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Message-ID: <CAHc6FU7YLUivXXa00dX1=DC1XaYLnh_j3QDvYcRoAji14WAyAQ@mail.gmail.com>
Date: Wed, 4 Aug 2021 17:24:55 +0200
From: Andreas Gruenbacher <agruenba@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Christoph Hellwig <hch@...radead.org>,
"Darrick J. Wong" <djwong@...nel.org>
Cc: Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
cluster-devel <cluster-devel@...hat.com>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>, ocfs2-devel@....oracle.com
Subject: Re: [PATCH v5 05/12] iov_iter: Introduce fault_in_iov_iter_writeable
On Tue, Aug 3, 2021 at 9:18 PM Andreas Gruenbacher <agruenba@...hat.com> wrote:
> Introduce a new fault_in_iov_iter_writeable helper for safely faulting in an
> iterator for writing. Uses get_user_pages() to fault in the pages without
> actually writing to them, which would be destructive.
>
> We'll use fault_in_iov_iter_writeable in gfs2 once we've determined that the
> iterator passed to .read_iter isn't in memory.
>
> Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
> ---
> include/linux/pagemap.h | 1 +
> include/linux/uio.h | 1 +
> lib/iov_iter.c | 41 +++++++++++++++++++++++++++
> mm/gup.c | 61 +++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 104 insertions(+)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 7c9edc9694d9..a629807edb8c 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -737,6 +737,7 @@ extern void add_page_wait_queue(struct page *page, wait_queue_entry_t *waiter);
> * Fault in userspace address range.
> */
> size_t fault_in_writeable(char __user *uaddr, size_t size);
> +size_t fault_in_safe_writeable(const char __user *uaddr, size_t size);
> size_t fault_in_readable(const char __user *uaddr, size_t size);
>
> int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index 12d30246c2e9..ffa431aeb067 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
> @@ -120,6 +120,7 @@ size_t copy_page_from_iter_atomic(struct page *page, unsigned offset,
> void iov_iter_advance(struct iov_iter *i, size_t bytes);
> void iov_iter_revert(struct iov_iter *i, size_t bytes);
> size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t bytes);
> +size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t bytes);
> size_t iov_iter_single_seg_count(const struct iov_iter *i);
> size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
> struct iov_iter *i);
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index c0fa1618561c..4ffc76801eaa 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -471,6 +471,47 @@ size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t size)
> }
> EXPORT_SYMBOL(fault_in_iov_iter_readable);
>
> +/*
> + * fault_in_iov_iter_writeable - fault in iov iterator for writing
> + * @i: iterator
> + * @size: maximum length
> + *
> + * Faults in the iterator using get_user_pages(), i.e., without triggering
> + * hardware page faults. This is primarily useful when we know that some or
> + * all of the pages in @i aren't in memory.
> + *
> + * Returns the number of bytes faulted in, or 0 if no bytes could be faulted in
> + * (i.e., because the address is invalid).
> + *
> + * Always returns the number of avaliable bytes for non-user space iterators.
> + */
> +size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t size)
> +{
> + if (size > i->count)
> + size = i->count;
> +
> + if (iter_is_iovec(i)) {
> + const struct iovec *p;
> + size_t bytes = size;
> + size_t skip;
> +
> + for (p = i->iov, skip = i->iov_offset; bytes; p++, skip = 0) {
> + size_t len = min(bytes, p->iov_len - skip);
> + size_t ret;
> +
> + if (unlikely(!len))
> + continue;
> + ret = fault_in_safe_writeable(p->iov_base + skip, len);
> + bytes -= ret;
> + if (ret != len)
> + break;
> + }
> + return size - bytes;
> + }
> + return size;
> +}
> +EXPORT_SYMBOL(fault_in_iov_iter_writeable);
> +
> void iov_iter_init(struct iov_iter *i, unsigned int direction,
> const struct iovec *iov, unsigned long nr_segs,
> size_t count)
> diff --git a/mm/gup.c b/mm/gup.c
> index d04984d5d93c..7218e27c2481 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1694,6 +1694,67 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)
> }
> EXPORT_SYMBOL(fault_in_writeable);
>
> +/**
> + * fault_in_safe_writeable - fault in an address range for writing
> + * @uaddr: start of address range
> + * @size: length of address range
> + *
> + * Faults in an address range using get_user_pages, i.e., without triggering
> + * hardware page faults. This is primarily useful when we know that some or
> + * all of the pages in the address range aren't in memory.
> + *
> + * Other than fault_in_writeable(), this function is non-destructive.
> + *
> + * Note that we don't pin or otherwise hold the pages referenced that we fault
> + * in. There's no guarantee that they'll stay in memory for any duration of
> + * time.
> + *
> + * Returns the number of bytes faulted in from @uaddr.
> + */
> +size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
> +{
> + unsigned long start = (unsigned long)uaddr;
> + unsigned long end, nstart, nend;
> + struct mm_struct *mm = current->mm;
> + struct vm_area_struct *vma = NULL;
> + int locked = 0;
> +
> + /* FIXME: Protect against overflow! */
> +
> + end = PAGE_ALIGN(start + size);
> + for (nstart = start & PAGE_MASK; nstart < end; nstart = nend) {
> + unsigned long nr_pages;
> + long ret;
> +
> + if (!locked) {
> + locked = 1;
> + mmap_read_lock(mm);
> + vma = find_vma(mm, nstart);
> + } else if (nstart >= vma->vm_end)
> + vma = vma->vm_next;
> + if (!vma || vma->vm_start >= end)
> + break;
> + nend = min(end, vma->vm_end);
> + if (vma->vm_flags & (VM_IO | VM_PFNMAP))
> + continue;
Shouldn't we disallow read()ing into those kinds of vmas? If we skip
over them here and then the actual write results in -EFAULT, we'll end
up in a loop.
> + if (nstart < vma->vm_start)
> + nstart = vma->vm_start;
Likewise, shouldn't we fail for memory ranges not covered by a vma?
> + nr_pages = (nend - nstart) / PAGE_SIZE;
> + ret = __get_user_pages_locked(mm, nstart, nr_pages,
> + NULL, NULL, &locked,
> + FOLL_TOUCH | FOLL_WRITE);
> + if (ret <= 0)
> + break;
> + nend = nstart + ret * PAGE_SIZE;
> + }
> + if (locked)
> + mmap_read_unlock(mm);
> + if (nstart > start)
> + return min(nstart - start, size);
> + return 0;
> +}
> +EXPORT_SYMBOL(fault_in_safe_writeable);
> +
> size_t fault_in_readable(const char __user *uaddr, size_t size)
> {
> const char __user *start = uaddr, *end;
> --
> 2.26.3
>
Thanks,
Andreas
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