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Re: [PATCH 0/9] overlay: fix inconsistency of ro file after copy-up

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On Fri, Feb 17, 2017 at 05:09:29PM +0100, Miklos Szeredi wrote:
> A file is opened for read-only, opened read-write (resulting in a copy up)
> and modified.  The data read back from the the read-only fd will be stale
> in this case (the read-only file descriptor still refers to the lower,
> unmodified file).
> 
> This patchset fixes issues related to this corner case.  This is a
> requirement from various parties for accepting overlayfs as a "POSIX"
> filesystem.
> 
> When an operation (read, mmap, fsync) is done on an overlay fd opened
> read-only that is referring to a lower file, check if it has been copied up
> in the mean time.  If so, open the upper file and use that for the operation.
> 
> To make the performance impact minimal for non-overlay case, use a flag in
> file->f_mode to indicate that this is an overlay file.

This is one hell of a DoS vector - it's really easy to eat tons of struct
file that way.  Preparatory parts of that series make sense on their own,
but your "let's allocate a struct file, call ->open() and schedule that
struct file for closing upon the exit to userland on each kernel_read()"
is not.

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[PATCH 0/9] overlay: fix inconsistency of ro file after copy-up Miklos Szeredi <mszeredi@redhat.com> - 2017-02-17 17:20 +0100
  [PATCH 9/9] vfs: intercept fsync on overlay files Miklos Szeredi <mszeredi@redhat.com> - 2017-02-17 17:20 +0100
  [PATCH 8/9] vfs: use helper for calling f_op->fsync() Miklos Szeredi <mszeredi@redhat.com> - 2017-02-17 17:20 +0100
  [PATCH 4/9] vfs: intercept reads to overlay files Miklos Szeredi <mszeredi@redhat.com> - 2017-02-17 17:20 +0100
    Re: [PATCH 4/9] vfs: intercept reads to overlay files Al Viro <viro@ZenIV.linux.org.uk> - 2017-02-19 10:10 +0100
      Re: [PATCH 4/9] vfs: intercept reads to overlay files Miklos Szeredi <mszeredi@redhat.com> - 2017-02-19 10:30 +0100
    Re: [PATCH 4/9] vfs: intercept reads to overlay files "zhangyi (F)" <yi.zhang@huawei.com> - 2017-02-20 09:10 +0100
      Re: [PATCH 4/9] vfs: intercept reads to overlay files Miklos Szeredi <miklos@szeredi.hu> - 2017-02-20 10:00 +0100
  Re: [PATCH 0/9] overlay: fix inconsistency of ro file after copy-up Al Viro <viro@ZenIV.linux.org.uk> - 2017-02-19 10:20 +0100
    Re: [PATCH 0/9] overlay: fix inconsistency of ro file after copy-up Miklos Szeredi <miklos@szeredi.hu> - 2017-02-20 16:20 +0100

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