arch/arm64/kernel/mte.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright (C) 2020 ARM Ltd.
  */

 #include <linux/bitops.h>
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/prctl.h>
 #include <linux/sched.h>
 #include <linux/sched/mm.h>
 #include <linux/string.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <linux/thread_info.h>
 #include <linux/types.h>
 #include <linux/uio.h>

 #include <asm/barrier.h>
 #include <asm/cpufeature.h>
 #include <asm/mte.h>
 #include <asm/ptrace.h>
 #include <asm/sysreg.h>

 u64 gcr_kernel_excl __ro_after_init;

 static bool report_fault_once = true;

 static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
 {
 	pte_t old_pte = READ_ONCE(*ptep);

 	if (check_swap && is_swap_pte(old_pte)) {
 		swp_entry_t entry = pte_to_swp_entry(old_pte);

 		if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
 			return;
 	}

 	page_kasan_tag_reset(page);
 	/*
 	 * We need smp_wmb() in between setting the flags and clearing the
 	 * tags because if another thread reads page->flags and builds a
 	 * tagged address out of it, there is an actual dependency to the
 	 * memory access, but on the current thread we do not guarantee that
 	 * the new page->flags are visible before the tags were updated.
 	 */
 	smp_wmb();
 	mte_clear_page_tags(page_address(page));
 }

 void mte_sync_tags(pte_t *ptep, pte_t pte)
 {
 	struct page *page = pte_page(pte);
 	long i, nr_pages = compound_nr(page);
 	bool check_swap = nr_pages == 1;

 	/* if PG_mte_tagged is set, tags have already been initialised */
 	for (i = 0; i < nr_pages; i++, page++) {
 		if (!test_and_set_bit(PG_mte_tagged, &page->flags))
 			mte_sync_page_tags(page, ptep, check_swap);
 	}
 }

 int memcmp_pages(struct page *page1, struct page *page2)
 {
 	char *addr1, *addr2;
 	int ret;

 	addr1 = page_address(page1);
 	addr2 = page_address(page2);
 	ret = memcmp(addr1, addr2, PAGE_SIZE);

 	if (!system_supports_mte() || ret)
 		return ret;

 	/*
 	 * If the page content is identical but at least one of the pages is
 	 * tagged, return non-zero to avoid KSM merging. If only one of the
 	 * pages is tagged, set_pte_at() may zero or change the tags of the
 	 * other page via mte_sync_tags().
 	 */
 	if (test_bit(PG_mte_tagged, &page1->flags) ||
 	    test_bit(PG_mte_tagged, &page2->flags))
 		return addr1 != addr2;

 	return ret;
 }

 void mte_init_tags(u64 max_tag)
 {
 	static bool gcr_kernel_excl_initialized;

 	if (!gcr_kernel_excl_initialized) {
 		/*
 		 * The format of the tags in KASAN is 0xFF and in MTE is 0xF.
 		 * This conversion extracts an MTE tag from a KASAN tag.
 		 */
 		u64 incl = GENMASK(FIELD_GET(MTE_TAG_MASK >> MTE_TAG_SHIFT,
 					     max_tag), 0);

 		gcr_kernel_excl = ~incl & SYS_GCR_EL1_EXCL_MASK;
 		gcr_kernel_excl_initialized = true;
 	}

 	/* Enable the kernel exclude mask for random tags generation. */
 	write_sysreg_s(SYS_GCR_EL1_RRND | gcr_kernel_excl, SYS_GCR_EL1);
 }

 void mte_enable_kernel(void)
 {
 	/* Enable MTE Sync Mode for EL1. */
 	sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
 	isb();
 }

 void mte_set_report_once(bool state)
 {
 	WRITE_ONCE(report_fault_once, state);
 }

 bool mte_report_once(void)
 {
 	return READ_ONCE(report_fault_once);
 }

 static void update_sctlr_el1_tcf0(u64 tcf0)
 {
 	/* ISB required for the kernel uaccess routines */
 	sysreg_clear_set(sctlr_el1, SCTLR_EL1_TCF0_MASK, tcf0);
 	isb();
 }

