[PATCH v3] userfaultfd: provide unmasked address on page-fault

[PATCH v3] userfaultfd: provide unmasked address on page-fault

[Nadav Amit]

From: Nadav Amit <namit@vmware.com>

Userfaultfd is supposed to provide the full address (i.e., unmasked) of
the faulting access back to userspace. However, that is not the case for
quite some time.

Even running "userfaultfd_demo" from the userfaultfd man page provides
the wrong output (and contradicts the man page). Notice that
"UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
and not the first read address (0x7fc5e30b300f).

	Address returned by mmap() = 0x7fc5e30b3000

	fault_handler_thread():
	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
		(uffdio_copy.copy returned 4096)
	Read address 0x7fc5e30b300f in main(): A
	Read address 0x7fc5e30b340f in main(): A
	Read address 0x7fc5e30b380f in main(): A
	Read address 0x7fc5e30b3c0f in main(): A

The exact address is useful for various reasons and specifically for
prefetching decisions. If it is known that the memory is populated by
certain objects whose size is not page-aligned, then based on the
faulting address, the uffd-monitor can decide whether to prefetch and
prefault the adjacent page.

This bug has been for quite some time in the kernel: since commit
1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
vmf->virtual_address"), which dates back to 2016. A concern has been
raised that existing userspace application might rely on the old/wrong
behavior in which the address is masked. Therefore, it was suggested to
provide the masked address unless the user explicitly asks for the exact
address.

Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
userfaultfd to provide the exact address. Add a new "real_address" field
to vmf to hold the unmasked address. Provide the address to userspace
accordingly.

Initialize real_address in various code-paths to be consistent with
address, even when it is not used, to be on the safe side.

Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Nadav Amit <namit@vmware.com>

---

v2->v3:
* Initialize real_address on all code paths [Jan]

v1->v2:
* Add uffd feature to selectively enable [David, Andrea]
---
 fs/userfaultfd.c                 | 5 ++++-
 include/linux/mm.h               | 3 ++-
 include/uapi/linux/userfaultfd.h | 8 +++++++-
 mm/hugetlb.c                     | 6 ++++--
 mm/memory.c                      | 1 +
 mm/swapfile.c                    | 1 +
 6 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index e26b10132d47..826927026fe7 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -198,6 +198,9 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
 	struct uffd_msg msg;
 	msg_init(&msg);
 	msg.event = UFFD_EVENT_PAGEFAULT;
+
+	if (!(features & UFFD_FEATURE_EXACT_ADDRESS))
+		address &= PAGE_MASK;
 	msg.arg.pagefault.address = address;
 	/*
 	 * These flags indicate why the userfault occurred:
@@ -482,7 +485,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 
 	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
 	uwq.wq.private = current;
-	uwq.msg = userfault_msg(vmf->address, vmf->flags, reason,
+	uwq.msg = userfault_msg(vmf->real_address, vmf->flags, reason,
 			ctx->features);
 	uwq.ctx = ctx;
 	uwq.waken = false;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 213cc569b192..27df0ca0a36a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -478,7 +478,8 @@ struct vm_fault {
 		struct vm_area_struct *vma;	/* Target VMA */
 		gfp_t gfp_mask;			/* gfp mask to be used for allocations */
 		pgoff_t pgoff;			/* Logical page offset based on vma */
-		unsigned long address;		/* Faulting virtual address */
+		unsigned long address;		/* Faulting virtual address - masked */
+		unsigned long real_address;	/* Faulting virtual address - unmaked */
 	};
 	enum fault_flag flags;		/* FAULT_FLAG_xxx flags
 					 * XXX: should really be 'const' */
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 05b31d60acf6..ef739054cb1c 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -32,7 +32,8 @@
 			   UFFD_FEATURE_SIGBUS |		\
 			   UFFD_FEATURE_THREAD_ID |		\
 			   UFFD_FEATURE_MINOR_HUGETLBFS |	\
-			   UFFD_FEATURE_MINOR_SHMEM)
+			   UFFD_FEATURE_MINOR_SHMEM |		\
+			   UFFD_FEATURE_EXACT_ADDRESS)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -189,6 +190,10 @@ struct uffdio_api {
 	 *
 	 * UFFD_FEATURE_MINOR_SHMEM indicates the same support as
 	 * UFFD_FEATURE_MINOR_HUGETLBFS, but for shmem-backed pages instead.
+	 *
+	 * UFFD_FEATURE_EXACT_ADDRESS indicates that the exact address of page
+	 * faults would be provided and the offset within the page would not be
+	 * masked.
 	 */
 #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
 #define UFFD_FEATURE_EVENT_FORK			(1<<1)
@@ -201,6 +206,7 @@ struct uffdio_api {
 #define UFFD_FEATURE_THREAD_ID			(1<<8)
 #define UFFD_FEATURE_MINOR_HUGETLBFS		(1<<9)
 #define UFFD_FEATURE_MINOR_SHMEM		(1<<10)
+#define UFFD_FEATURE_EXACT_ADDRESS		(1<<11)
 	__u64 features;
 
