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 d7a8cb93ef2d..a34ea2c35446 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 Mon, Mar 25, 2024 at 09:31:05AM +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.

This really should be run past whoever maintains build_bug.h and
also cc linux-kernel.  Please also split the addition and the user
in swiotlb into separate patches.

I plan to pick up patch 1 for now, and wait for the discussion on
this addition.

On Tue, 26 Mar 2024 08:56:06 +0100
Christoph Hellwig <hch@lst.de> wrote:

> On Mon, Mar 25, 2024 at 09:31:05AM +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.  
> 
> This really should be run past whoever maintains build_bug.h and
> also cc linux-kernel.  Please also split the addition and the user
> in swiotlb into separate patches.
> 
> I plan to pick up patch 1 for now, and wait for the discussion on
> this addition.

Fair enough. The use in swiotlb depends on patch 1 from this series,
but you're right, I can send the macro addition as a separate patch to
get some feedback.

Thank you!
Petr T