Re: [REGRESSION] ALSA: firewire-lib: heavy digital distortion with Fireface 800

[Takashi Iwai <tiwai@xxxxxxx>]

On Thu, 25 Jul 2024 17:11:31 +0200,
Gustavo A. R. Silva wrote:
> 
> 
> 
> On 25/07/24 08:08, Gustavo A. R. Silva wrote:
> > Hi!
> > 
> > On 25/07/24 07:07, Takashi Iwai wrote:
> >> On Thu, 25 Jul 2024 00:24:29 +0200,
> >> edmund.raile wrote:
> >>> 
> >>> Bisection revealed that the bitcrushing distortion with RME FireFace 800
> >>> was caused by 1d717123bb1a7555
> >>> ("ALSA: firewire-lib: Avoid -Wflex-array-member-not-at-end warning").
> >>> 
> >>> Reverting this commit yields restoration of clear audio output.
> >>> I will send in a patch reverting this commit for now, soonTM.
> >>> 
> >>> #regzbot introduced: 1d717123bb1a7555
> >> 
> >> While it's OK to have a quick revert, it'd be worth to investigate
> >> further what broke there; the change is rather trivial, so it might be
> >> something in the macro expansion or a use of flex array stuff.
> >> 
> > 
> > I wonder is there is any log that I can take a look at. That'd be really
> > helpful.
> 
> OK, I found a discrepancy in how the `header_length` field in the flexible
> structure (a struct that contains a flexible-array member) below is used:
> 
> include/linux/firewire.h:
> 458 struct fw_iso_packet {
> ...
> 465         u32 header_length:8;    /* Length of immediate header           */
> 466                                 /* tx: Top of 1394 isoch. data_block    */
> 467         u32 header[] __counted_by(header_length);
> 468 };
> 
> Take a look at the following piece of code:
> 
> sound/firewire/amdtp-stream.c:
> 1164         if (!(s->flags & CIP_NO_HEADER))
> 1165                 pkt_header_length = IT_PKT_HEADER_SIZE_CIP;
> 
> In the code above `pkt_header_length` is set to `IT_PKT_HEADER_SIZE_CIP`, which based
> on the following macros is 8 _bytes_:
> 
> sound/firewire/amdtp-stream.c:37:#define CIP_HEADER_QUADLETS	2
> sound/firewire/amdtp-stream.c:58:#define CIP_HEADER_SIZE		(sizeof(__be32) * CIP_HEADER_QUADLETS)
> sound/firewire/amdtp-stream.c:72:#define IT_PKT_HEADER_SIZE_CIP		CIP_HEADER_SIZE
> 
> Then we use the DEFINE_FLEX() macro, which internally sets `template->header_length`
> to `CIP_HEADER_QUADLETS`, which based on the macros above, takes the value
> of 2 _elements_. We set `header_length` because such variable is the _counter_
> used during the `__counted_by()` annotation in `struct fw_iso_packet`. The
> _counter_ is the variable that holds the number of _elements_ in the flex-array
> member at some point at run-time[1].
> 
> So, we set the counter to `CIP_HEADER_QUADLETS` because that's the total number
> of _elements_ allocated for the flexible-array member `header[]` by the DEFINE_FLEX()
> macro.
> 
> 1183                 DEFINE_FLEX(struct fw_iso_packet, template, header,
> 1184                             header_length, CIP_HEADER_QUADLETS);
> 1185                 bool sched_irq = false;
> 
> Then we call function `build_it_pkt_header()` and pass as arguments a pointer
> to `template`, and `pkt_header_length`, which at this point might hold the
> value of 8 _bytes_.
> 
> 1187                 build_it_pkt_header(s, desc->cycle, template, pkt_header_length,
> 1188                                     desc->data_blocks, desc->data_block_counter,
> 1189                                     desc->syt, i, curr_cycle_time);
> 
> Then inside function `build_it_pkt_header()`, the _counter_ is updated
> `params->header_length = header_length;`:
> 
>  680 static void build_it_pkt_header(struct amdtp_stream *s, unsigned int cycle,
>  681                                 struct fw_iso_packet *params, unsigned int header_length,
> ...
>  692         if (header_length > 0) {
>  693                 cip_header = (__be32 *)params->header;
>  694                 generate_cip_header(s, cip_header, data_block_counter, syt);
>  695                 params->header_length = header_length;
>  696         } else {
> 
> This causes `params->header_length == 8`; however, only enough space for 2
> _elements_ was allocated for the flex array (via DEFINE_FLEX()).
> 
> So, regardless of how `pkt_header_length` is intended to be used in the rest of
> the code inside `build_it_pkt_header()`, this last update to `params->header_length`
> seems to be incorrect.
> 
> So, my question here is whether this `header_length` struct member was originally
> intended to be used as a counter for the elements in the flex array or as size
> variable to hold the total number of bytes in the array?
> 
> Based on the comment "Length of immediate header", I suppose `header_length` would
> hold _elements_ not _bytes_.

Thanks, now I took a look over the whole picture, and I guess there
are two problems:

- The header_length should be in bytes, as far as I read the code in
  drivers/firwire/*.  So the assumption in the commit d3155742db89
  ("firewire: Annotate struct fw_iso_packet with __counted_by()") was
  already wrong, and it couldn't be annotated like that -- unless we
  fix up all users of header_length field.

- By the use of DEFINE_FLEX() in amdtp-stream.c, process_rx_packets()
  sets the header_length field to CIP_HEADER_QUADLETS (= 2) as
  default.  Meanwhile, build_it_pkt_header() doesn't touch
  header_length unless non-zero pkt_header_length is passed, supposing
  it being zero.  So this may lead to a bogus header_length, which is
  processed by the firewire core code wrongly.

The actual effect we see is likely the latter.  A simple fix would be
to use DEFINE_RAW_FLEX() instead of DEFINE_FLEX() like below.


thanks,

Takashi

-- 8< --
--- a/sound/firewire/amdtp-stream.c
+++ b/sound/firewire/amdtp-stream.c
@@ -1180,8 +1180,8 @@ static void process_rx_packets(struct fw_iso_context *context, u32 tstamp, size_
 		(void)fw_card_read_cycle_time(fw_parent_device(s->unit)->card, &curr_cycle_time);
 
 	for (i = 0; i < packets; ++i) {
-		DEFINE_FLEX(struct fw_iso_packet, template, header,
-			    header_length, CIP_HEADER_QUADLETS);
+		DEFINE_RAW_FLEX(struct fw_iso_packet, template, header,
+				CIP_HEADER_QUADLETS);
 		bool sched_irq = false;
 
 		build_it_pkt_header(s, desc->cycle, template, pkt_header_length,