linux-kernel - [PATCH v4 2/2] bug: introduce ASSERT_VAR_CAN

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Date: Mon, 25 Mar 2024 09:31:05 +0100
From: Petr Tesarik <petrtesarik@...weicloud.com>
To: Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	Michael Kelley <mhklinux@...look.com>,
	Will Deacon <will@...nel.org>,
	linux-kernel@...r.kernel.org (open list),
	iommu@...ts.linux.dev (open list:DMA MAPPING HELPERS)
Cc: Roberto Sassu <roberto.sassu@...weicloud.com>,
	Petr Tesarik <petr@...arici.cz>
Subject: [PATCH v4 2/2] bug: introduce ASSERT_VAR_CAN_HOLD()

From: Petr Tesarik <petr.tesarik1@...wei-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@...wei-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