 static void set_sctlr_el1_tcf0(u64 tcf0)
 {
 	/*
 	 * mte_thread_switch() checks current->thread.sctlr_tcf0 as an
 	 * optimisation. Disable preemption so that it does not see
 	 * the variable update before the SCTLR_EL1.TCF0 one.
 	 */
 	preempt_disable();
 	current->thread.sctlr_tcf0 = tcf0;
 	update_sctlr_el1_tcf0(tcf0);
 	preempt_enable();
 }

 static void update_gcr_el1_excl(u64 excl)
 {

 	/*
 	 * Note that the mask controlled by the user via prctl() is an
 	 * include while GCR_EL1 accepts an exclude mask.
 	 * No need for ISB since this only affects EL0 currently, implicit
 	 * with ERET.
 	 */
 	sysreg_clear_set_s(SYS_GCR_EL1, SYS_GCR_EL1_EXCL_MASK, excl);
 }

 static void set_gcr_el1_excl(u64 excl)
 {
 	current->thread.gcr_user_excl = excl;

 	/*
 	 * SYS_GCR_EL1 will be set to current->thread.gcr_user_excl value
 	 * by mte_set_user_gcr() in kernel_exit,
 	 */
 }

 void flush_mte_state(void)
 {
 	if (!system_supports_mte())
 		return;

 	/* clear any pending asynchronous tag fault */
 	dsb(ish);
 	write_sysreg_s(0, SYS_TFSRE0_EL1);
 	clear_thread_flag(TIF_MTE_ASYNC_FAULT);
 	/* disable tag checking */
 	set_sctlr_el1_tcf0(SCTLR_EL1_TCF0_NONE);
 	/* reset tag generation mask */
 	set_gcr_el1_excl(SYS_GCR_EL1_EXCL_MASK);
 }

 void mte_thread_switch(struct task_struct *next)
 {
 	if (!system_supports_mte())
 		return;

 	/* avoid expensive SCTLR_EL1 accesses if no change */
 	if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
 		update_sctlr_el1_tcf0(next->thread.sctlr_tcf0);
 }

 void mte_suspend_exit(void)
 {
 	if (!system_supports_mte())
 		return;

 	update_gcr_el1_excl(gcr_kernel_excl);
 }

 long set_mte_ctrl(struct task_struct *task, unsigned long arg)
 {
 	u64 tcf0;
 	u64 gcr_excl = ~((arg & PR_MTE_TAG_MASK) >> PR_MTE_TAG_SHIFT) &
 		       SYS_GCR_EL1_EXCL_MASK;

 	if (!system_supports_mte())
 		return 0;

 	switch (arg & PR_MTE_TCF_MASK) {
 	case PR_MTE_TCF_NONE:
 		tcf0 = SCTLR_EL1_TCF0_NONE;
 		break;
 	case PR_MTE_TCF_SYNC:
 		tcf0 = SCTLR_EL1_TCF0_SYNC;
 		break;
 	case PR_MTE_TCF_ASYNC:
 		tcf0 = SCTLR_EL1_TCF0_ASYNC;
 		break;
 	default:
 		return -EINVAL;
 	}

 	if (task != current) {
 		task->thread.sctlr_tcf0 = tcf0;
 		task->thread.gcr_user_excl = gcr_excl;
 	} else {
 		set_sctlr_el1_tcf0(tcf0);
 		set_gcr_el1_excl(gcr_excl);
 	}

 	return 0;
 }

 long get_mte_ctrl(struct task_struct *task)
 {
 	unsigned long ret;
 	u64 incl = ~task->thread.gcr_user_excl & SYS_GCR_EL1_EXCL_MASK;

 	if (!system_supports_mte())
 		return 0;

 	ret = incl << PR_MTE_TAG_SHIFT;

 	switch (task->thread.sctlr_tcf0) {
 	case SCTLR_EL1_TCF0_NONE:
 		ret |= PR_MTE_TCF_NONE;
 		break;
 	case SCTLR_EL1_TCF0_SYNC:
 		ret |= PR_MTE_TCF_SYNC;
 		break;
 	case SCTLR_EL1_TCF0_ASYNC:
 		ret |= PR_MTE_TCF_ASYNC;
 		break;
 	}

