Re: [PATCH AUTOSEL 4.4 080/100] char: hpet: Use flexible-array member

[Sasha Levin]

On Fri, Feb 14, 2020 at 09:43:14AM -0800, Eric Biggers wrote:
> On Fri, Feb 14, 2020 at 11:24:04AM -0500, Sasha Levin wrote:
> > From: "Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx>
> >
> > [ Upstream commit 987f028b8637cfa7658aa456ae73f8f21a7a7f6f ]
> >
> > Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
> > presence of a "variable length array":
> >
> > struct something {
> >     int length;
> >     u8 data[1];
> > };
> >
> > struct something *instance;
> >
> > instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> > instance->length = size;
> > memcpy(instance->data, source, size);
> >
> > There is also 0-byte arrays. Both cases pose confusion for things like
> > sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
> > to declare variable-length types such as the one above is a flexible array
> > member[2] which need to be the last member of a structure and empty-sized:
> >
> > struct something {
> >         int stuff;
> >         u8 data[];
> > };
> >
> > Also, by making use of the mechanism above, we will get a compiler warning
> > in case the flexible array does not occur last in the structure, which
> > will help us prevent some kind of undefined behavior bugs from being
> > unadvertenly introduced[3] to the codebase from now on.
> >
> > [1] https://github.com/KSPP/linux/issues/21
> > [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> >
> > Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/r/20200120235326.GA29231@xxxxxxxxxxxxxx
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > ---
> >  drivers/char/hpet.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
> > index 5b38d7a8202a1..38c2ae93ce492 100644
> > --- a/drivers/char/hpet.c
> > +++ b/drivers/char/hpet.c
> > @@ -112,7 +112,7 @@ struct hpets {
> >  	unsigned long hp_delta;
> >  	unsigned int hp_ntimer;
> >  	unsigned int hp_which;
> > -	struct hpet_dev hp_dev[1];
> > +	struct hpet_dev hp_dev[];
> >  };
>
> Umm, why are you backporting this without the commit that fixes it?  Does your

mhm, for some reason it failed to apply to 4.19 and older. I can look at
that.

> AUTOSEL process really still not pay attention to Fixes tags?  They are there
> for a reason.

Yes, it looks at the Fixes tag, thank you for the explanation.

> And for that matter, why are you backporting it all, given that this is a
> cleanup and not a fix?

If I recall correctly CONFIG_FORTIFY_SOURCE=y results in user visible
warnings, which we try to fix in the stable kernel.

--
Thanks,
Sasha