 	__u64 ioctls;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 61895cc01d09..16017f90568b 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5342,6 +5342,7 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
 						  pgoff_t idx,
 						  unsigned int flags,
 						  unsigned long haddr,
+						  unsigned long addr,
 						  unsigned long reason)
 {
 	vm_fault_t ret;
@@ -5349,6 +5350,7 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
 	struct vm_fault vmf = {
 		.vma = vma,
 		.address = haddr,
+		.real_address = addr,
 		.flags = flags,
 
 		/*
@@ -5417,7 +5419,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 		/* Check for page in userfault range */
 		if (userfaultfd_missing(vma)) {
 			ret = hugetlb_handle_userfault(vma, mapping, idx,
-						       flags, haddr,
+						       flags, haddr, address,
 						       VM_UFFD_MISSING);
 			goto out;
 		}
@@ -5481,7 +5483,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 			unlock_page(page);
 			put_page(page);
 			ret = hugetlb_handle_userfault(vma, mapping, idx,
-						       flags, haddr,
+						       flags, haddr, address,
 						       VM_UFFD_MINOR);
 			goto out;
 		}
diff --git a/mm/memory.c b/mm/memory.c
index c125c4969913..aae53fde13d9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4622,6 +4622,7 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
 	struct vm_fault vmf = {
 		.vma = vma,
 		.address = address & PAGE_MASK,
+		.real_address = address,
 		.flags = flags,
 		.pgoff = linear_page_index(vma, address),
 		.gfp_mask = __get_fault_gfp_mask(vma),
diff --git a/mm/swapfile.c b/mm/swapfile.c
index bf0df7aa7158..33c7abb16610 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1951,6 +1951,7 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 			struct vm_fault vmf = {
 				.vma = vma,
 				.address = addr,
+				.real_address = addr,
 				.pmd = pmd,
 			};
 
-- 
2.25.1

Re: [PATCH v3] userfaultfd: provide unmasked address on page-fault

[Mike Rapoport]

On Sat, Feb 26, 2022 at 02:26:55AM +0000, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> Userfaultfd is supposed to provide the full address (i.e., unmasked) of
> the faulting access back to userspace. However, that is not the case for
> quite some time.
> 
> Even running "userfaultfd_demo" from the userfaultfd man page provides
> the wrong output (and contradicts the man page). Notice that
> "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
> and not the first read address (0x7fc5e30b300f).
> 
> 	Address returned by mmap() = 0x7fc5e30b3000
> 
> 	fault_handler_thread():
> 	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
> 	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
> 		(uffdio_copy.copy returned 4096)
> 	Read address 0x7fc5e30b300f in main(): A
> 	Read address 0x7fc5e30b340f in main(): A
> 	Read address 0x7fc5e30b380f in main(): A
> 	Read address 0x7fc5e30b3c0f in main(): A
> 
> The exact address is useful for various reasons and specifically for
> prefetching decisions. If it is known that the memory is populated by
> certain objects whose size is not page-aligned, then based on the
> faulting address, the uffd-monitor can decide whether to prefetch and
> prefault the adjacent page.
> 
> This bug has been for quite some time in the kernel: since commit
> 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
> vmf->virtual_address"), which dates back to 2016. A concern has been
> raised that existing userspace application might rely on the old/wrong
> behavior in which the address is masked. Therefore, it was suggested to
> provide the masked address unless the user explicitly asks for the exact
> address.
> 
> Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
> userfaultfd to provide the exact address. Add a new "real_address" field
> to vmf to hold the unmasked address. Provide the address to userspace
> accordingly.
> 
> Initialize real_address in various code-paths to be consistent with
> address, even when it is not used, to be on the safe side.
> 
> Acked-by: Peter Xu <peterx@redhat.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Cc: Jan Kara <jack@suse.cz>
> Signed-off-by: Nadav Amit <namit@vmware.com>

