Fwd: Re: [PATCH v2 1/2] [PATCH] cdrom: extra upper bound check for logical block address.

[Felix Busch <felixbusch470@gmail.com>]

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Hi,
sorry for this confusion.
I somehow had in mind while reading this spec that when the capacity of the
disk
is a multiple of the block size, then the sector size would also be a
multiple
of the block size, which is of course not true. Then, of course this needs
to
have a bit more of searching.

Regards
Felix

Am Do., 26. Feb. 2026 um 21:28 Uhr schrieb Phillip Potter <
phil@philpotter.co.uk>:

> On Tue, Feb 24, 2026 at 05:16:12PM +0100, Felix Busch wrote:
> > Hi Phillip,
> >
> > Thank you for your feedback. I’ve updated everything based on your
> suggestions and hope
> > I’ve covered them all.
> >
> > It has been built and run and tested in the following:
> >
> > 1) This has been tested on a real CD-ROM with 700MB capacity, containing
> 2097151
> > sectors in my case. Since 2097151 is not divisible by a blocksize of
> 2048, the if condition is not satisfied.
> >
> > 2) When using a smaller emulated CD-ROM with QEMU with the same
> blocksize,
> > the if condition is triggered when having a too big logical block
> addresses. For example, this occurs when minutes,
> > seconds, and frames are set to UINT8_MAX via an ioctl call.
> >
> > According to this SCSI-2 spec, chapter 14 on this web page (date
> 23.2.2026):
> > https://www.staff.uni-mainz.de/tacke/scsi/SCSI2-introduction.html
> >
> > They are stating:
> > "
> > Logical addressing of CD-ROM information may use any logical block
> length. When the specified logical block
> > length is an exact divisor or integral multiple of the selected number
> of bytes per CD-ROM sector,
> > the device shall map (one to one) the bytes transferred from CD-ROM
> sectors to the bytes of logical blocks.
> > For instance, if 2 048 bytes are transferred from each CD-ROM sector
> (specified by the CD-ROM density code value),
> > and the logical block length is 512 bytes, then each CD-ROM sector shall
> map to exactly four logical blocks.
> > This International Standard does not define the mapping of logical block
> lengths which do not evenly divide
> > or are not exact multiples of the selected number of bytes per CD-ROM
> sector.
> > "
> >
> > So, I do refer to this SCSI-2 spec.
> > Using this information it's possible to use logical_blocks =
> cd_nr_sectors / blocksize to get the number of blocks that are be able to
> read due to this mapping.
> > For now, I decided using this special checking only, due to the given
> statement in the SCSI-2 spec.
> >
> > If lba > blocksize * (logical_blocks - 1), then it should not be
> possible to continue reading a full block, because then, as I understand,
> > it would "overflow" the region it can read from the disk.
> >
>
> I've seen your V3 - please make sure the relevant mailing list is copied
> in when posting patches, as development is ideally done in the open. As
> for that and for this version, I'm sorry but I'm still not convinced of
> the approach unless I'm missing something.
>
> You are taking units of two different things and dividing one by the
> other. Dividing the number of sectors (2,097,151) by the detected
> blocksize (2048 bytes) is not meaningful. The statement in the SCSI-2
> spec simply refers to the difference between CD-ROM sector size in bytes
> and the logical block size in bytes.
>
> Here, you are dividing one thing (number of sectors) by another (block
> size). Do you see what I mean?
>
> Finally, please make sure cover letters of a patch series are labels
> PATCH 0.
>
> Regards,
> Phil
>

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