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    Subject[PATCH 2/2] Perform check in iov_iter_fault_in_readable() by check_readable_bytes()
    Date
    This patch changes iov_iter_fault_in_readable() to use check_readable_bytes()
    instead of fault_in_pages_readable(). It allows iov_iter_fault_in_readable()
    callers to know how much data is accessible and proceed with it. It makes
    sys_write() more POSIX-friendly.

    Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>
    ---
    fs/fuse/file.c | 6 ++++--
    mm/filemap.c | 13 +++++++++----
    2 files changed, 13 insertions(+), 6 deletions(-)

    diff --git a/fs/fuse/file.c b/fs/fuse/file.c
    index 06f30e9..c74ef04 100644
    --- a/fs/fuse/file.c
    +++ b/fs/fuse/file.c
    @@ -782,13 +782,15 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
    pgoff_t index = pos >> PAGE_CACHE_SHIFT;
    size_t bytes = min_t(size_t, PAGE_CACHE_SIZE - offset,
    iov_iter_count(ii));
    -
    + long accessible;
    bytes = min_t(size_t, bytes, fc->max_write - count);

    again:
    err = -EFAULT;
    - if (iov_iter_fault_in_readable(ii, bytes))
    + accessible = iov_iter_fault_in_readable(ii, bytes);
    + if (accessible < 0)
    break;
    + bytes = accessible;

    err = -ENOMEM;
    page = grab_cache_page_write_begin(mapping, index, 0);
    diff --git a/mm/filemap.c b/mm/filemap.c
    index 379ff0b..ef598a1 100644
    --- a/mm/filemap.c
    +++ b/mm/filemap.c
    @@ -1947,8 +1947,9 @@ EXPORT_SYMBOL(iov_iter_advance);

    /*
    * Fault in the first iovec of the given iov_iter, to a maximum length
    - * of bytes. Returns 0 on success, or non-zero if the memory could not be
    - * accessed (ie. because it is an invalid address).
    + * of bytes. Returns count of accessible bytes on success, or -EFAULT
    + * if the memory could not be accessed (ie. because it is an invalid
    + * address).
    *
    * writev-intensive code may want this to prefault several iovecs -- that
    * would be possible (callers must not rely on the fact that _only_ the
    @@ -1958,7 +1959,7 @@ int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes)
    {
    char __user *buf = i->iov->iov_base + i->iov_offset;
    bytes = min(bytes, i->iov->iov_len - i->iov_offset);
    - return fault_in_pages_readable(buf, bytes);
    + return check_readable_bytes(buf, bytes);
    }
    EXPORT_SYMBOL(iov_iter_fault_in_readable);

    @@ -2215,6 +2216,7 @@ static ssize_t generic_perform_write(struct file *file,
    unsigned long offset; /* Offset into pagecache page */
    unsigned long bytes; /* Bytes to write to page */
    size_t copied; /* Bytes copied from user */
    + long accessible;
    void *fsdata;

    offset = (pos & (PAGE_CACHE_SIZE - 1));
    @@ -2234,10 +2236,13 @@ again:
    * to check that the address is actually valid, when atomic
    * usercopies are used, below.
    */
    - if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
    +
    + accessible = iov_iter_fault_in_readable(i, bytes);
    + if (accessible < 0) {
    status = -EFAULT;
    break;
    }
    + bytes = accessible;

    status = a_ops->write_begin(file, mapping, pos, bytes, flags,
    &page, &fsdata);
    --
    1.6.2.2


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