Acked-by: Mike Rapoport <rppt@linux.ibm.com>

> 
> ---
> 
> v2->v3:
> * Initialize real_address on all code paths [Jan]
> 
> v1->v2:
> * Add uffd feature to selectively enable [David, Andrea]
> ---
>  fs/userfaultfd.c                 | 5 ++++-
>  include/linux/mm.h               | 3 ++-
>  include/uapi/linux/userfaultfd.h | 8 +++++++-
>  mm/hugetlb.c                     | 6 ++++--
>  mm/memory.c                      | 1 +
>  mm/swapfile.c                    | 1 +
>  6 files changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index e26b10132d47..826927026fe7 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -198,6 +198,9 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
>  	struct uffd_msg msg;
>  	msg_init(&msg);
>  	msg.event = UFFD_EVENT_PAGEFAULT;
> +
> +	if (!(features & UFFD_FEATURE_EXACT_ADDRESS))
> +		address &= PAGE_MASK;
>  	msg.arg.pagefault.address = address;
>  	/*
>  	 * These flags indicate why the userfault occurred:
> @@ -482,7 +485,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> 
>  	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
>  	uwq.wq.private = current;
> -	uwq.msg = userfault_msg(vmf->address, vmf->flags, reason,
> +	uwq.msg = userfault_msg(vmf->real_address, vmf->flags, reason,
>  			ctx->features);
>  	uwq.ctx = ctx;
>  	uwq.waken = false;
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 213cc569b192..27df0ca0a36a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -478,7 +478,8 @@ struct vm_fault {
>  		struct vm_area_struct *vma;	/* Target VMA */
>  		gfp_t gfp_mask;			/* gfp mask to be used for allocations */
>  		pgoff_t pgoff;			/* Logical page offset based on vma */
> -		unsigned long address;		/* Faulting virtual address */
> +		unsigned long address;		/* Faulting virtual address - masked */
> +		unsigned long real_address;	/* Faulting virtual address - unmaked */
>  	};
>  	enum fault_flag flags;		/* FAULT_FLAG_xxx flags
>  					 * XXX: should really be 'const' */
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 05b31d60acf6..ef739054cb1c 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -32,7 +32,8 @@
>  			   UFFD_FEATURE_SIGBUS |		\
>  			   UFFD_FEATURE_THREAD_ID |		\
>  			   UFFD_FEATURE_MINOR_HUGETLBFS |	\
> -			   UFFD_FEATURE_MINOR_SHMEM)
> +			   UFFD_FEATURE_MINOR_SHMEM |		\
> +			   UFFD_FEATURE_EXACT_ADDRESS)
>  #define UFFD_API_IOCTLS				\
>  	((__u64)1 << _UFFDIO_REGISTER |		\
>  	 (__u64)1 << _UFFDIO_UNREGISTER |	\
> @@ -189,6 +190,10 @@ struct uffdio_api {
>  	 *
>  	 * UFFD_FEATURE_MINOR_SHMEM indicates the same support as
>  	 * UFFD_FEATURE_MINOR_HUGETLBFS, but for shmem-backed pages instead.
> +	 *
> +	 * UFFD_FEATURE_EXACT_ADDRESS indicates that the exact address of page
> +	 * faults would be provided and the offset within the page would not be
> +	 * masked.
>  	 */
>  #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
>  #define UFFD_FEATURE_EVENT_FORK			(1<<1)
> @@ -201,6 +206,7 @@ struct uffdio_api {
>  #define UFFD_FEATURE_THREAD_ID			(1<<8)
>  #define UFFD_FEATURE_MINOR_HUGETLBFS		(1<<9)
>  #define UFFD_FEATURE_MINOR_SHMEM		(1<<10)
> +#define UFFD_FEATURE_EXACT_ADDRESS		(1<<11)
>  	__u64 features;
> 
>  	__u64 ioctls;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 61895cc01d09..16017f90568b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5342,6 +5342,7 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
>  						  pgoff_t idx,
>  						  unsigned int flags,
>  						  unsigned long haddr,
> +						  unsigned long addr,
>  						  unsigned long reason)
>  {
>  	vm_fault_t ret;
> @@ -5349,6 +5350,7 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
>  	struct vm_fault vmf = {
>  		.vma = vma,
>  		.address = haddr,
> +		.real_address = addr,
>  		.flags = flags,
> 
>  		/*
> @@ -5417,7 +5419,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
>  		/* Check for page in userfault range */
>  		if (userfaultfd_missing(vma)) {
>  			ret = hugetlb_handle_userfault(vma, mapping, idx,
> -						       flags, haddr,
> +						       flags, haddr, address,
>  						       VM_UFFD_MISSING);
>  			goto out;
>  		}
> @@ -5481,7 +5483,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
>  			unlock_page(page);
>  			put_page(page);
>  			ret = hugetlb_handle_userfault(vma, mapping, idx,
> -						       flags, haddr,
> +						       flags, haddr, address,
>  						       VM_UFFD_MINOR);
>  			goto out;
>  		}
> diff --git a/mm/memory.c b/mm/memory.c
> index c125c4969913..aae53fde13d9 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4622,6 +4622,7 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  	struct vm_fault vmf = {
>  		.vma = vma,
>  		.address = address & PAGE_MASK,
> +		.real_address = address,
>  		.flags = flags,
>  		.pgoff = linear_page_index(vma, address),
>  		.gfp_mask = __get_fault_gfp_mask(vma),
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index bf0df7aa7158..33c7abb16610 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1951,6 +1951,7 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  			struct vm_fault vmf = {
>  				.vma = vma,
>  				.address = addr,
> +				.real_address = addr,
>  				.pmd = pmd,
>  			};
> 
> -- 
> 2.25.1
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH v3] userfaultfd: provide unmasked address on page-fault

