linux-kernel - Re: [PATCH] mtd: spi-nor: avoid holes in struct spi_mem

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Message-ID: <20230720104832.15a4d4ab@xps-13>
Date:   Thu, 20 Jul 2023 10:48:32 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Tudor Ambarus <tudor.ambarus@...aro.org>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Pratyush Yadav <pratyush@...nel.org>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Foley <pefoley2@...oley.com>,
        Pedro Falcato <pedro.falcato@...il.com>,
        Michael Walle <michael@...le.cc>,
        Mark Brown <broonie@...nel.org>,
        Takahiro Kuwano <Takahiro.Kuwano@...ineon.com>,
        Dhruva Gole <d-gole@...com>, linux-mtd@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org
Subject: Re: [PATCH] mtd: spi-nor: avoid holes in struct spi_mem_op

Hi Tudor,

tudor.ambarus@...aro.org wrote on Thu, 20 Jul 2023 07:50:33 +0100:

> On 7/19/23 20:00, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> > 
> > gcc gets confused when -ftrivial-auto-var-init=pattern is used on sparse
> > bit fields such as 'struct spi_mem_op', which caused the previous false
> > positive warning about an uninitialized variable:
> > 
> > drivers/mtd/spi-nor/spansion.c: error: 'op' is used uninitialized [-Werror=uninitialized]
> > 
> > In fact, the variable is fully initialized and gcc does not see it being
> > used, so the warning is entirely bogus. The problem appears to be
> > a misoptimization in the initialization of single bit fields when the
> > rest of the bytes are not initialized.
> > 
> > A previous workaround added another initialization, which ended up
> > shutting up the warning in spansion.c, though it apparently still happens
> > in other files as reported by Peter Foley in the gcc bugzilla. The
> > workaround of adding a fake initialization seems particularly bad
> > because it would set values that can never be correct but prevent the
> > compiler from warning about actually missing initializations.
> > 
> > Revert the broken workaround and instead pad the structure to only
> > have bitfields that add up to full bytes, which should avoid this
> > behavior in all drivers.
> > 
> > I also filed a new bug against gcc with what I found, so this can
> > hopefully be addressed in future gcc releases. At the moment, only
> > gcc-12 and gcc-13 are affected.
> > 
> > Cc: Peter Foley <pefoley2@...oley.com>
> > Cc: Pedro Falcato <pedro.falcato@...il.com>
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110743
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108402
> > Link: https://godbolt.org/z/efMMsG1Kx
> > Fixes: 420c4495b5e56 ("mtd: spi-nor: spansion: make sure local struct does not contain garbage")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>  
> 
> Acked-by: Tudor Ambarus <tudor.ambarus@...aro.org>
> 
> Miquel, would you please take this through mtd/fixes?

I will!

Thanks,
Miquèl