Re: [PATCH v3] smp: Fix a potential usage of stale nr_cpus
* Thomas Gleixner wrote:
> Ingo Molnar writes:
> >> - get_option(, _cpus);
> >> + if (get_option(, _cpus) != 1)
> >> + return -EINVAL;
> >> +
> >>if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
> >>nr_cpu_ids = nr_cpus;
> >> + else
> >> + return -EINVAL;
> >
> > Exactly what does 'not valid' mean, and why doesn't get_option()
> > return -EINVAL in that case?
>
> What's unclear about invalid? If you specify nr_cpus=-1 or
> nr_cpus=200 the its obviously invalid.
So this was the old (buggy) code:
> {
> int nr_cpus;
>
> get_option(, _cpus);
> if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
> nr_cpu_ids = nr_cpus;
And this was the explanation given in the changelog:
>> When the cmdline of "nr_cpus" is not valid, the @nr_cpu_ids is
>> assigned a stale value. The nr_cpus is only valid when get_option()
>> return 1. So check the return value to prevent this.
The answer to my question is that the bug is that the return value of
get_option() wasn't checked properly, and if get_option() returns an
error then the nr_cpus local variable is not set - but we used it in
the old code, which can result in essentially a random value for
nr_cpu_ids.
> How should get_option() know that this is invalid? get_option() is a
> number parser and does not know about any restrictions on the parsed
> value obviously.
But that's apparently not the bug here, 'invalid' here was meant as
per the parser's syntax. If nr_cpus is out of range (like the 200
example you gave), then nr_cpu_ids might not be set at all, and
remains at the 0 initialized value. Which isn't good but not 'stale'
either.
This is why I was puzzled where a 'stale' value might come from, at
first sight I was assuming that some large value was written, like
your 20 example. The "stale value" happens if it's invalid syntax
and get_option() returns an error, in which case 'nr_cpus' remains
uninitialized.
And this is the explanation I didn't find at first reading, and which
explanation future changelogs should perhaps include.
The new code does this:
int nr_cpus;
if (get_option(, _cpus) != 1)
return -EINVAL;
if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
nr_cpu_ids = nr_cpus;
else
return -EINVAL;
Which does all the proper error handling and fixes the uninitialized
'nr_cpus' local variable usage. So I agree with the fix:
Reviewed-by: Ingo Molnar
Thanks,
Ingo
Re: [PATCH v3] smp: Fix a potential usage of stale nr_cpus
Ingo Molnar writes: >> -get_option(, _cpus); >> +if (get_option(, _cpus) != 1) >> +return -EINVAL; >> + >> if (nr_cpus > 0 && nr_cpus < nr_cpu_ids) >> nr_cpu_ids = nr_cpus; >> +else >> +return -EINVAL; > > Exactly what does 'not valid' mean, and why doesn't get_option() > return -EINVAL in that case? What's unclear about invalid? If you specify nr_cpus=-1 or nr_cpus=200 the its obviously invalid. How should get_option() know that this is invalid? get_option() is a number parser and does not know about any restrictions on the parsed value obviously. get_option() returns string parsing information: 0 -> not integer found 1 -> integer found, no trailing comma or hyphen 2 -> integer found and trailing comma 3 -> integer found and traling hyphen (range parsing) And that's what is checked in if (get_option() != 1), i.e. anything else than a plain integer is invalid for this command line option. Thanks, tglx
Re: [PATCH v3] smp: Fix a potential usage of stale nr_cpus
* Muchun Song wrote:
> When the cmdline of "nr_cpus" is not valid, the @nr_cpu_ids is assigned
> a stale value. The nr_cpus is only valid when get_option() return 1. So
> check the return value to prevent this.
>
> Signed-off-by: Muchun Song
> ---
> changelog in v3:
> 1) Return -EINVAL when the parameter is bogus.
>
> changelog in v2:
> 1) Rework the commit log.
> 2) Rework the return value check.
>
> kernel/smp.c | 6 +-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/smp.c b/kernel/smp.c
> index a5a66fc28f4e..0dacfcfcf00b 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -772,9 +772,13 @@ static int __init nrcpus(char *str)
> {
> int nr_cpus;
>
> - get_option(, _cpus);
> + if (get_option(, _cpus) != 1)
> + return -EINVAL;
> +
> if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
> nr_cpu_ids = nr_cpus;
> + else
> + return -EINVAL;
Exactly what does 'not valid' mean, and why doesn't get_option()
return -EINVAL in that case?
Thanks,
Ingo
[PATCH v3] smp: Fix a potential usage of stale nr_cpus
When the cmdline of "nr_cpus" is not valid, the @nr_cpu_ids is assigned
a stale value. The nr_cpus is only valid when get_option() return 1. So
check the return value to prevent this.
Signed-off-by: Muchun Song
---
changelog in v3:
1) Return -EINVAL when the parameter is bogus.
changelog in v2:
1) Rework the commit log.
2) Rework the return value check.
kernel/smp.c | 6 +-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/smp.c b/kernel/smp.c
index a5a66fc28f4e..0dacfcfcf00b 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -772,9 +772,13 @@ static int __init nrcpus(char *str)
{
int nr_cpus;
- get_option(, _cpus);
+ if (get_option(, _cpus) != 1)
+ return -EINVAL;
+
if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
nr_cpu_ids = nr_cpus;
+ else
+ return -EINVAL;
return 0;
}
--
2.11.0
