Re: [PATCH] mtd: spi-nor: avoid holes in struct spi_mem_op

[Miquel Raynal <miquel.raynal@xxxxxxxxxxx>]

Hi Tudor,

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

> On 7/19/23 20:00, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@xxxxxxxx>
> > 
> > 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@xxxxxxxxxxx>
> > Cc: Pedro Falcato <pedro.falcato@xxxxxxxxx>
> > 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@xxxxxxxx>  
> 
> Acked-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>
> 
> Miquel, would you please take this through mtd/fixes?

I will!

Thanks,
Miquèl