Re: [Intel-gfx] [PATCH] drm/i915/hwmon: Silence UBSAN uninitialized bool variable warning

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Wed, 10 May 2023 11:36:06 -0700, Ashutosh Dixit wrote:
>
> Loading i915 on UBSAN enabled kernels (CONFIG_UBSAN/CONFIG_UBSAN_BOOL)
> causes the following warning:
>
>   UBSAN: invalid-load in drivers/gpu/drm/i915/gt/uc/intel_uc.c:558:2
>   load of value 255 is not a valid value for type '_Bool'
>   Call Trace:
>    dump_stack_lvl+0x57/0x7d
>    ubsan_epilogue+0x5/0x40
>    __ubsan_handle_load_invalid_value.cold+0x43/0x48
>    __uc_init_hw+0x76a/0x903 [i915]
>    ...
>    i915_driver_probe+0xfb1/0x1eb0 [i915]
>    i915_pci_probe+0xbe/0x2d0 [i915]
>
> The warning happens because during probe i915_hwmon is still not available
> which results in the output boolean variable *old remaining
> uninitialized.

Note that the variable was uninitialized in this case but it was never used
uninitialized (the variable was not needed when it was uninitialized). So
there was no bug in the code. UBSAN warning is just complaining about the
uninitialized variable being passed into a function (where it is not used).

Also the variable can be initialized in the caller (__uc_init_hw) too and
it will fix this issue. However in __uc_init_hw the assumption is that the
variable will be initialized in the callee (i915_hwmon_power_max_disable),
so that is how I have done it in this patch.

I thought these clarifications will help with the review.

Thanks.
--
Ashutosh

> Silence the warning by initializing the variable to an arbitrary value.
>
> Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_hwmon.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
> index a3bdd9f68a458..685663861bc0b 100644
> --- a/drivers/gpu/drm/i915/i915_hwmon.c
> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> @@ -502,8 +502,11 @@ void i915_hwmon_power_max_disable(struct drm_i915_private *i915, bool *old)
>	struct i915_hwmon *hwmon = i915->hwmon;
>	u32 r;
>
> -	if (!hwmon || !i915_mmio_reg_valid(hwmon->rg.pkg_rapl_limit))
> +	if (!hwmon || !i915_mmio_reg_valid(hwmon->rg.pkg_rapl_limit)) {
> +		/* Fix uninitialized bool variable warning */
> +		*old = false;
>		return;
> +	}
>
>	mutex_lock(&hwmon->hwmon_lock);
>
> --
> 2.38.0
>