 	return ret;
 }

 /*
  * Access MTE tags in another process' address space as given in mm. Update
  * the number of tags copied. Return 0 if any tags copied, error otherwise.
  * Inspired by __access_remote_vm().
  */
 static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
 				struct iovec *kiov, unsigned int gup_flags)
 {
 	struct vm_area_struct *vma;
 	void __user *buf = kiov->iov_base;
 	size_t len = kiov->iov_len;
 	int ret;
 	int write = gup_flags & FOLL_WRITE;

 	if (!access_ok(buf, len))
 		return -EFAULT;

 	if (mmap_read_lock_killable(mm))
 		return -EIO;

 	while (len) {
 		unsigned long tags, offset;
 		void *maddr;
 		struct page *page = NULL;

 		ret = get_user_pages_remote(mm, addr, 1, gup_flags, &page,
 					    &vma, NULL);
 		if (ret <= 0)
 			break;

 		/*
 		 * Only copy tags if the page has been mapped as PROT_MTE
 		 * (PG_mte_tagged set). Otherwise the tags are not valid and
 		 * not accessible to user. Moreover, an mprotect(PROT_MTE)
 		 * would cause the existing tags to be cleared if the page
 		 * was never mapped with PROT_MTE.
 		 */
 		if (!(vma->vm_flags & VM_MTE)) {
 			ret = -EOPNOTSUPP;
 			put_page(page);
 			break;
 		}
 		WARN_ON_ONCE(!test_bit(PG_mte_tagged, &page->flags));

 		/* limit access to the end of the page */
 		offset = offset_in_page(addr);
 		tags = min(len, (PAGE_SIZE - offset) / MTE_GRANULE_SIZE);

 		maddr = page_address(page);
 		if (write) {
 			tags = mte_copy_tags_from_user(maddr + offset, buf, tags);
 			set_page_dirty_lock(page);
 		} else {
 			tags = mte_copy_tags_to_user(buf, maddr + offset, tags);
 		}
 		put_page(page);

 		/* error accessing the tracer's buffer */
 		if (!tags)
 			break;

 		len -= tags;
 		buf += tags;
 		addr += tags * MTE_GRANULE_SIZE;
 	}
 	mmap_read_unlock(mm);

 	/* return an error if no tags copied */
 	kiov->iov_len = buf - kiov->iov_base;
 	if (!kiov->iov_len) {
 		/* check for error accessing the tracee's address space */
 		if (ret <= 0)
 			return -EIO;
 		else
 			return -EFAULT;
 	}

 	return 0;
 }

 /*
  * Copy MTE tags in another process' address space at 'addr' to/from tracer's
  * iovec buffer. Return 0 on success. Inspired by ptrace_access_vm().
  */
 static int access_remote_tags(struct task_struct *tsk, unsigned long addr,
 			      struct iovec *kiov, unsigned int gup_flags)
 {
 	struct mm_struct *mm;
 	int ret;

 	mm = get_task_mm(tsk);
 	if (!mm)
 		return -EPERM;

 	if (!tsk->ptrace || (current != tsk->parent) ||
 	    ((get_dumpable(mm) != SUID_DUMP_USER) &&
 	     !ptracer_capable(tsk, mm->user_ns))) {
 		mmput(mm);
 		return -EPERM;
 	}

 	ret = __access_remote_tags(mm, addr, kiov, gup_flags);
 	mmput(mm);

 	return ret;
 }

 int mte_ptrace_copy_tags(struct task_struct *child, long request,
 			 unsigned long addr, unsigned long data)
 {
 	int ret;
 	struct iovec kiov;
 	struct iovec __user *uiov = (void __user *)data;
 	unsigned int gup_flags = FOLL_FORCE;

 	if (!system_supports_mte())
 		return -EIO;

 	if (get_user(kiov.iov_base, &uiov->iov_base) ||
 	    get_user(kiov.iov_len, &uiov->iov_len))
 		return -EFAULT;

 	if (request == PTRACE_POKEMTETAGS)
 		gup_flags |= FOLL_WRITE;

