	default y if ARM_ARCH_TIMER
+	select GENERIC_TIME_VSYSCALL
+	help
+	  Place in the process address space an ELF shared object
+	  providing fast implementations of several system calls,
+	  including gettimeofday and clock_gettime.  Systems that
+	  implement the ARM architected timer will receive maximum
+	  benefit.
+
+	  You must have glibc 2.21 or later for programs to seamlessly
+	  take advantage of this.
+
 config DMA_CACHE_RWFO
 	bool "Enable read/write for ownership DMA cache maintenance"
 	depends on CPU_V6K && SMP

1.9.3