From: Petr Tesarik <petr.tesarik1@huawei-partners.com>

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@huawei-partners.com>
---
 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 ".");	\
+})
+
 #endif	/* _LINUX_BUILD_BUG_H */
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 3779a48eec9b..8256fcdc0cf6 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -285,6 +285,8 @@ static void swiotlb_init_io_tlb_pool(struct io_tlb_pool *mem, phys_addr_t start,
 		mem->areas[i].used = 0;
 	}
 
+	ASSERT_VAR_CAN_HOLD(mem->slots[0].list, IO_TLB_SEGSIZE);
+	ASSERT_VAR_CAN_HOLD(mem->slots[0].pad_slots, IO_TLB_SEGSIZE);
 	for (i = 0; i < mem->nslabs; i++) {
 		mem->slots[i].list = min(IO_TLB_SEGSIZE - io_tlb_offset(i),
 					 mem->nslabs - i);
-- 
2.34.1

On Thu, Mar 21, 2024 at 06:19:02PM +0100, Petr Tesarik wrote:
> From: Petr Tesarik <petr.tesarik1@huawei-partners.com>
> 
> 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@huawei-partners.com>
> ---
>  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.

Will

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

> On Thu, Mar 21, 2024 at 06:19:02PM +0100, Petr Tesarik wrote:
> > From: Petr Tesarik <petr.tesarik1@huawei-partners.com>
> > 
> > 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@huawei-partners.com>
> > ---
> >  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