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 <ajones@ventanamicro.com>
Cc: Yury Norov <yury.norov@gmail.com>
---
 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

On Wed, Oct 12, 2022 at 10:19:05AM +0200, Andrew Jones wrote:
> 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 <ajones@ventanamicro.com>
> 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]

>  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
>

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:
> > 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 <ajones@ventanamicro.com>
> > 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]
> 
> >  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
> >