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Subject[PATCH v2 21/26] userfaultfd: wp: add the writeprotect API to userfaultfd ioctl
Date
From: Andrea Arcangeli <aarcange@redhat.com>

v1: From: Shaohua Li <shli@fb.com>

v2: cleanups, remove a branch.

[peterx writes up the commit message, as below...]

This patch introduces the new uffd-wp APIs for userspace.

Firstly, we'll allow to do UFFDIO_REGISTER with write protection
tracking using the new UFFDIO_REGISTER_MODE_WP flag. Note that this
flag can co-exist with the existing UFFDIO_REGISTER_MODE_MISSING, in
which case the userspace program can not only resolve missing page
faults, and at the same time tracking page data changes along the way.

Secondly, we introduced the new UFFDIO_WRITEPROTECT API to do page
level write protection tracking. Note that we will need to register
the memory region with UFFDIO_REGISTER_MODE_WP before that.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
[peterx: remove useless block, write commit message, check against
VM_MAYWRITE rather than VM_WRITE when register]
Signed-off-by: Peter Xu <peterx@redhat.com>
---
fs/userfaultfd.c | 82 +++++++++++++++++++++++++-------
include/uapi/linux/userfaultfd.h | 11 +++++
2 files changed, 77 insertions(+), 16 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 3092885c9d2c..81962d62520c 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -304,8 +304,11 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
if (!pmd_present(_pmd))
goto out;

- if (pmd_trans_huge(_pmd))
+ if (pmd_trans_huge(_pmd)) {
+ if (!pmd_write(_pmd) && (reason & VM_UFFD_WP))
+ ret = true;
goto out;
+ }

/*
* the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
@@ -318,6 +321,8 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
*/
if (pte_none(*pte))
ret = true;
+ if (!pte_write(*pte) && (reason & VM_UFFD_WP))
+ ret = true;
pte_unmap(pte);

out:
@@ -1251,10 +1256,13 @@ static __always_inline int validate_range(struct mm_struct *mm,
return 0;
}

-static inline bool vma_can_userfault(struct vm_area_struct *vma)
+static inline bool vma_can_userfault(struct vm_area_struct *vma,
+ unsigned long vm_flags)
{
- return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
- vma_is_shmem(vma);
+ /* FIXME: add WP support to hugetlbfs and shmem */
+ return vma_is_anonymous(vma) ||
+ ((is_vm_hugetlb_page(vma) || vma_is_shmem(vma)) &&
+ !(vm_flags & VM_UFFD_WP));
}

static int userfaultfd_register(struct userfaultfd_ctx *ctx,
@@ -1286,15 +1294,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
vm_flags = 0;
if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
vm_flags |= VM_UFFD_MISSING;
- if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
+ if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP)
vm_flags |= VM_UFFD_WP;
- /*
- * FIXME: remove the below error constraint by
- * implementing the wprotect tracking mode.
- */
- ret = -EINVAL;
- goto out;
- }

ret = validate_range(mm, uffdio_register.range.start,
uffdio_register.range.len);
@@ -1342,7 +1343,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,

/* check not compatible vmas */
ret = -EINVAL;
- if (!vma_can_userfault(cur))
+ if (!vma_can_userfault(cur, vm_flags))
goto out_unlock;

/*
@@ -1370,6 +1371,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
if (end & (vma_hpagesize - 1))
goto out_unlock;
}
+ if ((vm_flags & VM_UFFD_WP) && !(cur->vm_flags & VM_MAYWRITE))
+ goto out_unlock;

/*
* Check that this vma isn't already owned by a
@@ -1399,7 +1402,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
do {
cond_resched();

- BUG_ON(!vma_can_userfault(vma));
+ BUG_ON(!vma_can_userfault(vma, vm_flags));
BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
vma->vm_userfaultfd_ctx.ctx != ctx);
WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
@@ -1534,7 +1537,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
* provides for more strict behavior to notice
* unregistration errors.
*/
- if (!vma_can_userfault(cur))
+ if (!vma_can_userfault(cur, cur->vm_flags))
goto out_unlock;

found = true;
@@ -1548,7 +1551,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
do {
cond_resched();

- BUG_ON(!vma_can_userfault(vma));
+ BUG_ON(!vma_can_userfault(vma, vma->vm_flags));

/*
* Nothing to do: this vma is already registered into this
@@ -1761,6 +1764,50 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
return ret;
}

+static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
+ unsigned long arg)
+{
+ int ret;
+ struct uffdio_writeprotect uffdio_wp;
+ struct uffdio_writeprotect __user *user_uffdio_wp;
+ struct userfaultfd_wake_range range;
+
+ if (READ_ONCE(ctx->mmap_changing))
+ return -EAGAIN;
+
+ user_uffdio_wp = (struct uffdio_writeprotect __user *) arg;
+
+ if (copy_from_user(&uffdio_wp, user_uffdio_wp,
+ sizeof(struct uffdio_writeprotect)))
+ return -EFAULT;
+
+ ret = validate_range(ctx->mm, uffdio_wp.range.start,
+ uffdio_wp.range.len);
+ if (ret)
+ return ret;
+
+ if (uffdio_wp.mode & ~(UFFDIO_WRITEPROTECT_MODE_DONTWAKE |
+ UFFDIO_WRITEPROTECT_MODE_WP))
+ return -EINVAL;
+ if ((uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP) &&
+ (uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE))
+ return -EINVAL;
+
+ ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
+ uffdio_wp.range.len, uffdio_wp.mode &
+ UFFDIO_WRITEPROTECT_MODE_WP,
+ &ctx->mmap_changing);
+ if (ret)
+ return ret;
+
+ if (!(uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE)) {
+ range.start = uffdio_wp.range.start;
+ range.len = uffdio_wp.range.len;
+ wake_userfault(ctx, &range);
+ }
+ return ret;
+}
+
static inline unsigned int uffd_ctx_features(__u64 user_features)
{
/*
@@ -1838,6 +1885,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
case UFFDIO_ZEROPAGE:
ret = userfaultfd_zeropage(ctx, arg);
break;
+ case UFFDIO_WRITEPROTECT:
+ ret = userfaultfd_writeprotect(ctx, arg);
+ break;
}
return ret;
}
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 297cb044c03f..1b977a7a4435 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -52,6 +52,7 @@
#define _UFFDIO_WAKE (0x02)
#define _UFFDIO_COPY (0x03)
#define _UFFDIO_ZEROPAGE (0x04)
+#define _UFFDIO_WRITEPROTECT (0x06)
#define _UFFDIO_API (0x3F)

/* userfaultfd ioctl ids */
@@ -68,6 +69,8 @@
struct uffdio_copy)
#define UFFDIO_ZEROPAGE _IOWR(UFFDIO, _UFFDIO_ZEROPAGE, \
struct uffdio_zeropage)
+#define UFFDIO_WRITEPROTECT _IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \
+ struct uffdio_writeprotect)

/* read() structure */
struct uffd_msg {
@@ -232,4 +235,12 @@ struct uffdio_zeropage {
__s64 zeropage;
};

+struct uffdio_writeprotect {
+ struct uffdio_range range;
+ /* !WP means undo writeprotect. DONTWAKE is valid only with !WP */
+#define UFFDIO_WRITEPROTECT_MODE_WP ((__u64)1<<0)
+#define UFFDIO_WRITEPROTECT_MODE_DONTWAKE ((__u64)1<<1)
+ __u64 mode;
+};
+
#endif /* _LINUX_USERFAULTFD_H */
--
2.17.1
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