 	/* align addr to the MTE tag granule */
 	addr &= MTE_GRANULE_MASK;

 	ret = access_remote_tags(child, addr, &kiov, gup_flags);
 	if (!ret)
 		ret = put_user(kiov.iov_len, &uiov->iov_len);

 	return ret;
 }
	// SPDX-License-Identifier: GPL-2.0-only
	/*
	* Copyright (C) 2020 ARM Ltd.
	*/

	#include <linux/bitops.h>
	#include <linux/kernel.h>
	#include <linux/mm.h>
	#include <linux/prctl.h>
	#include <linux/sched.h>
	#include <linux/sched/mm.h>
	#include <linux/string.h>
	#include <linux/swap.h>
	#include <linux/swapops.h>
	#include <linux/thread_info.h>
	#include <linux/types.h>
	#include <linux/uio.h>

	#include <asm/barrier.h>
	#include <asm/cpufeature.h>
	#include <asm/mte.h>
	#include <asm/ptrace.h>
	#include <asm/sysreg.h>

	u64 gcr_kernel_excl __ro_after_init;

	static bool report_fault_once = true;

	static void mte_sync_page_tags(struct page page, pte_t ptep, bool check_swap)
	{
	pte_t old_pte = READ_ONCE(*ptep);

	if (check_swap && is_swap_pte(old_pte)) {
	swp_entry_t entry = pte_to_swp_entry(old_pte);

	if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
	return;
	}

	page_kasan_tag_reset(page);
	/*
	* We need smp_wmb() in between setting the flags and clearing the
	* tags because if another thread reads page->flags and builds a
	* tagged address out of it, there is an actual dependency to the
	* memory access, but on the current thread we do not guarantee that
	* the new page->flags are visible before the tags were updated.
	*/
	smp_wmb();
	mte_clear_page_tags(page_address(page));
	}

	void mte_sync_tags(pte_t *ptep, pte_t pte)
	{
	struct page *page = pte_page(pte);
	long i, nr_pages = compound_nr(page);
	bool check_swap = nr_pages == 1;

	/* if PG_mte_tagged is set, tags have already been initialised */
	for (i = 0; i < nr_pages; i++, page++) {
	if (!test_and_set_bit(PG_mte_tagged, &page->flags))
	mte_sync_page_tags(page, ptep, check_swap);
	}
	}

	int memcmp_pages(struct page page1, struct page page2)
	{
	char addr1, addr2;
	int ret;

	addr1 = page_address(page1);
	addr2 = page_address(page2);
	ret = memcmp(addr1, addr2, PAGE_SIZE);

	if (!system_supports_mte() \|\| ret)
	return ret;

	/*
	* If the page content is identical but at least one of the pages is
	* tagged, return non-zero to avoid KSM merging. If only one of the
	* pages is tagged, set_pte_at() may zero or change the tags of the
	* other page via mte_sync_tags().
	*/
	if (test_bit(PG_mte_tagged, &page1->flags) \|\|
	test_bit(PG_mte_tagged, &page2->flags))
	return addr1 != addr2;

	return ret;
	}

	void mte_init_tags(u64 max_tag)
	{
	static bool gcr_kernel_excl_initialized;

	if (!gcr_kernel_excl_initialized) {
	/*
	* The format of the tags in KASAN is 0xFF and in MTE is 0xF.
	* This conversion extracts an MTE tag from a KASAN tag.
	*/
	u64 incl = GENMASK(FIELD_GET(MTE_TAG_MASK >> MTE_TAG_SHIFT,
	max_tag), 0);

	gcr_kernel_excl = ~incl & SYS_GCR_EL1_EXCL_MASK;
	gcr_kernel_excl_initialized = true;
	}

