linux-kernel - Re: [PATCH] staging: exfat: add exfat filesystem code to staging

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Message-ID: <81682.1567082044@turing-police>
Date:   Thu, 29 Aug 2019 08:34:04 -0400
From:   "Valdis KlÄ“tnieks" <valdis.kletnieks@...edu>
To:     Pali RohÃ¡r <pali.rohar@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        devel@...verdev.osuosl.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sasha Levin <alexander.levin@...rosoft.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] staging: exfat: add exfat filesystem code to staging

On Thu, 29 Aug 2019 14:14:35 +0200, Pali Rohár said:
> On Wednesday 28 August 2019 18:08:17 Greg Kroah-Hartman wrote:
> > The full specification of the filesystem can be found at:
> >   https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification
>
> This is not truth. This specification is not "full". There are missing
> important details, like how is TexFAT implemented. 

Well..given that the spec says it's an extension used by Windows CE...

> 1.5 Windows CE and TexFAT

> TexFAT is an extension to exFAT that adds transaction-safe operational
> semantics on top of the base file system. TexFAT is used by Windows CE. TexFAT
> requires the use of the two FATs and allocation bitmaps for use in
> transactions. It also defines several additional structures including padding
> descriptors and security descriptors.

And these two pieces of info:

> 3.1.13.1 ActiveFat Field

> The ActiveFat field shall describe which FAT and Allocation Bitmap are active
> (and implementations shall use), as follows:

> 0, which means the First FAT and First Allocation Bitmap are active

> 1, which means the Second FAT and Second Allocation Bitmap are active and is
> possible only when the NumberOfFats field contains the value 2

> Implementations shall consider the inactive FAT and Allocation Bitmap as stale.
> Only TexFAT-aware implementations shall switch the active FAT and Allocation
> Bitmaps (see Section 7.1).

> 3.1.16 NumberOfFats Field
> The NumberOfFats field shall describe the number of FATs and Allocation Bitmaps
> the volume contains.

> The valid range of values for this field shall be:

> 1, which indicates the volume only contains the First FAT and First Allocation Bitmap

> 2, which indicates the volume contains the First FAT, Second FAT, First
> Allocation Bitmap, and Second Allocation Bitmap; this value is only valid for
> TexFAT volumes

I think we're OK if we just set ActiveFat to 0 and NumberOfFats to 1.

Unless somebody has actual evidence of a non-WindowsCE extfat that has
NumberOfFats == 2....

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