[v2] hw: misc: edu: fix 2 off-by-one errors

[Chris Friedt]

From: Christopher Friedt <cfriedt@meta.com>

In the case that size1 was zero, because of the explicit
'end1 > addr' check, the range check would fail and the error
message would read as shown below. The correct comparison
is 'end1 >= addr' (or 'addr <= end1').

EDU: DMA range 0x40000-0x3ffff out of bounds (0x40000-0x40fff)!

At the opposite end, in the case that size1 was 4096, within()
would fail because of the non-inclusive check 'end1 < end2',
which should have been 'end1 <= end2'. The error message would
previously say

EDU: DMA range 0x40000-0x40fff out of bounds (0x40000-0x40fff)!

This change
1. renames local variables to be more less ambiguous
2. fixes the two off-by-one errors described above.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1254

Signed-off-by: Christopher Friedt <cfriedt@meta.com>
---
 hw/misc/edu.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/hw/misc/edu.c b/hw/misc/edu.c
index e935c418d4..52afbd792a 100644
--- a/hw/misc/edu.c
+++ b/hw/misc/edu.c
@@ -103,25 +103,24 @@ static void edu_lower_irq(EduState *edu, uint32_t val)
     }
 }
 
-static bool within(uint64_t addr, uint64_t start, uint64_t end)
+static void edu_check_range(uint64_t xfer_start, uint64_t xfer_size,
+                uint64_t dma_start, uint64_t dma_size)
 {
-    return start <= addr && addr < end;
-}
-
-static void edu_check_range(uint64_t addr, uint64_t size1, uint64_t start,
-                uint64_t size2)
-{
-    uint64_t end1 = addr + size1;
-    uint64_t end2 = start + size2;
-
-    if (within(addr, start, end2) &&
-            end1 > addr && within(end1, start, end2)) {
+    uint64_t xfer_end = xfer_start + xfer_size;
+    uint64_t dma_end = dma_start + dma_size;
+
+    /*
+     * 1. ensure we aren't overflowing
+     * 2. ensure that xfer is within dma address range
+     */
+    if (dma_end >= dma_start && xfer_end >= xfer_start &&
+        xfer_start >= dma_start && xfer_end <= dma_end) {
         return;
     }
 
     hw_error("EDU: DMA range 0x%016"PRIx64"-0x%016"PRIx64
              " out of bounds (0x%016"PRIx64"-0x%016"PRIx64")!",
-            addr, end1 - 1, start, end2 - 1);
+            xfer_start, xfer_end - 1, dma_start, dma_end - 1);
 }
 
 static dma_addr_t edu_clamp_addr(const EduState *edu, dma_addr_t addr)
-- 
2.36.1