	/* Enable the kernel exclude mask for random tags generation. */
	write_sysreg_s(SYS_GCR_EL1_RRND \| gcr_kernel_excl, SYS_GCR_EL1);
	}

	void mte_enable_kernel(void)
	{
	/* Enable MTE Sync Mode for EL1. */
	sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
	isb();
	}

	void mte_set_report_once(bool state)
	{
	WRITE_ONCE(report_fault_once, state);
	}

	bool mte_report_once(void)
	{
	return READ_ONCE(report_fault_once);
	}

	static void update_sctlr_el1_tcf0(u64 tcf0)
	{
	/* ISB required for the kernel uaccess routines */
	sysreg_clear_set(sctlr_el1, SCTLR_EL1_TCF0_MASK, tcf0);
	isb();
	}

	static void set_sctlr_el1_tcf0(u64 tcf0)
	{
	/*
	* mte_thread_switch() checks current->thread.sctlr_tcf0 as an
	* optimisation. Disable preemption so that it does not see
	* the variable update before the SCTLR_EL1.TCF0 one.
	*/
	preempt_disable();
	current->thread.sctlr_tcf0 = tcf0;
	update_sctlr_el1_tcf0(tcf0);
	preempt_enable();
	}

	static void update_gcr_el1_excl(u64 excl)
	{

	/*
	* Note that the mask controlled by the user via prctl() is an
	* include while GCR_EL1 accepts an exclude mask.
	* No need for ISB since this only affects EL0 currently, implicit
	* with ERET.
	*/
	sysreg_clear_set_s(SYS_GCR_EL1, SYS_GCR_EL1_EXCL_MASK, excl);
	}

	static void set_gcr_el1_excl(u64 excl)
	{
	current->thread.gcr_user_excl = excl;

	/*
	* SYS_GCR_EL1 will be set to current->thread.gcr_user_excl value
	* by mte_set_user_gcr() in kernel_exit,
	*/
	}

	void flush_mte_state(void)
	{
	if (!system_supports_mte())
	return;

	/* clear any pending asynchronous tag fault */
	dsb(ish);
	write_sysreg_s(0, SYS_TFSRE0_EL1);
	clear_thread_flag(TIF_MTE_ASYNC_FAULT);
	/* disable tag checking */
	set_sctlr_el1_tcf0(SCTLR_EL1_TCF0_NONE);
	/* reset tag generation mask */
	set_gcr_el1_excl(SYS_GCR_EL1_EXCL_MASK);
	}

	void mte_thread_switch(struct task_struct *next)
	{
	if (!system_supports_mte())
	return;

	/* avoid expensive SCTLR_EL1 accesses if no change */
	if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0)
	update_sctlr_el1_tcf0(next->thread.sctlr_tcf0);
	}

	void mte_suspend_exit(void)
	{
	if (!system_supports_mte())
	return;

	update_gcr_el1_excl(gcr_kernel_excl);
	}

	long set_mte_ctrl(struct task_struct *task, unsigned long arg)
	{
	u64 tcf0;
	u64 gcr_excl = ~((arg & PR_MTE_TAG_MASK) >> PR_MTE_TAG_SHIFT) &
	SYS_GCR_EL1_EXCL_MASK;

	if (!system_supports_mte())
	return 0;

	switch (arg & PR_MTE_TCF_MASK) {
	case PR_MTE_TCF_NONE:
	tcf0 = SCTLR_EL1_TCF0_NONE;
	break;
	case PR_MTE_TCF_SYNC:
	tcf0 = SCTLR_EL1_TCF0_SYNC;
	break;
	case PR_MTE_TCF_ASYNC:
	tcf0 = SCTLR_EL1_TCF0_ASYNC;
	break;
	default:
	return -EINVAL;
	}

	if (task != current) {
	task->thread.sctlr_tcf0 = tcf0;
	task->thread.gcr_user_excl = gcr_excl;
	} else {
	set_sctlr_el1_tcf0(tcf0);
	set_gcr_el1_excl(gcr_excl);
	}

	return 0;
	}

	long get_mte_ctrl(struct task_struct *task)
	{
	unsigned long ret;
	u64 incl = ~task->thread.gcr_user_excl & SYS_GCR_EL1_EXCL_MASK;

	if (!system_supports_mte())
	return 0;

	ret = incl << PR_MTE_TAG_SHIFT;

	switch (task->thread.sctlr_tcf0) {
	case SCTLR_EL1_TCF0_NONE:
	ret \|= PR_MTE_TCF_NONE;
	break;
	case SCTLR_EL1_TCF0_SYNC:
	ret \|= PR_MTE_TCF_SYNC;
	break;
	case SCTLR_EL1_TCF0_ASYNC:
	ret \|= PR_MTE_TCF_ASYNC;
	break;
	}

