Re: [PATCH for-5.10] spi: gpio: Don't leak SPI master in probe error path

[Lukas Wunner <lukas@wunner.de>]

On Mon, Nov 16, 2020 at 11:23:43AM -0800, Andrey Smirnov wrote:
> On Mon, Nov 16, 2020 at 12:44 AM Lukas Wunner <lukas@wunner.de> wrote:
> > If the call to devm_spi_register_master() fails on probe of the GPIO SPI
> > driver, the spi_master struct is erroneously not freed:
> >
> > After allocating the spi_master, its reference count is 1.  The driver
> > unconditionally decrements the reference count on unbind using a devm
> > action.  Before calling devm_spi_register_master(), the driver
> > unconditionally increments the reference count because on success,
> > that function will decrement the reference count on unbind.  However on
> > failure, devm_spi_register_master() does *not* decrement the reference
> > count, so the spi_master is leaked.
> 
> Not sure I fully understand this. On failure
> devm_spi_register_master() will return a negative error code which
> should result in probe failure and release of devres resource, right?

Yes, but that just decrements the refcount from 2 to 1:

    /* refcount initialized to 1 */
    master = spi_alloc_master(dev, sizeof(*spi_gpio));

    ...

    /* refcount incremented to 2 */
    return devm_spi_register_master(&pdev->dev, spi_master_get(master));

    ...

    /* on failure of devm_spi_register_master(), refcount decremented to 1
       by devres action */
    spi_gpio_put()


> > The issue was introduced by commits 8b797490b4db ("spi: gpio: Make sure
> > spi_master_put() is called in every error path") and 79567c1a321e ("spi:
> > gpio: Use devm_spi_register_master()"), which sought to plug leaks
> > introduced by 9b00bc7b901f ("spi: spi-gpio: Rewrite to use GPIO
> > descriptors") but missed this remaining leak.
> 
> That extra spi_master_get() that might be problematic was present in
> the code before 8b797490b4db ("spi: gpio: Make sure spi_master_put()
> is called in every error path") and I think was first introduced in
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers?h=v5.9-rc4&id=702a4879ec337463f858c8ab467482cce260bf18
> 
> Or am I missing something?

The extra spi_master_get() was introduced by 79567c1a321e.
I don't see it in spi-gpio.c before that commit.

Its quite possible that I missed something myself, nobody's perfect.
But just from code inspection it seems wrong the way it is right now.

Shout if I failed to explain it properly and I'll try again. :)

Thanks,

Lukas