[Jan Kara]

On Sat 26-02-22 02:26:55, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> Userfaultfd is supposed to provide the full address (i.e., unmasked) of
> the faulting access back to userspace. However, that is not the case for
> quite some time.
> 
> Even running "userfaultfd_demo" from the userfaultfd man page provides
> the wrong output (and contradicts the man page). Notice that
> "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
> and not the first read address (0x7fc5e30b300f).
> 
> 	Address returned by mmap() = 0x7fc5e30b3000
> 
> 	fault_handler_thread():
> 	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
> 	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
> 		(uffdio_copy.copy returned 4096)
> 	Read address 0x7fc5e30b300f in main(): A
> 	Read address 0x7fc5e30b340f in main(): A
> 	Read address 0x7fc5e30b380f in main(): A
> 	Read address 0x7fc5e30b3c0f in main(): A
> 
> The exact address is useful for various reasons and specifically for
> prefetching decisions. If it is known that the memory is populated by
> certain objects whose size is not page-aligned, then based on the
> faulting address, the uffd-monitor can decide whether to prefetch and
> prefault the adjacent page.
> 
> This bug has been for quite some time in the kernel: since commit
> 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
> vmf->virtual_address"), which dates back to 2016. A concern has been
> raised that existing userspace application might rely on the old/wrong
> behavior in which the address is masked. Therefore, it was suggested to
> provide the masked address unless the user explicitly asks for the exact
> address.
> 
> Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
> userfaultfd to provide the exact address. Add a new "real_address" field
> to vmf to hold the unmasked address. Provide the address to userspace
> accordingly.
> 
> Initialize real_address in various code-paths to be consistent with
> address, even when it is not used, to be on the safe side.
> 
> Acked-by: Peter Xu <peterx@redhat.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Cc: Jan Kara <jack@suse.cz>
> Signed-off-by: Nadav Amit <namit@vmware.com>
> 
> ---
> 
> v2->v3:
> * Initialize real_address on all code paths [Jan]
> 
> v1->v2:
> * Add uffd feature to selectively enable [David, Andrea]

I've just noticed one typo below. Otherwise the patch looks good to me.
Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>


> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 213cc569b192..27df0ca0a36a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -478,7 +478,8 @@ struct vm_fault {
>  		struct vm_area_struct *vma;	/* Target VMA */
>  		gfp_t gfp_mask;			/* gfp mask to be used for allocations */
>  		pgoff_t pgoff;			/* Logical page offset based on vma */
> -		unsigned long address;		/* Faulting virtual address */
> +		unsigned long address;		/* Faulting virtual address - masked */
> +		unsigned long real_address;	/* Faulting virtual address - unmaked */
										^^ typo here

									Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH v3] userfaultfd: provide unmasked address on page-fault

[Peter Xu]

On Sat, Feb 26, 2022 at 02:26:55AM +0000, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
> 
> Userfaultfd is supposed to provide the full address (i.e., unmasked) of
> the faulting access back to userspace. However, that is not the case for
> quite some time.
> 
> Even running "userfaultfd_demo" from the userfaultfd man page provides
> the wrong output (and contradicts the man page). Notice that
> "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
> and not the first read address (0x7fc5e30b300f).
> 
> 	Address returned by mmap() = 0x7fc5e30b3000
> 
> 	fault_handler_thread():
> 	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
> 	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
> 		(uffdio_copy.copy returned 4096)
> 	Read address 0x7fc5e30b300f in main(): A
> 	Read address 0x7fc5e30b340f in main(): A
> 	Read address 0x7fc5e30b380f in main(): A
> 	Read address 0x7fc5e30b3c0f in main(): A
> 
> The exact address is useful for various reasons and specifically for
> prefetching decisions. If it is known that the memory is populated by
> certain objects whose size is not page-aligned, then based on the
> faulting address, the uffd-monitor can decide whether to prefetch and
> prefault the adjacent page.
> 
> This bug has been for quite some time in the kernel: since commit
> 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
> vmf->virtual_address"), which dates back to 2016. A concern has been
> raised that existing userspace application might rely on the old/wrong
> behavior in which the address is masked. Therefore, it was suggested to
> provide the masked address unless the user explicitly asks for the exact
> address.
> 
> Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
> userfaultfd to provide the exact address. Add a new "real_address" field
> to vmf to hold the unmasked address. Provide the address to userspace
> accordingly.
> 
> Initialize real_address in various code-paths to be consistent with
> address, even when it is not used, to be on the safe side.
> 
> Acked-by: Peter Xu <peterx@redhat.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Cc: Jan Kara <jack@suse.cz>
> Signed-off-by: Nadav Amit <namit@vmware.com>

Hi, Andrew,

Just a heads-up that this version has not yet been updated in -mm I think,
while the queued one is the old version.

IOW, uffd is currently broken on latest linux-next on hugetlb.

Thanks,

-- 
Peter Xu

Re: [PATCH v3] userfaultfd: provide unmasked address on page-fault

[Nadav Amit]

> On Mar 3, 2022, at 12:03 AM, Peter Xu <peterx@redhat.com> wrote:
> 
> On Sat, Feb 26, 2022 at 02:26:55AM +0000, Nadav Amit wrote:
>> From: Nadav Amit <namit@vmware.com>
>> 
>> Userfaultfd is supposed to provide the full address (i.e., unmasked) of
>> the faulting access back to userspace. However, that is not the case for
>> quite some time.
>> 
>> Even running "userfaultfd_demo" from the userfaultfd man page provides
>> the wrong output (and contradicts the man page). Notice that
>> "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
>> and not the first read address (0x7fc5e30b300f).
>> 
>> 	Address returned by mmap() = 0x7fc5e30b3000
>> 
>> 	fault_handler_thread():
>> 	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
>> 	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
>> 		(uffdio_copy.copy returned 4096)
>> 	Read address 0x7fc5e30b300f in main(): A
>> 	Read address 0x7fc5e30b340f in main(): A
>> 	Read address 0x7fc5e30b380f in main(): A
>> 	Read address 0x7fc5e30b3c0f in main(): A
>> 
>> The exact address is useful for various reasons and specifically for
>> prefetching decisions. If it is known that the memory is populated by
>> certain objects whose size is not page-aligned, then based on the
>> faulting address, the uffd-monitor can decide whether to prefetch and
>> prefault the adjacent page.
>> 
>> This bug has been for quite some time in the kernel: since commit
>> 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
>> vmf->virtual_address"), which dates back to 2016. A concern has been
>> raised that existing userspace application might rely on the old/wrong
>> behavior in which the address is masked. Therefore, it was suggested to
>> provide the masked address unless the user explicitly asks for the exact
>> address.
>> 
>> Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
>> userfaultfd to provide the exact address. Add a new "real_address" field
>> to vmf to hold the unmasked address. Provide the address to userspace
>> accordingly.
>> 
>> Initialize real_address in various code-paths to be consistent with
>> address, even when it is not used, to be on the safe side.
>> 
>> Acked-by: Peter Xu <peterx@redhat.com>
>> Reviewed-by: David Hildenbrand <david@redhat.com>
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
>> Cc: Jan Kara <jack@suse.cz>
>> Signed-off-by: Nadav Amit <namit@vmware.com>
> 
> Hi, Andrew,
> 
> Just a heads-up that this version has not yet been updated in -mm I think,
> while the queued one is the old version.
> 
> IOW, uffd is currently broken on latest linux-next on hugetlb.

