Hi James, On Tue, Nov 07, 2017 at 07:04:23PM -0800, James Bottomley wrote: > On Wed, 2017-11-08 at 09:15 +0800, Ming Lei wrote: > > On Wed, Nov 08, 2017 at 01:06:44AM +0000, Bart Van Assche wrote: > > > > > > On Wed, 2017-11-08 at 08:59 +0800, Ming Lei wrote: > > > > > > > > On Tue, Nov 07, 2017 at 04:13:48PM +0000, Bart Van Assche wrote: > > > > > > > > > > On Tue, 2017-11-07 at 23:21 +0800, Ming Lei wrote: > > > > > > > > > > > > diff --git a/drivers/scsi/scsi_debugfs.c > > > > > > b/drivers/scsi/scsi_debugfs.c > > > > > > index 5e9755008aed..7a50878446b4 100644 > > > > > > --- a/drivers/scsi/scsi_debugfs.c > > > > > > +++ b/drivers/scsi/scsi_debugfs.c > > > > > > @@ -9,7 +9,10 @@ void scsi_show_rq(struct seq_file *m, struct > > > > > > request *rq) > > > > > > int msecs = jiffies_to_msecs(jiffies - cmd- > > > > > > >jiffies_at_alloc); > > > > > > char buf[80]; > > > > > > > > > > > > - __scsi_format_command(buf, sizeof(buf), cmd->cmnd, > > > > > > cmd->cmd_len); > > > > > > + if (cmd->cmnd == scsi_req(rq)->cmd) > > > > > > + __scsi_format_command(buf, sizeof(buf), cmd- > > > > > > >cmnd, cmd->cmd_len); > > > > > > + else > > > > > > + strcpy(buf, "unknown"); > > > > > > seq_printf(m, ", .cmd=%s, .retries=%d, allocated > > > > > > %d.%03d s ago", buf, > > > > > > cmd->retries, msecs / 1000, msecs % > > > > > > 1000); > > > > > > } > > > > > > > > > > This change introduces a new bug, namely that "unknown" will > > > > > appear in the debugfs output if (cmd->cmnd != scsi_req(rq)- > > > > > >cmd). Have you considered to use > > > > > > > > Because there isn't reliable way to get the command safely, and I > > > > don't think it is a new bug. > > > > > > > > > > > > > > the test (cmd->cmnd != NULL) instead? > > > > > > > > No, that is worse, because you may cause use-after-free if you > > > > read a freed buffer. > > > > > > It seems like your operating mode is still to contradict all > > > feedback you get instead of trying to address the feedback you get? > > > > > > Anyway, this is a debugging facility so I'm not convinced that > > > avoiding a (very sporadic) use-after-free in this code is better > > > than omitting very useful output. > > > > OK, if no one objects the use-after-free, because this way may > > trigger warning from some utility, such as KASAN. > > Good grief, this list is supposed to be for technical discussions not > kindergarten playground squabbles; could you both please act your age? In my reply, I mentioned I don't object to check NULL any more, and only provide one extra input about possible KASAN's complaint, which may annoy people. > > The patch as proposed would lose us all information about PI commands, > hence the objection. For the concern about use after free on the NULL > check, what about modifying sd_uninit_command() to NULL SCpnt->cmnd > before it calls mempool_free()? That will likely eliminate the race > window for printing the command. That may decrease the window, but can't eliminate it because of write/read can be reordered. But looks better, will cook a patch soon, together with using READ_ONCE() and WRITE_ONCE(). Thanks, Ming
