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

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

[Hungyu Lin]

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@gmail.com>
---
 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

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

[Tobias Klauser]

On 2026-10-20 at 19:01:26 +0200, Hungyu Lin <dennylin0707@gmail.com> 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@gmail.com>

Reviewed-by: Tobias Klauser <tklauser@distanz.ch>

Thanks

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@gmail.com>

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