linux-kernel - Re: [PATCH] scsi: sg: fix v3 compat read/write interface

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Date:   Wed, 4 Dec 2019 10:32:56 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     James Bottomley <James.Bottomley@...senpartnership.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Doug Gilbert <dgilbert@...erlog.com>
Subject: Re: [PATCH] scsi: sg: fix v3 compat read/write interface

On Wed, Dec 4, 2019 at 6:08 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> To address both of these, move the definition of compat_sg_io_hdr
> into a scsi/sg.h to make it visible to sg.c and rewrite the logic
> for reading req_pack_id as well as the size check to a simpler
> version that gets the expected results.

I think the patch is a good thing, except for this part:

> @@ -575,6 +561,14 @@ sg_new_read(Sg_fd * sfp, char __user *buf, size_t count, Sg_request * srp)
>         int err = 0, err2;
>         int len;
>
> +#ifdef CONFIG_COMPAT
> +       if (in_compat_syscall()) {
> +               if (count < sizeof(struct compat_sg_io_hdr)) {
> +                       err = -EINVAL;
> +                       goto err_out;
> +               }
> +       } else
> +#endif
>         if (count < SZ_SG_IO_HDR) {
>                 err = -EINVAL;
>                 goto err_out;

Yes, yes, I know we do things like that in some other places too, but
I really detest this kind of ifdeffery.

That

         } else
  #endif
         if (count < SZ_SG_IO_HDR) {

is just evil. Please don't add things like this where the #ifdef
section has subtle semantic continuations outside of it. If somebody
adds a statement in between there, it now acts completely wrong.

I think you can remove the #ifdef entirely. If CONFIG_COMPAT isn't
set, I think in_compat_syscall() just turns to 0, and the code gets
optimized away.

Hmm?

               Linus