Re: [PATCH] [SCSI] sg: fix read() error reporting

[Douglas Gilbert]

On 15-02-11 01:13 PM, Tony Battersby wrote:
> On 02/11/2015 12:45 PM, Douglas Gilbert wrote:
> > On 15-02-11 11:32 AM, Tony Battersby wrote:
> > > Fix SCSI generic read() incorrectly returning success after detecting an
> > > error.
> > >
> > > Cc: <stable@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Tony Battersby <tonyb@xxxxxxxxxxxxxxx>
> > > ---
> > >
> > > For inclusion in kernel 3.20.
> > >
> > > --- linux-3.19.0/drivers/scsi/sg.c.orig	2015-02-08 21:54:22.000000000 -0500
> > > +++ linux-3.19.0/drivers/scsi/sg.c	2015-02-10 09:26:09.000000000 -0500
> > > @@ -546,7 +546,7 @@ static ssize_t
> > >    sg_new_read(Sg_fd * sfp, char __user *buf, size_t count, Sg_request * srp)
> > >    {
> > >    	sg_io_hdr_t *hp = &srp->header;
> > > -	int err = 0;
> > > +	int err = 0, err2;
> > >    	int len;
> > >
> > >    	if (count < SZ_SG_IO_HDR) {
> > > @@ -575,8 +575,8 @@ sg_new_read(Sg_fd * sfp, char __user *bu
> > >    		goto err_out;
> > >    	}
> > >    err_out:
> > > -	err = sg_finish_rem_req(srp);
> > > -	return (0 == err) ? count : err;
> > > +	err2 = sg_finish_rem_req(srp);
> > > +	return err ? : err2 ? : count;
> > Tony,
> > Your point is well made.
> >
> > I just don't like that last line, using a gcc extension that
> > hasn't even made it into C11 (or C++11). Wouldn't:
> >       return err ? err : (err2 ? err2 : count);
> >
> > be a bit better? I think the following snippet makes the intent
> > clear but would it generate any more code:
> >       if (err || err2)

An improvement on the above line would be:
      if (unlikely(err || err2))

> > 	return err ? err : err2;
> >       else
> >           return count;

Another approach is to return the good path prior to the
err_out label. This would require two calls to
sg_finish_rem_req().

However Tony's clean-up is sufficient.

> I checked before submitting, and the foo ? : bar construct is already
> used at least 455 times in the kernel, so it is nothing new.
>
> find linux-3.19 -type f -name "*.[ch]" | xargs grep '\? :' | wc -l
> 455
>
> But you are probably right; I think that with such small variable names
> your version is more readable due to the nesting.  Updated patch below.
>
> ---
>
> Fix SCSI generic read() incorrectly returning success after detecting an
> error.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Tony Battersby <tonyb@xxxxxxxxxxxxxxx>

Acked-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>

> ---
>
> --- linux-3.19.0/drivers/scsi/sg.c.orig	2015-02-08 21:54:22.000000000 -0500
> +++ linux-3.19.0/drivers/scsi/sg.c	2015-02-11 13:02:56.000000000 -0500
> @@ -546,7 +546,7 @@ static ssize_t
>   sg_new_read(Sg_fd * sfp, char __user *buf, size_t count, Sg_request * srp)
>   {
>   	sg_io_hdr_t *hp = &srp->header;
> -	int err = 0;
> +	int err = 0, err2;
>   	int len;
>
>   	if (count < SZ_SG_IO_HDR) {
> @@ -575,8 +575,8 @@ sg_new_read(Sg_fd * sfp, char __user *bu
>   		goto err_out;
>   	}
>   err_out:
> -	err = sg_finish_rem_req(srp);
> -	return (0 == err) ? count : err;
> +	err2 = sg_finish_rem_req(srp);
> +	return err ? err : (err2 ? err2 : count);
>   }
>
>   static ssize_t
>
> --

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