linux-kernel - Re: [PATCH 4.19 262/276] media: saa7146: avoid high stack usage with clang

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Date:   Thu, 6 Jun 2019 13:44:39 +0200
From:   Pavel Machek <pavel@...x.de>
To:     pavel@....cz
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 262/276] media: saa7146: avoid high stack usage with
 clang

Hi!

> Two saa7146/hexium files contain a construct that causes a warning
> when built with clang:
> 
> drivers/media/pci/saa7146/hexium_orion.c:210:12: error: stack frame size of 2272 bytes in function 'hexium_probe'
>       [-Werror,-Wframe-larger-than=]
> static int hexium_probe(struct saa7146_dev *dev)
>            ^
> drivers/media/pci/saa7146/hexium_gemini.c:257:12: error: stack frame size of 2304 bytes in function 'hexium_attach'
>       [-Werror,-Wframe-larger-than=]
> static int hexium_attach(struct saa7146_dev *dev, struct saa7146_pci_extension_data *info)
>            ^
> 
> This one happens regardless of KASAN, and the problem is that a
> constructor to initialize a dynamically allocated structure leads
> to a copy of that structure on the stack, whereas gcc initializes
> it in place.
> 
> Link: https://bugs.llvm.org/show_bug.cgi?id=40776

> --- a/drivers/media/pci/saa7146/hexium_gemini.c
> +++ b/drivers/media/pci/saa7146/hexium_gemini.c
> @@ -270,9 +270,8 @@ static int hexium_attach(struct saa7146_dev *dev, struct saa7146_pci_extension_d
>  	/* enable i2c-port pins */
>  	saa7146_write(dev, MC1, (MASK_08 | MASK_24 | MASK_10 | MASK_26));
>  
> -	hexium->i2c_adapter = (struct i2c_adapter) {
> -		.name = "hexium gemini",
> -	};
> +	strscpy(hexium->i2c_adapter.name, "hexium gemini",
> +		sizeof(hexium->i2c_adapter.name));
>  	saa7146_i2c_adapter_prepare(dev, &hexium->i2c_adapter, SAA7146_I2C_BUS_BIT_RATE_480);
>  	if (i2c_add_adapter(&hexium->i2c_adapter) < 0) {
>  		DEB_S("cannot register i2c-device. skipping.\n");

As a sideeffect, this removes zero-initialization from
hexium->i2c_adapter.

Is that intended / correct?

[I tried looked at saa7146_i2c_adapter_prepare(), and that does not
initialize all the fields, either.]

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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