On Wed, Jul 28, 2021 at 4:59 AM Xianting Tian
<xianting.tian@xxxxxxxxxxxxxxxxx> wrote:
>
> Arnd, thanks for your quick reply,
>
> As we know put_chars() of virtio-console is registered to hvc framework.
> I go throughed the code, actually there are totally three places that
> put_chars() is called in hvc driver, but only 1 has issue which is
> fixed by commit c4baad5029.
Ah, good. Knowing what the callers are definitely helps. ;-)
> So I think the scenario that the buf is from "ioremap(), kmap_atomic() ,
> fixmap, loadable module" doesn't exist for virtio-console.
> If there is something wrong about above description, please correct me,
> thanks.
The description is good then.
> Three places that put_chars() is called in hvc driver:
> 1， it is on stack buf, it is not ok for dma
> hvc_console_print():
> char c[N_OUTBUF] __ALIGNED__;
> cons_ops[index]->put_chars(vtermnos[index], c, i);
>
> 2， just one byte, no issue for dma
> static void hvc_poll_put_char(struct tty_driver *driver, int line,
> char ch)
> {
> struct tty_struct *tty = driver->ttys[0];
> struct hvc_struct *hp = tty->driver_data;
> int n;
>
> do {
> n = hp->ops->put_chars(hp->vtermno, &ch, 1);
> } while (n <= 0);
> }
This is actually the same as the first, taking the address of a
function argument forces it onto the stack.
> 3, hp->outbuf is allocated in hvc_alloc() via kzalloc(), no issue for dma
> static int hvc_push(struct hvc_struct *hp)
> {
> int n;
>
> n = hp->ops->put_chars(hp->vtermno, hp->outbuf, hp->n_outbuf);
> …
> }
ok.
I have a new question then: are there any other hvc backends that do
DMA, or is the virtio-console driver the only one? If there are any others,
I think this should better be fixed in the hvc framework, by changing it
to never pass stack data into the put_chars() function in the first place.
It may be possible to just use the 'hp->n_outbuf' buffer in all three cases.
Arnd
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