mm/userfaultfd: provide unmasked address on page-fault
Subsystems (4)
PATCH 1/1 mm/userfaultfd: provide unmasked address on page-fault
From: Nadav Amit <[email protected]>
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.
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
Add a new "real_address" field to vmf to hold the unmasked address. It is possible to keep the unmasked address in the existing address field (and mask whenever necessary) instead, but this is likely to cause backporting problems of this patch.
--- fs/userfaultfd.c | 2 +- include/linux/mm.h | 3 ++- mm/memory.c | 1 + 3 files changed, 4 insertions(+), 2 deletions(-)
fs/userfaultfd.c +1 -1
@@ -481,7 +481,7 @@ 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;
include/linux/mm.h +2 -1
@@ -523,7 +523,8 @@ 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' */
mm/memory.c +1 -0
@@ -4594,6 +4594,7 @@ 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),
Signature · 1 line
2.25.1
Quoted 21 lines · Nadav Amit
From: Nadav Amit <[email protected]>
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_PAGEFAULTevent" shows the masked address.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
Quoted 4 lines
Add a new "
real_address" field to vmf to hold the unmasked address. It is possible to keep the unmasked address in the existing address field (and mask whenever necessary) instead, but this is likely to cause backporting problems of this patch.
Can we be sure that no existing users will rely on this behavior that has been the case since end of 2016 IIRC, one year after UFFD was upstreamed? I do wonder what the official ABI nowadays is, because man pages aren't necessarily the source of truth.
I checked QEMU (postcopy live migration), and I think it should be fine with this change.
If we don't want to change the current ABI behavior, we could add a new feature flag to change behavior.
@Peter, what are your thoughts?
Signature · 3 lines
Thanks, David / dhildenb
Quoted 26 lines · David Hildenbrand
Quoted 16 lines · Nadav Amit
From: Nadav Amit <[email protected]>
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_PAGEFAULTevent" shows the masked address.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(): AQuoted 4 lines
Add a new "
real_address" field to vmf to hold the unmasked address. It is possible to keep the unmasked address in the existing address field (and mask whenever necessary) instead, but this is likely to cause backporting problems of this patch.
Quoted 1 line
Can we be sure that no existing users will rely on this behavior that has been the case since end of 2016 IIRC, one year after UFFD was upstreamed?
Let me to blow off your mind: how do you be sure that the current behavior does not make applications to misbehave? It might cause performance issues as it did for me or hidden correctness issues.
Quoted 1 line
I do wonder what the official ABI nowadays is, because man pages aren't necessarily the source of truth.
Documentation/admin-guide/mm/userfaultfd.rst says: "You get the address of the access that triggered the missing page event”.
So it is a bug.
Quoted 26 lines · Nadav Amit
Quoted 26 lines · David Hildenbrand
Quoted 20 lines · Nadav Amit
From: Nadav Amit <[email protected]>
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_PAGEFAULTevent" shows the masked address.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(): AAdd a new "
real_address" field to vmf to hold the unmasked address. It is possible to keep the unmasked address in the existing address field (and mask whenever necessary) instead, but this is likely to cause backporting problems of this patch.
Quoted 4 lines
Quoted 1 line
Can we be sure that no existing users will rely on this behavior that has been the case since end of 2016 IIRC, one year after UFFD was upstreamed?
Let me to blow off your mind: how do you be sure that the current behavior does not make applications to misbehave? It might cause performance issues as it did for me or hidden correctness issues.
Fair point, but now we can speculate what's more likely:
Having an app rely on >4 year old kernel behavior just after the feature was released or having and app rely on kernel behavior that was the case for the last 4 years?
<offtopic>
Someone once told me about the unwritten way to remove things from the kernel. 1) Silently break it upstream 2) Wait 2 kernel releases 3) Propose removal of the feature because it's broken and nobody complained. <\offtopic>
You might ask "why does David even care?", here is why:
For the records, I *do* have a prototype from last year that breaks with this new behavior as far as I can tell: using uffd in the context of virtio-balloon in QEMU. I just pushed the latest state to a !private github tree:
https://github.com/davidhildenbrand/qemu/tree/virtio-balloon-uffd
In that code, I made sure that I'm only dealing with 4k pages (because that's the only thing virtio-balloon really can deal with), and during the debugging I figured that the kernel always returns 4k aligned page fault addresses, so I didn't care about masking. I'll reuse the unmodified fault address for UFFDIO_ZEROPAGE()/UFFDIO_COPY()/... which should then fail because:
"
EINVAL The start or the len field of the ufdio_range structure
was not a multiple of the system page size; or len was zero; or the specified range was otherwise invalid.
"
If I'm too lazy to read all documentation, I'm quite sure that there are other people that don't. I don't care to much if this patch breaks that prototype, it's just a prototype after all, but I am concerned that we might break other users in a similar way.
Quoted 6 lines
Quoted 1 line
I do wonder what the official ABI nowadays is, because man pages aren't necessarily the source of truth.
Documentation/admin-guide/mm/userfaultfd.rstsays: "You get the address of the access that triggered the missing page event”.So it is a bug.
The least thing I would expect in the patch description is a better motivation ("who cares and why" -- I know you have a better motivation that making the doc correct :) ) and a discussion on the chances of this actually breaking other apps (see my example).
I'd sleep better if we'd glue the changed behavior to a new feature flag, but that's just my 2 cents.
Signature · 3 lines
Thanks, David / dhildenb
Quoted 27 lines · David Hildenbrand
Quoted 21 lines · Nadav Amit
From: Nadav Amit <[email protected]>
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_PAGEFAULTevent" shows the masked address.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(): AQuoted 4 lines
Add a new "
real_address" field to vmf to hold the unmasked address. It is possible to keep the unmasked address in the existing address field (and mask whenever necessary) instead, but this is likely to cause backporting problems of this patch.
Quoted 7 lines
Can we be sure that no existing users will rely on this behavior that has been the case since end of 2016 IIRC, one year after UFFD was upstreamed? I do wonder what the official ABI nowadays is, because man pages aren't necessarily the source of truth.
I checked QEMU (postcopy live migration), and I think it should be fine with this change.
CRIU is Ok with this change, we anyway mask the address.
Signature · 2 lines
Sincerely yours, Mike.