Thanks Peter for reminding Andrew.

Andrew, please acknowledge it would be queue for the next version and
I will submit a patch to the man pages.
 

Re: [PATCH v3] userfaultfd: provide unmasked address on page-fault

[Nadav Amit]

> On Mar 3, 2022, at 11:05 AM, Nadav Amit <namit@vmware.com> wrote:
> 
> 
> 
>> On Mar 3, 2022, at 12:03 AM, Peter Xu <peterx@redhat.com> wrote:
>> 
>> On Sat, Feb 26, 2022 at 02:26:55AM +0000, Nadav Amit wrote:
>>> From: Nadav Amit <namit@vmware.com>
>>> 
>>> Userfaultfd is supposed to provide the full address (i.e., unmasked) of
>>> the faulting access back to userspace. However, that is not the case for
>>> quite some time.
>>> 
>>> Even running "userfaultfd_demo" from the userfaultfd man page provides
>>> the wrong output (and contradicts the man page). Notice that
>>> "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
>>> and not the first read address (0x7fc5e30b300f).
>>> 
>>> 	Address returned by mmap() = 0x7fc5e30b3000
>>> 
>>> 	fault_handler_thread():
>>> 	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
>>> 	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
>>> 		(uffdio_copy.copy returned 4096)
>>> 	Read address 0x7fc5e30b300f in main(): A
>>> 	Read address 0x7fc5e30b340f in main(): A
>>> 	Read address 0x7fc5e30b380f in main(): A
>>> 	Read address 0x7fc5e30b3c0f in main(): A
>>> 
>>> The exact address is useful for various reasons and specifically for
>>> prefetching decisions. If it is known that the memory is populated by
>>> certain objects whose size is not page-aligned, then based on the
>>> faulting address, the uffd-monitor can decide whether to prefetch and
>>> prefault the adjacent page.
>>> 
>>> This bug has been for quite some time in the kernel: since commit
>>> 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
>>> vmf->virtual_address"), which dates back to 2016. A concern has been
>>> raised that existing userspace application might rely on the old/wrong
>>> behavior in which the address is masked. Therefore, it was suggested to
>>> provide the masked address unless the user explicitly asks for the exact
>>> address.
>>> 
>>> Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
>>> userfaultfd to provide the exact address. Add a new "real_address" field
>>> to vmf to hold the unmasked address. Provide the address to userspace
>>> accordingly.
>>> 
>>> Initialize real_address in various code-paths to be consistent with
>>> address, even when it is not used, to be on the safe side.
>>> 
>>> Acked-by: Peter Xu <peterx@redhat.com>
>>> Reviewed-by: David Hildenbrand <david@redhat.com>
>>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>>> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
>>> Cc: Jan Kara <jack@suse.cz>
>>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> 
>> Hi, Andrew,
>> 
>> Just a heads-up that this version has not yet been updated in -mm I think,
>> while the queued one is the old version.
>> 
>> IOW, uffd is currently broken on latest linux-next on hugetlb.
> 
> Thanks Peter for reminding Andrew.
> 
> Andrew, please acknowledge it would be queue for the next version and
> I will submit a patch to the man pages.

Peter (et. al),

I’ll send it in a more orderly fashion later, but let me know if I got
something completely wrong for the man page change:

[ Thanks as usual; sorry - limited experience changing man pages ]

-- >8 --

From: Nadav Amit <namit@vmware.com>
Date: Thu, 3 Mar 2022 19:44:37 +0000
Subject: [PATCH] ioctl_userfaultfd: add UFFD_FEATURE_EXACT_ADDRESS

Describe the new UFFD_FEATURE_EXACT_ADDRESS API feature.

