Re: [PATCH] ALSA: line6: Bound capture copies by the PCM buffer size

[Sinan Turunc <enscrpt@pm.me>]

> Does clamping frames to runtime->buffer_size here cause the capture
> stream to stall whenever an incoming packet exceeds the buffer size
> (for example, when a user configures a 1-period 64-byte capture buffer
> of 11 or 16 frames and receives a regular 44- or 48-frame 1 ms packet
> or a larger USB/IP packet)?

Yes. Clamping frames to buffer_size adds exactly one buffer to pos_done
and then subtracts it, so the ring position never advances and the
stream makes no progress. The same holds for a buffer of exactly one
packet, so the bound has to be two packets.

v2 takes a different approach: snd_line6_capture_open() now constrains
SNDRV_PCM_HW_PARAM_BUFFER_BYTES to at least twice the larger of the two
endpoints' packet sizes, so hw_params rejects a buffer that cannot hold
two packets, and line6_capture_copy() keeps only a guard that drops an
oversized packet with a ratelimited error instead of clamping it, for
the case that should no longer be reachable.

> Also, fsize is not read again after frames is computed at the top of
> line6_capture_copy(), so is the assignment to fsize here unused?

fsize is read in the single-chunk branch (memcpy(..., fbuf, fsize)),
which runs when pos_done is zero, so the assignment was used. It goes
away in v2 with the clamp.

v2 tested on 7.3-rc7 against 1024-byte packets: a 1024-byte buffer is
now refused at hw_params with -EINVAL, and a 2048-byte buffer streams
for 30 s with no KASAN report, with hw_ptr sampled once a second at
positions 0 and 256 inside the 512-frame buffer and advancing by about
450000 frames per second.