Re: [Patch] sysctl: forbid too long numbers

Amerigo Wang
2009-07-02T10:01:25Z

Andrew Morton wrote:
> Also, fixing this is a non-backward-compatible change which could break
> existing userspace. Although the chances of this seem fairly small.
>
> Or are they? One could imagine a script which was tested and developed
> on a 64-bit system, which writes a >4G number into a pseudo file. That
> script happens to work on 32-bit systems (it might not work _well_, but
> it'll work). With this change, the write will fail on the 32-bit
> system and the entire application could bale out or something.
>
> I'm not saying that this is a reason to avoid making the change, but
> it's all a worry and needs consideration.
>
>

Ah, I didn't consider this situation...
Hmm... but only taking the lower 32-bits really looks strange.

> The other worrisome thing about this change is that there may well be
> existing userspace which does
>
> echo 42foo > /proc/whatever
>
> and the conversion to strict_strtoul() will cause that script to
> newly fail.
>
> And the chances that there are scripts which do this are pretty darned
> good - it's fairly easy to accidentally leave junk like this in strings
> when hacking stuff together in scripting languages.
>
>

Yeah, maybe, but that is really tricky...
>
>
>> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
>> index 756ccaf..ff2ca5c 100644
>> --- a/lib/vsprintf.c
>> +++ b/lib/vsprintf.c
>> @@ -163,11 +163,14 @@ int strict_strtoul(const char *cp, unsigned int base, unsigned long *res)
>> char *tail;
>> unsigned long val;
>> size_t len;
>> + char tmp[32];
>>
>> *res = 0;
>> len = strlen(cp);
>> if (len == 0)
>> return -EINVAL;
>> + if (len > snprintf(tmp, "%ld", ULONG_MAX))
>> + return -EINVAL;
>>
>> val = simple_strtoul(cp, &tail, base);
>> if (tail == cp)
>>
>
> And here we're doing a check for that overflow, yes?
>
> - We don't need tmp[]. vsnprintf(NULL, ...) can be used to query the
> length of an sprintf. See how kvasprintf() does this.
>
> - The strict_strtoul() documentation should be updated?
>
> - The above change affects strict_strtol() too.
>
> - The same change should be made to strict_strtoull() and hence
> strict_strtoll()?
>
>
Good points!
I agree, so maybe we only need to change this part?
Hmm, I need to check the callers of strict_strtol()...

Thank you!