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Date: Fri, 8 Mar 2024 11:37:15 +0100
To: Jeffrey Altman <jaltman AT secure-endpoints DOT com>
Subject: Re: ls/stat on OneDrive causes download of files
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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Hi Jeffrey,

On Mar  6 13:55, Jeffrey Altman via Cygwin wrote:
> On 3/6/2024 12:19 PM, Corinna Vinschen via Cygwin wrote:
> > We can add an explicit call to
> > 
> >    RtlSetProcessPlaceholderCompatibilityMode (PHCM_EXPOSE_PLACEHOLDERS);
> > [...]
> Files and directories that are placeholders should have either the
> FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS or FILE_ATTRIBUTE_RECALL_ON_OPEN file
> attributes set. When these attributes are set, applications and mini filters
> are advised not to "read" or "open" the files or directories unless they
> absolutely need to because doing so will cause the placeholder to be
> replaced by an object containing the actual data which might take a long
> time to fetch,

Yesterday I stumbled over a certain NtCreateFile flag:

  FILE_OPEN_NO_RECALL (0x00400000)

    Instructs any filters that perform offline storage or virtualization
    to not recall the contents of the file as a result of this open.

MS-CIFS described it like this:

  FILE_OPEN_NO_RECALL
  0x00400000

    In a hierarchical storage management environment, this option
    requests that the file SHOULD NOT be recalled from tertiary storage
    such as tape. A file recall can take up to several minutes in a
    hierarchical storage management environment. The clients can specify
    this option to avoid such delays.

This sounds like we could simply add this flag to all NtOpenFile
used for path conversion or stat-like calls, without having to care
for any file attributes specificially.

Does that make sense?


Thanks,
Corinna

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