Re: [PATCH v2] x86: Fix /proc/cpuinfo cpumask warning

Adding more people and lists to CC in order to point this patch out,
because, based on a quick grep of cpuinfo seq operations, I think at least
openrisc, powerpc, and s390 also need an equivalent patch (and I've
already sent a patch for riscv). While the test is simple (see next
paragraph) I'm not equipped to test on each architecture.

To test, just build a kernel with DEBUG_PER_CPU_MAPS enabled, boot to a
shell, do 'cat /proc/cpuinfo', and look for a kernel warning.

Thanks,
drew

On Wed, Oct 12, 2022 at 10:24:22AM +0200, Andrew Jones wrote:

On Wed, Oct 12, 2022 at 10:19:05AM +0200, Andrew Jones wrote:

quoted

Commit 78e5a3399421 ("cpumask: fix checking valid cpu range") has
started issuing warnings[*] when cpu indices equal to nr_cpu_ids - 1
are passed to cpumask_next* functions. 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 issue the warning when reading /proc/cpuinfo. Ensure no
warning is generated by validating the cpu index before calling
cpumask_next().

[*] Warnings will only appear with DEBUG_PER_CPU_MAPS enabled.

Signed-off-by: Andrew Jones <redacted>
Cc: Yury Norov <yury.norov@gmail.com>
---

Forgot the changelog...

v2:
  - Added all the information I should have in the first place to the
    commit message [Boris]
  - Changed style of fix [Boris]

quoted

 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..94ac02c8dd6f 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