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Date:   Mon, 28 Feb 2022 13:56:50 +0200
From:   Abel Vesa <abel.vesa@nxp.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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Subject: Re: [PATCH v3 01/11] driver: platform: Add helper for safer setting
 of driver_override
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References: <20220227135214.145599-1-krzysztof.kozlowski@canonical.com>
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On 22-02-28 12:30:45, Krzysztof Kozlowski wrote:
> On 28/02/2022 11:51, Abel Vesa wrote:
> > On 22-02-27 14:52:04, Krzysztof Kozlowski wrote:
> >> Several core drivers and buses expect that driver_override is a
> >> dynamically allocated memory thus later they can kfree() it.
> >>
> >> However such assumption is not documented, there were in the past and
> >> there are already users setting it to a string literal. This leads to
> >> kfree() of static memory during device release (e.g. in error paths or
> >> during unbind):
> >>
> >>     kernel BUG at ../mm/slub.c:3960!
> >>     Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
> >>     ...
> >>     (kfree) from [<c058da50>] (platform_device_release+0x88/0xb4)
> >>     (platform_device_release) from [<c0585be0>] (device_release+0x2c/0x90)
> >>     (device_release) from [<c0a69050>] (kobject_put+0xec/0x20c)
> >>     (kobject_put) from [<c0f2f120>] (exynos5_clk_probe+0x154/0x18c)
> >>     (exynos5_clk_probe) from [<c058de70>] (platform_drv_probe+0x6c/0xa4)
> >>     (platform_drv_probe) from [<c058b7ac>] (really_probe+0x280/0x414)
> >>     (really_probe) from [<c058baf4>] (driver_probe_device+0x78/0x1c4)
> >>     (driver_probe_device) from [<c0589854>] (bus_for_each_drv+0x74/0xb8)
> >>     (bus_for_each_drv) from [<c058b48c>] (__device_attach+0xd4/0x16c)
> >>     (__device_attach) from [<c058a638>] (bus_probe_device+0x88/0x90)
> >>     (bus_probe_device) from [<c05871fc>] (device_add+0x3dc/0x62c)
> >>     (device_add) from [<c075ff10>] (of_platform_device_create_pdata+0x94/0xbc)
> >>     (of_platform_device_create_pdata) from [<c07600ec>] (of_platform_bus_create+0x1a8/0x4fc)
> >>     (of_platform_bus_create) from [<c0760150>] (of_platform_bus_create+0x20c/0x4fc)
> >>     (of_platform_bus_create) from [<c07605f0>] (of_platform_populate+0x84/0x118)
> >>     (of_platform_populate) from [<c0f3c964>] (of_platform_default_populate_init+0xa0/0xb8)
> >>     (of_platform_default_populate_init) from [<c01031f8>] (do_one_initcall+0x8c/0x404)
> >>     (do_one_initcall) from [<c0f012c0>] (kernel_init_freeable+0x3d0/0x4d8)
> >>     (kernel_init_freeable) from [<c0a7def0>] (kernel_init+0x8/0x114)
> >>     (kernel_init) from [<c01010b4>] (ret_from_fork+0x14/0x20)
> >>
> >> Provide a helper which clearly documents the usage of driver_override.
> >> This will allow later to reuse the helper and reduce amount of
> >> duplicated code.
> >>
> >> Convert the platform driver to use new helper and make the
> >> driver_override field const char (it is not modified by the core).
> >>
> >> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
> >> ---
> >>  drivers/base/driver.c           | 51 +++++++++++++++++++++++++++++++++
> >>  drivers/base/platform.c         | 28 +++---------------
> >>  include/linux/device/driver.h   |  2 ++
> >>  include/linux/platform_device.h |  7 ++++-
> >>  4 files changed, 63 insertions(+), 25 deletions(-)
> >>
> >> diff --git a/drivers/base/driver.c b/drivers/base/driver.c
> >> index 8c0d33e182fd..353750b0bbc5 100644
> >> --- a/drivers/base/driver.c
> >> +++ b/drivers/base/driver.c
> >> @@ -30,6 +30,57 @@ static struct device *next_device(struct klist_iter *i)
> >>  	return dev;
> >>  }
> >>  
> >> +/**
> >> + * driver_set_override() - Helper to set or clear driver override.
> >> + * @dev: Device to change
> >> + * @override: Address of string to change (e.g. &device->driver_override);
> >> + *            The contents will be freed and hold newly allocated override.
> >> + * @s: NUL terminated string, new driver name to force a match, pass empty
> >> + *     string to clear it
> >> + * @len: length of @s
> >> + *
> >> + * Helper to set or clear driver override in a device, intended for the cases
> >> + * when the driver_override field is allocated by driver/bus code.
> >> + *
> >> + * Returns: 0 on success or a negative error code on failure.
> >> + */
> >> +int driver_set_override(struct device *dev, const char **override,
> >> +			const char *s, size_t len)
> > 
> > TBH, I think it would make more sense to have this generic
> > driver_set_override receive only the dev and the string. And then,
> > each bus type will have their own implementation that handle things
> > their own way. This would allow all the drivers that will use this to
> > do something like this:
> > 
> > 	ret = driver_set_override(&pdev->dev, "override_string");
> > 
> > I think it would look more cleaner.
> > 
> 
> The interface in general is not for the drivers. Drivers use it in
> exceptions (few cases in entire kernel) but many times they actually do
> not need to.
> 
> Adding a dedicated driver_set_override() brings intention that such
> usage is welcomed... but it's not. :)

I understand that. Anyway, I was more focused on this looking cleaner.
But I'll let the others weigh in on this before applying the imx clk
patch.

Thanks.

> 
> 
> Best regards,
> Krzysztof