linux-kernel - [PATCH 2/2][next] ALSA: firewire-lib: Avoid -Wflex-array-member-not-at-end warning

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Message-ID: <d3a764eb76909b16b8a22d9ff530e5edf0e59e6b.1709658886.git.gustavoars@kernel.org>
Date: Tue, 5 Mar 2024 11:25:32 -0600
From: "Gustavo A. R. Silva" <gustavoars@...nel.org>
To: Takashi Sakamoto <o-takashi@...amocchi.jp>,
	Clemens Ladisch <clemens@...isch.de>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>
Cc: Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-hardening@...r.kernel.org, alsa-devel@...a-project.org,
	linux-sound@...r.kernel.org, linux1394-devel@...ts.sourceforge.net
Subject: [PATCH 2/2][next] ALSA: firewire-lib: Avoid
 -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

There is currently a local structure `template` that is using a flexible
`struct fw_iso_packet` as a header for an on-stack array
`__be32 header[CIP_HEADER_QUADLETS];`.

struct {
	struct fw_iso_packet params;
	__be32 header[CIP_HEADER_QUADLETS];
} template = { {0}, {0} };

However, we are deprecating flexible arrays in the middle of another
struct. So, in order to avoid this, we use the `struct_group_tagged()`
helper to separate the flexible array from the rest of the members in
the flexible structure:

struct fw_iso_packet {
        struct_group_tagged(fw_iso_packet_hdr, hdr,
                ... the rest of the members
        );
        u32 header[];           /* tx: Top of 1394 isoch. data_block    */
};

With the change described above, we can now declare an object of the
type of the tagged struct, without embedding the flexible array in the
middle of another struct:

struct {
	struct fw_iso_packet_hdr params;
	__be32 header[CIP_HEADER_QUADLETS];
} template = { {0}, {0} };

We also use `container_of()` whenever we need to retrieve a pointer to
the flexible structure.

So, with these changes, fix the following warning:

sound/firewire/amdtp-stream.c: In function ‘process_rx_packets’:
sound/firewire/amdtp-stream.c:1184:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
 1184 |                         struct fw_iso_packet params;
      |

Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 sound/firewire/amdtp-stream.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/sound/firewire/amdtp-stream.c b/sound/firewire/amdtp-stream.c
index c9f153f85ae6..7ba1cd64d7f1 100644
--- a/sound/firewire/amdtp-stream.c
+++ b/sound/firewire/amdtp-stream.c
@@ -1181,12 +1181,14 @@ static void process_rx_packets(struct fw_iso_context *context, u32 tstamp, size_
 
 	for (i = 0; i < packets; ++i) {
 		struct {
-			struct fw_iso_packet params;
+			struct fw_iso_packet_hdr params;
 			__be32 header[CIP_HEADER_QUADLETS];
 		} template = { {0}, {0} };
+		struct fw_iso_packet *params =
+			container_of(&template.params, struct fw_iso_packet, hdr);
 		bool sched_irq = false;
 
-		build_it_pkt_header(s, desc->cycle, &template.params, pkt_header_length,
+		build_it_pkt_header(s, desc->cycle, params, pkt_header_length,
 				    desc->data_blocks, desc->data_block_counter,
 				    desc->syt, i, curr_cycle_time);
 
@@ -1198,7 +1200,7 @@ static void process_rx_packets(struct fw_iso_context *context, u32 tstamp, size_
 			}
 		}
 
-		if (queue_out_packet(s, &template.params, sched_irq) < 0) {
+		if (queue_out_packet(s, params, sched_irq) < 0) {
 			cancel_stream(s);
 			return;
 		}
-- 
2.34.1