[PATCH 09/15] udf: avoid gcc-10 zero-length-bounds warnings

[Arnd Bergmann]

gcc-10 warns about writes to the empty freeSpaceTable[] array, with
many instances like:

fs/udf/balloc.c: In function 'udf_bitmap_new_block':
fs/udf/balloc.c:101:36: error: array subscript 65535 is outside the bounds of an interior zero-length array '__le32[0]' {aka 'unsigned int[0]'} [-Werror=zero-length-bounds]
  101 |  le32_add_cpu(&lvid->freeSpaceTable[partition], cnt);
      |                ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
In file included from fs/udf/udfdecl.h:7,
                 from fs/udf/balloc.c:22:
fs/udf/ecma_167.h:363:11: note: while referencing 'freeSpaceTable'
  363 |  __le32   freeSpaceTable[0];
      |           ^~~~~~~~~~~~~~

These can all be avoided by using a flexible array member instead.

Another warning is a bit more obscure:

fs/udf/super.c: In function 'udf_count_free':
fs/udf/super.c:2521:26: warning: array subscript '(<unknown>) + 4294967295' is outside the bounds of an interior zero-length array '__le32[0]' {aka 'unsigned int[0]'} [-Wzero-length-bounds]
 2521 |      lvid->freeSpaceTable[part]);

Work around this one by changing the array access to equivalent
pointer arithmetic, as there cannot be multiple flexible-array
members in a single struct.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
 fs/udf/ecma_167.h | 2 +-
 fs/udf/super.c    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/udf/ecma_167.h b/fs/udf/ecma_167.h
index 736ebc5dc441..14ffe27342bc 100644
--- a/fs/udf/ecma_167.h
+++ b/fs/udf/ecma_167.h
@@ -360,9 +360,9 @@ struct logicalVolIntegrityDesc {
 	uint8_t			logicalVolContentsUse[32];
 	__le32			numOfPartitions;
 	__le32			lengthOfImpUse;
-	__le32			freeSpaceTable[0];
 	__le32			sizeTable[0];
 	uint8_t			impUse[0];
+	__le32			freeSpaceTable[];
 } __packed;
 
 /* Integrity Type (ECMA 167r3 3/10.10.3) */
diff --git a/fs/udf/super.c b/fs/udf/super.c
index f747bf72edbe..379867888c36 100644
--- a/fs/udf/super.c
+++ b/fs/udf/super.c
@@ -117,7 +117,7 @@ struct logicalVolIntegrityDescImpUse *udf_sb_lvidiu(struct super_block *sb)
 	}
 	/* The offset is to skip freeSpaceTable and sizeTable arrays */
 	offset = partnum * 2 * sizeof(uint32_t);
-	return (struct logicalVolIntegrityDescImpUse *)&(lvid->impUse[offset]);
+	return (struct logicalVolIntegrityDescImpUse *)(lvid->impUse + offset);
 }
 
 /* UDF filesystem type */
-- 
2.26.0