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Date: Fri, 8 Mar 2024 13:52:20 +0100
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Subject: Re: ls/stat on OneDrive causes download of files
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From: Thomas Wolff via Cygwin <cygwin@cygwin.com>
Reply-To: Thomas Wolff <towo@towo.net>
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Sender: "Cygwin" <cygwin-bounces+archive-cygwin=delorie.com@cygwin.com>


Am 08.03.2024 um 11:37 schrieb Corinna Vinschen via Cygwin:
> 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?
Sounds good, without even studying the other details...
I speculate some more handling would still be needed to avoid executable
detection via magic tags.

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