	return ret;
	}

	/*
	* Access MTE tags in another process' address space as given in mm. Update
	* the number of tags copied. Return 0 if any tags copied, error otherwise.
	* Inspired by __access_remote_vm().
	*/
	static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
	struct iovec *kiov, unsigned int gup_flags)
	{
	struct vm_area_struct *vma;
	void __user *buf = kiov->iov_base;
	size_t len = kiov->iov_len;
	int ret;
	int write = gup_flags & FOLL_WRITE;

	if (!access_ok(buf, len))
	return -EFAULT;

	if (mmap_read_lock_killable(mm))
	return -EIO;

	while (len) {
	unsigned long tags, offset;
	void *maddr;
	struct page *page = NULL;

	ret = get_user_pages_remote(mm, addr, 1, gup_flags, &page,
	&vma, NULL);
	if (ret <= 0)
	break;

	/*
	* Only copy tags if the page has been mapped as PROT_MTE
	* (PG_mte_tagged set). Otherwise the tags are not valid and
	* not accessible to user. Moreover, an mprotect(PROT_MTE)
	* would cause the existing tags to be cleared if the page
	* was never mapped with PROT_MTE.
	*/
	if (!(vma->vm_flags & VM_MTE)) {
	ret = -EOPNOTSUPP;
	put_page(page);
	break;
	}
	WARN_ON_ONCE(!test_bit(PG_mte_tagged, &page->flags));

	/* limit access to the end of the page */
	offset = offset_in_page(addr);
	tags = min(len, (PAGE_SIZE - offset) / MTE_GRANULE_SIZE);

	maddr = page_address(page);
	if (write) {
	tags = mte_copy_tags_from_user(maddr + offset, buf, tags);
	set_page_dirty_lock(page);
	} else {
	tags = mte_copy_tags_to_user(buf, maddr + offset, tags);
	}
	put_page(page);

	/* error accessing the tracer's buffer */
	if (!tags)
	break;

	len -= tags;
	buf += tags;
	addr += tags * MTE_GRANULE_SIZE;
	}
	mmap_read_unlock(mm);

	/* return an error if no tags copied */
	kiov->iov_len = buf - kiov->iov_base;
	if (!kiov->iov_len) {
	/* check for error accessing the tracee's address space */
	if (ret <= 0)
	return -EIO;
	else
	return -EFAULT;
	}

	return 0;
	}

	/*
	* Copy MTE tags in another process' address space at 'addr' to/from tracer's
	* iovec buffer. Return 0 on success. Inspired by ptrace_access_vm().
	*/
	static int access_remote_tags(struct task_struct *tsk, unsigned long addr,
	struct iovec *kiov, unsigned int gup_flags)
	{
	struct mm_struct *mm;
	int ret;

	mm = get_task_mm(tsk);
	if (!mm)
	return -EPERM;

	if (!tsk->ptrace \|\| (current != tsk->parent) \|\|
	((get_dumpable(mm) != SUID_DUMP_USER) &&
	!ptracer_capable(tsk, mm->user_ns))) {
	mmput(mm);
	return -EPERM;
	}

	ret = __access_remote_tags(mm, addr, kiov, gup_flags);
	mmput(mm);

	return ret;
	}

	int mte_ptrace_copy_tags(struct task_struct *child, long request,
	unsigned long addr, unsigned long data)
	{
	int ret;
	struct iovec kiov;
	struct iovec __user uiov = (void __user )data;
	unsigned int gup_flags = FOLL_FORCE;

	if (!system_supports_mte())
	return -EIO;

	if (get_user(kiov.iov_base, &uiov->iov_base) \|\|
	get_user(kiov.iov_len, &uiov->iov_len))
	return -EFAULT;

	if (request == PTRACE_POKEMTETAGS)
	gup_flags \|= FOLL_WRITE;

	/* align addr to the MTE tag granule */
	addr &= MTE_GRANULE_MASK;

	ret = access_remote_tags(child, addr, &kiov, gup_flags);
	if (!ret)
	ret = put_user(kiov.iov_len, &uiov->iov_len);

	return ret;
	}