Re: [PATCH] ASoC: fsl_sai: Fix 32 slots TDM broken by integer shift UB in xMR write

[Christophe Leroy (CS GROUP)]

Le 29/05/2026 à 10:50, Chancel Liu a écrit :
> When configuring 32 slots TDM (channels == slots == 32), the xMR
> (Mask Register) write used:
> ~0UL - ((1 << min(channels, slots)) - 1)
>
> The literal '1' is a signed 32-bit int. Shifting it by 32 positions is
> undefined behaviour which may set this register to 0xFFFFFFFF, masking
> all 32 slots.
>
> Use 1ULL so the shift is carried out in 64 bits. For 32 slots this
> produces a zero mask after truncation to the 32-bit register:
> ~0ULL - ((1ULL << 32) - 1)
>    = 0xFFFFFFFFFFFFFFFF - (0x100000000 - 1)
>    = 0xFFFFFFFFFFFFFFFF - 0xFFFFFFFF
>    = 0xFFFFFFFF00000000
>    -> Truncates to 0x00000000
> Behaviour for fewer than 32 slots is unchanged.

Why not use macro GENMASK_U32() instead ?

> Fixes: 770f58d7d2c5 ("ASoC: fsl_sai: Support multiple data channel enable bits")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chancel Liu <chancel.liu@xxxxxxx>
> ---
>   sound/soc/fsl/fsl_sai.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/sound/soc/fsl/fsl_sai.c b/sound/soc/fsl/fsl_sai.c
> index d6dd95680892..821e3bd51b6e 100644
> --- a/sound/soc/fsl/fsl_sai.c
> +++ b/sound/soc/fsl/fsl_sai.c
> @@ -797,7 +797,7 @@ static int fsl_sai_hw_params(struct snd_pcm_substream *substream,
>   				   FSL_SAI_CR4_FSD_MSTR, FSL_SAI_CR4_FSD_MSTR);
>   
>   	regmap_write(sai->regmap, FSL_SAI_xMR(tx),
> -		     ~0UL - ((1 << min(channels, slots)) - 1));
> +		     ~0ULL - ((1ULL << min(channels, slots)) - 1));
>   
>   	return 0;
>   }