Signed-off-by: Nadav Amit <namit@vmware.com>
---
 man2/ioctl_userfaultfd.2 | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index 504f61d4b..2d065504e 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/man2/ioctl_userfaultfd.2
@@ -214,6 +214,12 @@ memory accesses to the regions registered with userfaultfd.
 If this feature bit is set,
 .I uffd_msg.pagefault.feat.ptid
 will be set to the faulted thread ID for each page-fault message.
+.TP
+.BR UFFD_FEATURE_EXACT_ADDRESS " (since Linux 5.18)"
+If this feature bit is set,
+.I uffd_msg.pagefault.address
+will be set to the exact page-fault address that was reported by the hardware,
+and will not mask the offset within the page.
 .PP
 The returned
 .I ioctls
-- 
2.25.1

Re: [PATCH v3] userfaultfd: provide unmasked address on page-fault

[Andrew Morton]

On Thu, 3 Mar 2022 19:05:36 +0000 Nadav Amit <namit@vmware.com> wrote:

> 
> 
> >> Reviewed-by: David Hildenbrand <david@redhat.com>
> >> Cc: Andrea Arcangeli <aarcange@redhat.com>
> >> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> >> Cc: Jan Kara <jack@suse.cz>
> >> Signed-off-by: Nadav Amit <namit@vmware.com>
> > 
> > Hi, Andrew,
> > 
> > Just a heads-up that this version has not yet been updated in -mm I think,
> > while the queued one is the old version.
> > 
> > IOW, uffd is currently broken on latest linux-next on hugetlb.
> 
> Thanks Peter for reminding Andrew.
> 
> Andrew, please acknowledge it would be queue for the next version and
> I will submit a patch to the man pages.

Queued now, thanks.  Also a fix for Jan's comment typo.

Re: [PATCH v3] userfaultfd: provide unmasked address on page-fault

[Peter Xu]

On Thu, Mar 03, 2022 at 07:51:26PM +0000, Nadav Amit wrote:
> Peter (et. al),
> 
> I’ll send it in a more orderly fashion later, but let me know if I got
> something completely wrong for the man page change:
> 
> [ Thanks as usual; sorry - limited experience changing man pages ]
> 
> -- >8 --
> 
> From: Nadav Amit <namit@vmware.com>
> Date: Thu, 3 Mar 2022 19:44:37 +0000
> Subject: [PATCH] ioctl_userfaultfd: add UFFD_FEATURE_EXACT_ADDRESS
> 
> Describe the new UFFD_FEATURE_EXACT_ADDRESS API feature.
> 
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
>  man2/ioctl_userfaultfd.2 | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index 504f61d4b..2d065504e 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -214,6 +214,12 @@ memory accesses to the regions registered with userfaultfd.
>  If this feature bit is set,
>  .I uffd_msg.pagefault.feat.ptid
>  will be set to the faulted thread ID for each page-fault message.
> +.TP
> +.BR UFFD_FEATURE_EXACT_ADDRESS " (since Linux 5.18)"
> +If this feature bit is set,
> +.I uffd_msg.pagefault.address
> +will be set to the exact page-fault address that was reported by the hardware,
> +and will not mask the offset within the page.
>  .PP
>  The returned
>  .I ioctls
> -- 
> 2.25.1

Looks good to me, thanks!

Acked-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu

Re: [PATCH v3] userfaultfd: provide unmasked address on page-fault

[David Hildenbrand]

