Re: [PATCH v3 2/2] bug: introduce ASSERT_VAR_CAN_HOLD()

[Petr Tesařík]

On Fri, 22 Mar 2024 15:37:38 +0000
Will Deacon <will@xxxxxxxxxx> wrote:

> On Thu, Mar 21, 2024 at 06:19:02PM +0100, Petr Tesarik wrote:
> > From: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
> > 
> > Introduce an ASSERT_VAR_CAN_HOLD() macro to check at build time that a
> > variable can hold the given value.
> > 
> > Use this macro in swiotlb to make sure that the list and pad_slots fields
> > of struct io_tlb_slot are big enough to hold the maximum possible value of
> > IO_TLB_SEGSIZE.
> > 
> > Signed-off-by: Petr Tesarik <petr.tesarik1@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  include/linux/build_bug.h | 10 ++++++++++
> >  kernel/dma/swiotlb.c      |  2 ++
> >  2 files changed, 12 insertions(+)
> > 
> > diff --git a/include/linux/build_bug.h b/include/linux/build_bug.h
> > index 3aa3640f8c18..6e2486508af0 100644
> > --- a/include/linux/build_bug.h
> > +++ b/include/linux/build_bug.h
> > @@ -86,4 +86,14 @@
> >  		"Offset of " #field " in " #type " has changed.")
> >  
> >  
> > +/*
> > + * Compile time check that a variable can hold the given value
> > + */
> > +#define ASSERT_VAR_CAN_HOLD(var, value) ({		\
> > +	typeof(value) __val = (value);			\
> > +	typeof(var) __tmp = __val;			\
> > +	BUILD_BUG_ON_MSG(__tmp != __val,		\
> > +		#var " cannot hold " #value ".");	\
> > +})  
> 
> nit, but I think this prevents putting negative values into unsigned
> types. Not sure whether we care? Arguably it's even correct to complain.
> 
> e.g.
> 
> 	u16 s;
> 	ASSERT_VAR_CAN_HOLD(s, -1);
> 
> explodes for me.

Then it works as intended. I specifically aimed at making a macro that
checks at build time whether a given constant is within the value range
of a variable, so it explodes for a signed overflow (in either
direction).

To check the size of a variable, I could have gone with something
simpler like BUG_ON(sizeof(var) < sizeof(val)).

Petr T