linux-kernel - Re: [alsa-devel] [PATCH v7 8/9] ALSA: add new 32-bit layout for snd_pcm_mmap

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Message-Id: <29QBMJU8DE71E.2YZSH8IHT5HMH@mforney.org>
Date:   Wed, 06 Oct 2021 10:49:17 -0700
From:   Michael Forney <mforney@...rney.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     alsa-devel@...a-project.org, Takashi Iwai <tiwai@...e.com>,
        Baolin Wang <baolin.wang@...aro.org>, y2038@...ts.linaro.org,
        linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>,
        Baolin Wang <baolin.wang7@...il.com>
Subject: Re: [alsa-devel] [PATCH v7 8/9] ALSA: add new 32-bit layout for
 snd_pcm_mmap_status/control

Arnd Bergmann <arnd@...db.de> wrote:
> +#if defined(__BYTE_ORDER) ? __BYTE_ORDER == __BIG_ENDIAN : defined(__BIG_ENDIAN)
> +typedef char __pad_before_uframe[sizeof(__u64) - sizeof(snd_pcm_uframes_t)];
> +typedef char __pad_after_uframe[0];
> +#endif
> +
> +#if defined(__BYTE_ORDER) ? __BYTE_ORDER == __LITTLE_ENDIAN : defined(__LITTLE_ENDIAN)
> +typedef char __pad_before_uframe[0];
> +typedef char __pad_after_uframe[sizeof(__u64) - sizeof(snd_pcm_uframes_t)];
> +#endif
> +
> +struct __snd_pcm_mmap_status64 {
> +	__s32 state;			/* RO: state - SNDRV_PCM_STATE_XXXX */
> +	__u32 pad1;			/* Needed for 64 bit alignment */
> +	__pad_before_uframe __pad1;
> +	snd_pcm_uframes_t hw_ptr;	/* RO: hw ptr (0...boundary-1) */
> +	__pad_after_uframe __pad2;
> +	struct __snd_timespec64 tstamp;	/* Timestamp */
> +	__s32 suspended_state;		/* RO: suspended stream state */
> +	__u32 pad3;			/* Needed for 64 bit alignment */
> +	struct __snd_timespec64 audio_tstamp; /* sample counter or wall clock */
> +};
> +
> +struct __snd_pcm_mmap_control64 {
> +	__pad_before_uframe __pad1;
> +	snd_pcm_uframes_t appl_ptr;	 /* RW: appl ptr (0...boundary-1) */
> +	__pad_before_uframe __pad2;

I was looking through this header and happened to notice that this
padding is wrong. I believe it should be __pad_after_uframe here.

I'm not sure of the implications of this typo, but I suspect it
breaks something on 32-bit systems with 64-bit time (regardless of
the endianness, since it changes the offset of avail_min).

> +
> +	__pad_before_uframe __pad3;
> +	snd_pcm_uframes_t  avail_min;	 /* RW: min available frames for wakeup */
> +	__pad_after_uframe __pad4;
> +};