On 07/11/2018 02:45 AM, Christoph Hellwig wrote: > On Tue, Jul 10, 2018 at 05:53:18PM -0400, Tony Battersby wrote: >> At my job (https://www.cybernetics.com/), I use the write()/read() >> interface to the SCSI generic driver for access to tape drives and tape >> medium changers. For example, the write()/read() interface is useful >> for implementing RAID-like functionality for tape drives since a single >> thread can send commands to multiple tape drives at once and poll() for >> command completion. We have a lot of code invested in this interface, >> so it would be a huge pain for us if it were removed. But in our case, >> everything runs as root (as the firmware of an embedded storage >> appliance), so extra permission checks should be OK. > Do you just use read/write on /dev/sg or also on /dev/bsg? Because > I started a discussion to kill the read/write support for the latter > even before Linus brought it up here.. (and we have the same fix > pending for /dev/bsg) > The read/write interface on /dev/bsg is impossible to use safely because the list of completed commands is per-device (bd->done_list) rather than per-fd like it is with /dev/sg. So if program A and program B are both using the write/read interface on the same bsg device, then their command responses will get mixed up, and program A will read() some command results from program B and vice versa. So no, I don't use read/write on /dev/bsg. From a security standpoint, it should definitely be fixed or removed. Another issue with the read/write interface of /dev/bsg was this: [PATCH] [SCSI] bsg: fix unkillable I/O wait deadlock with scsi-mq https://marc.info/?l=linux-scsi&m=142367231311098&w=2 My similar patch to sg.c was accepted as commit 7568615c1054 ("sg: fix unkillable I/O wait deadlock with scsi-mq"), but my bsg patch was never applied. I do not know if the problem still exists in the current kernel or if some other change to scsi-mq has fixed it. I do have a forward-port of the patch to 4.16, but it no longer applies to 4.17. --- A while ago there was a commit that broke the read/write interface of /dev/sg (but not SG_IO), and some other people complained in the following bugzilla entry, indicating that there are other users also: https://bugzilla.kernel.org/show_bug.cgi?id=198081 The commit that broke it was: 109bade9c625 ("scsi: sg: use standard lists for sg_requests") The commit that fixed it was: 48ae8484e9fc ("scsi: sg: don't return bogus Sg_requests") (sg_get_rq_mark() is called only from sg_read(), so if the patch fixed the problem that people were complaining about in bugzilla, then that indicates that they are using the read/write interface also) See my explanation here: https://marc.info/?l=linux-scsi&m=152227354106242 (basically, the commit that broke it got backported to -stable, but the fix didn't, so various -stable kernels were broken for a while, and people complained) Tony Battersby Cybernetics
