[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@xxxxxxxxx>
---
 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