On 03.03.22 20:51, Nadav Amit wrote:
> 
> 
>> On Mar 3, 2022, at 11:05 AM, Nadav Amit <namit@vmware.com> wrote:
>>
>>
>>
>>> On Mar 3, 2022, at 12:03 AM, Peter Xu <peterx@redhat.com> wrote:
>>>
>>> On Sat, Feb 26, 2022 at 02:26:55AM +0000, Nadav Amit wrote:
>>>> From: Nadav Amit <namit@vmware.com>
>>>>
>>>> Userfaultfd is supposed to provide the full address (i.e., unmasked) of
>>>> the faulting access back to userspace. However, that is not the case for
>>>> quite some time.
>>>>
>>>> Even running "userfaultfd_demo" from the userfaultfd man page provides
>>>> the wrong output (and contradicts the man page). Notice that
>>>> "UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000)
>>>> and not the first read address (0x7fc5e30b300f).
>>>>
>>>> 	Address returned by mmap() = 0x7fc5e30b3000
>>>>
>>>> 	fault_handler_thread():
>>>> 	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
>>>> 	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
>>>> 		(uffdio_copy.copy returned 4096)
>>>> 	Read address 0x7fc5e30b300f in main(): A
>>>> 	Read address 0x7fc5e30b340f in main(): A
>>>> 	Read address 0x7fc5e30b380f in main(): A
>>>> 	Read address 0x7fc5e30b3c0f in main(): A
>>>>
>>>> The exact address is useful for various reasons and specifically for
>>>> prefetching decisions. If it is known that the memory is populated by
>>>> certain objects whose size is not page-aligned, then based on the
>>>> faulting address, the uffd-monitor can decide whether to prefetch and
>>>> prefault the adjacent page.
>>>>
>>>> This bug has been for quite some time in the kernel: since commit
>>>> 1a29d85eb0f1 ("mm: use vmf->address instead of of vmf->virtual_address")
>>>> vmf->virtual_address"), which dates back to 2016. A concern has been
>>>> raised that existing userspace application might rely on the old/wrong
>>>> behavior in which the address is masked. Therefore, it was suggested to
>>>> provide the masked address unless the user explicitly asks for the exact
>>>> address.
>>>>
>>>> Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
>>>> userfaultfd to provide the exact address. Add a new "real_address" field
>>>> to vmf to hold the unmasked address. Provide the address to userspace
>>>> accordingly.
>>>>
>>>> Initialize real_address in various code-paths to be consistent with
>>>> address, even when it is not used, to be on the safe side.
>>>>
>>>> Acked-by: Peter Xu <peterx@redhat.com>
>>>> Reviewed-by: David Hildenbrand <david@redhat.com>
>>>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>>>> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
>>>> Cc: Jan Kara <jack@suse.cz>
>>>> Signed-off-by: Nadav Amit <namit@vmware.com>
>>>
>>> Hi, Andrew,
>>>
>>> Just a heads-up that this version has not yet been updated in -mm I think,
>>> while the queued one is the old version.
>>>
>>> IOW, uffd is currently broken on latest linux-next on hugetlb.
>>
>> Thanks Peter for reminding Andrew.
>>
>> Andrew, please acknowledge it would be queue for the next version and
>> I will submit a patch to the man pages.
> 
> Peter (et. al),
> 
> I’ll send it in a more orderly fashion later, but let me know if I got
> something completely wrong for the man page change:
> 
> [ Thanks as usual; sorry - limited experience changing man pages ]
> 
> -- >8 --
> 
> From: Nadav Amit <namit@vmware.com>
> Date: Thu, 3 Mar 2022 19:44:37 +0000
> Subject: [PATCH] ioctl_userfaultfd: add UFFD_FEATURE_EXACT_ADDRESS
> 
> Describe the new UFFD_FEATURE_EXACT_ADDRESS API feature.
> 
> Signed-off-by: Nadav Amit <namit@vmware.com>
> ---
>  man2/ioctl_userfaultfd.2 | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index 504f61d4b..2d065504e 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -214,6 +214,12 @@ memory accesses to the regions registered with userfaultfd.
>  If this feature bit is set,
>  .I uffd_msg.pagefault.feat.ptid
>  will be set to the faulted thread ID for each page-fault message.
> +.TP
> +.BR UFFD_FEATURE_EXACT_ADDRESS " (since Linux 5.18)"
> +If this feature bit is set,
> +.I uffd_msg.pagefault.address
> +will be set to the exact page-fault address that was reported by the hardware,
> +and will not mask the offset within the page.
>  .PP
>  The returned
>  .I ioctls

Do we want to add a comment about early uffd code that did this as well?

"Note that old Linux versions might indicate the exact address as well,
even though the feature bit is not set."

-- 
Thanks,

David / dhildenb

Re: [PATCH v3] userfaultfd: provide unmasked address on page-fault

[Nadav Amit]

> On Mar 4, 2022, at 2:38 AM, David Hildenbrand <david@redhat.com> wrote:
> 
> Do we want to add a comment about early uffd code that did this as well?
> 
> "Note that old Linux versions might indicate the exact address as well,
> even though the feature bit is not set."

Thanks! I thought how to phrase something non-committing as you did and
gave up. I will go with yours.