[PATCH v4 1/1] x86: cpuinfo: Ensure inputs to cpumask_next are valid

[Andrew Jones <ajones@xxxxxxxxxxxxxxxx>]

The valid cpumask range is [0, nr_cpu_ids) and cpumask_next()
currently calls find_next_bit() with its input CPU ID number plus one
for the bit number, giving cpumask_next() the range [-1, nr_cpu_ids - 1).
seq_read_iter() and cpuinfo's start and next seq operations implement a
pattern like

  n = cpumask_next(n - 1, mask);
  show(n);
  while (1) {
      ++n;
      n = cpumask_next(n - 1, mask);
      if (n >= nr_cpu_ids)
          break;
      show(n);
  }

which will eventually result in cpumask_next() being called with
nr_cpu_ids - 1. A kernel compiled with commit 78e5a3399421 ("cpumask:
fix checking valid cpu range"), but not its revert, commit
80493877d7d0 ("Revert "cpumask: fix checking valid cpu range"."),
will generate a warning when DEBUG_PER_CPU_MAPS is enabled each time
/proc/cpuinfo is read. Future-proof cpuinfo by checking its input to
cpumask_next() is valid.

Signed-off-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
Cc: Yury Norov <yury.norov@xxxxxxxxx>
---
 arch/x86/kernel/cpu/proc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
index 099b6f0d96bd..de3f93ac6e49 100644
--- a/arch/x86/kernel/cpu/proc.c
+++ b/arch/x86/kernel/cpu/proc.c
@@ -153,6 +153,9 @@ static int show_cpuinfo(struct seq_file *m, void *v)
 
 static void *c_start(struct seq_file *m, loff_t *pos)
 {
+	if (*pos == nr_cpu_ids)
+		return NULL;
+
 	*pos = cpumask_next(*pos - 1, cpu_online_mask);
 	if ((*pos) < nr_cpu_ids)
 		return &cpu_data(*pos);
-- 
2.37.3