[PATCH v2][next] scsi: aacraid: Replace one-element array with flexible-array member

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of a flexible-array member in
struct aac_raw_io2 instead of one-element array, and use the
struct_size() and flex_array_size() helpers.

Also, this helps with the ongoing efforts to enable -Warray-bounds by
fixing the following warnings:

drivers/scsi/aacraid/aachba.c: In function ‘aac_build_sgraw2’:
drivers/scsi/aacraid/aachba.c:3970:18: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 3970 |     if (rio2->sge[j].length % (i*PAGE_SIZE)) {
      |         ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:3974:27: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 3974 |     nseg_new += (rio2->sge[j].length / (i*PAGE_SIZE));
      |                  ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:4011:28: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 4011 |   for (j = 0; j < rio2->sge[i].length / (pages * PAGE_SIZE); ++j) {
      |                   ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:4012:24: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 4012 |    addr_low = rio2->sge[i].addrLow + j * pages * PAGE_SIZE;
      |               ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:4014:33: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 4014 |    sge[pos].addrHigh = rio2->sge[i].addrHigh;
      |                        ~~~~~~~~~^~~
drivers/scsi/aacraid/aachba.c:4015:28: warning: array subscript 1 is above array bounds of ‘struct sge_ieee1212[1]’ [-Warray-bounds]
 4015 |    if (addr_low < rio2->sge[i].addrLow)
      |                   ~~~~~~~~~^~~

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Build-tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60414244.ur4%2FkI+fBF1ohKZs%25lkp@intel.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
 - Add code comment for clarification.

 drivers/scsi/aacraid/aachba.c  | 17 +++++++++++------
 drivers/scsi/aacraid/aacraid.h |  2 +-
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/scsi/aacraid/aachba.c b/drivers/scsi/aacraid/aachba.c
index 8e06604370c4..2816a15d5633 100644
--- a/drivers/scsi/aacraid/aachba.c
+++ b/drivers/scsi/aacraid/aachba.c
@@ -1235,8 +1235,8 @@ static int aac_read_raw_io(struct fib * fib, struct scsi_cmnd * cmd, u64 lba, u3
 		if (ret < 0)
 			return ret;
 		command = ContainerRawIo2;
-		fibsize = sizeof(struct aac_raw_io2) +
-			((le32_to_cpu(readcmd2->sgeCnt)-1) * sizeof(struct sge_ieee1212));
+		fibsize = struct_size(readcmd2, sge,
+				     le32_to_cpu(readcmd2->sgeCnt));
 	} else {
 		struct aac_raw_io *readcmd;
 		readcmd = (struct aac_raw_io *) fib_data(fib);
@@ -1366,8 +1366,8 @@ static int aac_write_raw_io(struct fib * fib, struct scsi_cmnd * cmd, u64 lba, u
 		if (ret < 0)
 			return ret;
 		command = ContainerRawIo2;
-		fibsize = sizeof(struct aac_raw_io2) +
-			((le32_to_cpu(writecmd2->sgeCnt)-1) * sizeof(struct sge_ieee1212));
+		fibsize = struct_size(writecmd2, sge,
+				      le32_to_cpu(writecmd2->sgeCnt));
 	} else {
 		struct aac_raw_io *writecmd;
 		writecmd = (struct aac_raw_io *) fib_data(fib);
@@ -4003,7 +4003,7 @@ static int aac_convert_sgraw2(struct aac_raw_io2 *rio2, int pages, int nseg, int
 	if (aac_convert_sgl == 0)
 		return 0;
 
-	sge = kmalloc_array(nseg_new, sizeof(struct sge_ieee1212), GFP_ATOMIC);
+	sge = kmalloc_array(nseg_new, sizeof(*sge), GFP_ATOMIC);
 	if (sge == NULL)
 		return -ENOMEM;
 
@@ -4020,7 +4020,12 @@ static int aac_convert_sgraw2(struct aac_raw_io2 *rio2, int pages, int nseg, int
 		}
 	}
 	sge[pos] = rio2->sge[nseg-1];
-	memcpy(&rio2->sge[1], &sge[1], (nseg_new-1)*sizeof(struct sge_ieee1212));
+	/*
+	 * Notice that, in this case, flex_array_size() evaluates to
+	 * (nseg_new - 1) number of sge objects of type struct sge_ieee1212.
+	 */
+	memcpy(&rio2->sge[1], &sge[1],
+	       flex_array_size(rio2, sge, nseg_new - 1));
 
 	kfree(sge);
 	rio2->sgeCnt = cpu_to_le32(nseg_new);
diff --git a/drivers/scsi/aacraid/aacraid.h b/drivers/scsi/aacraid/aacraid.h
index e3e4ecbea726..3733df77bc65 100644
--- a/drivers/scsi/aacraid/aacraid.h
+++ b/drivers/scsi/aacraid/aacraid.h
@@ -1929,7 +1929,7 @@ struct aac_raw_io2 {
 	u8		bpComplete;	/* reserved for F/W use */
 	u8		sgeFirstIndex;	/* reserved for F/W use */
 	u8		unused[4];
-	struct sge_ieee1212	sge[1];
+	struct sge_ieee1212	sge[];
 };
 
 #define CT_FLUSH_CACHE 129
-- 
2.27.0