New: SCSI_IOCTL_GET_IDLUN only returns 8 bits for the SCSI Target value

https://bugzilla.kernel.org/show_bug.cgi?id=88591

            Bug ID: 88591
           Summary: SCSI_IOCTL_GET_IDLUN only returns 8 bits for the SCSI
                    Target value
           Product: SCSI Drivers
           Version: 2.5
    Kernel Version: 3.14.22
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Other
          Assignee: [email protected]
          Reporter: [email protected]
        Regression: No

However, very large configurations can have target ids that exceed 255. Or, in the process of devices dropping offline and being rediscovered, they can be assigned target ids greater than 255.

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If that ioctl has not been deprecated, it should be. Fixing it would likely break existing apps so I doubt that will happen. Introducing new ioctls is discouraged.

Also the 8 bit LUN "feature" with this ioctl was documented over 10 years ago, about the time that a new ioctl was proposed to address this issue.

The answer is to use sysfs. Now you would think sysfs would have a pseudo file called "lun" in some obvious place that would contain the 64 bit LUN value in binary, hex or decimal. Even better would be a string of eight bytes to match T10's LUN representation.
No, that would be too simple.

One way to find a file descriptor's LUN is to use stat() to
get its major and minor device numbers. For example if you had
opened /dev/sde that would usually be 8,64 . Then go searching in sysfs for a "dev" file that contains "8,64" ***, then look at
the real path of the match (i.e. without any symlinks).
That real path might be:
/sys/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:4/block/sde/dev

Back up that real path until you find /<h>:<c>:<t>:<l>/ and there is your LUN: <l>
That would be "4" for the path show above.

As you may have noticed sysfs is a pretty bad mechanism for answering this type of question, compared to an ioctl.

Doug Gilbert

*** the details of the search differ depending on whether the
driver is bsg, sg, sd, st, ses ....

BTW Thanks to udev, just because a disk is known as /dev/sde
in the user space, doesn't mean that the kernel (and sysfs) will know it as "sde", it could be "sdg". That is the reason for searching with major,minor numbers rather than its name.

https://bugzilla.kernel.org/show_bug.cgi?id=88591

--- Comment #1 from d gilbert <[email protected]> ---

If that ioctl has not been deprecated, it should be. Fixing it would likely break existing apps so I doubt that will happen. Introducing new ioctls is discouraged.

Also the 8 bit LUN "feature" with this ioctl was documented over 10 years ago, about the time that a new ioctl was proposed to address this issue.

The answer is to use sysfs. Now you would think sysfs would have a pseudo file called "lun" in some obvious place that would contain the 64 bit LUN value in binary, hex or decimal. Even better would be a string of eight bytes to match T10's LUN representation.
No, that would be too simple.

One way to find a file descriptor's LUN is to use stat() to get its major and minor device numbers. For example if you had opened /dev/sde that would usually be 8,64 . Then go searching in sysfs for a "dev" file that contains "8,64" ***, then look at the real path of the match (i.e. without any symlinks). That real path might be:

/sys/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:4/block/sde/dev

Back up that real path until you find /<h>:<c>:<t>:<l>/ and there is your LUN: <l>
That would be "4" for the path show above.

As you may have noticed sysfs is a pretty bad mechanism for answering this type of question, compared to an ioctl.

Doug Gilbert

*** the details of the search differ depending on whether the
driver is bsg, sg, sd, st, ses ....

BTW Thanks to udev, just because a disk is known as /dev/sde
in the user space, doesn't mean that the kernel (and sysfs) will know it as "sde", it could be "sdg". That is the reason for searching with major,minor numbers rather than its name.

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https://bugzilla.kernel.org/show_bug.cgi?id=88591

Lei, Ni <[email protected]> changed:

What |Removed |Added
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CC| |[email protected]

--- Comment #2 from Lei, Ni <[email protected]> --- (In reply to d gilbert from comment #1)

Since sysfs is not a good mechanism, why should this ioctl command be deprecated ?

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https://bugzilla.kernel.org/show_bug.cgi?id=88591

Alan <[email protected]> changed:

What |Removed |Added
----------------------------------------------------------------------------
CC| |[email protected]

--- Comment #3 from Alan <[email protected]> ---
IMHO the ioctl ought to be sorted - it's otherwise hard to fix up existing code, or to handle platforms that don't use sysfs crap but where you can get regular pulls/rescans of devices (eg some embedded)

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