Re: [PATCH] cpumask: Preserve short reads in cpumap printing helpers

[Yury Norov]

On Tue, Oct 20, 2026 at 05:01:26PM +0000, Hungyu Lin wrote:
> The cpumap printing helpers unconditionally subtract one from the bitmap
> helper's return value to exclude a terminating NUL. A partial read may
> not reach that NUL, so this also subtracts a data byte from the returned
> length. A one-byte read therefore returns zero even when data remains.
> For example, this command prints nothing despite the file being nonempty:
> 
>   head -c 1 /sys/devices/system/cpu/cpu0/topology/core_cpus_list
> 
> Subtract one only when the copied range actually ends in NUL. This
> preserves short reads, EOF and negative error returns while continuing
> to exclude the terminator from complete sysfs reads.
> 
> Fixes: c86a2d9058c5 ("cpumask: Omit terminating null byte in cpumap_print_{list,bitmask}_to_buf")
> Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>

Added in bitmap-for-next. Thanks!

> ---
>  include/linux/cpumask.h | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 4c8bb6953107..2caeae982faf 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -1334,8 +1334,14 @@ static __always_inline
>  ssize_t cpumap_print_bitmask_to_buf(char *buf, const struct cpumask *mask,
>  				    loff_t off, size_t count)
>  {
> -	return bitmap_print_bitmask_to_buf(buf, cpumask_bits(mask),
> -				   nr_cpu_ids, off, count) - 1;
> +	ssize_t n;
> +
> +	n = bitmap_print_bitmask_to_buf(buf, cpumask_bits(mask),
> +					nr_cpu_ids, off, count);
> +	if (n > 0 && buf[n - 1] == '\0')
> +		n--;
> +
> +	return n;
>  }
>  
>  /**
> @@ -1356,8 +1362,14 @@ static __always_inline
>  ssize_t cpumap_print_list_to_buf(char *buf, const struct cpumask *mask,
>  				 loff_t off, size_t count)
>  {
> -	return bitmap_print_list_to_buf(buf, cpumask_bits(mask),
> -				   nr_cpu_ids, off, count) - 1;
> +	ssize_t n;
> +
> +	n = bitmap_print_list_to_buf(buf, cpumask_bits(mask),
> +				     nr_cpu_ids, off, count);
> +	if (n > 0 && buf[n - 1] == '\0')
> +		n--;
> +
> +	return n;
>  }
>  
>  #if NR_CPUS <= BITS_PER_LONG
> 
> base-commit: a73b4d1fa7d334a5f7de5bb0a14726530f66196e