[RFC patch] vsprintf: Allow %pe to print non PTR_ERR %pe uses as decimal

[Rasmus Villemoes]

On 24/03/2021 23.18, Joe Perches wrote:
> On Wed, 2021-03-24 at 22:27 +0100, Rasmus Villemoes wrote:
>> On 24/03/2021 20.24, Joe Perches wrote:
>>> On Wed, 2021-03-24 at 18:33 +0100, Rasmus Villemoes wrote:
>>>> On 24/03/2021 18.20, Joe Perches wrote:
>>>>
>>>>>
>>>>> Maybe it's better to output non PTR_ERR %pe uses as decimal so this
>>>>> sort of code would work.
>>>>
>>>> No, because that would leak the pointer value when somebody has
>>>> accidentally passed a real kernel pointer to %pe.
>>>
>>> I think it's not really an issue.
>>>
>>> _All_ code that uses %p<foo> extensions need inspection anyway.
>>
>> There are now a bunch of sanity checks in place that catch e.g. an
>> ERR_PTR passed to an extension that would derefence the pointer;
>> enforcing that only ERR_PTRs are passed to %pe (or falling back to %p)
>> is another of those safeguards.
>>
>>> It's already possible to intentionally 'leak' the ptr value
>>> by using %pe, -ptr so I think that's not really an issue.
>>>
>>
>> Huh, what? I assume -ptr is shorthand for (void*)-(unsigned long)ptr.
>> How would that leak the value if ptr is an ordinary kernel pointer?
>> That's not an ERR_PTR unless (unsigned long)ptr is < 4095 or so.
> 
> You are confusing ERR_PTR with IS_ERR

No I'm not, I'm just being slightly sloppy - obviously when I say "not
an ERR_PTR" I mean "not the result of ERR_PTR applied to a negative
errno value", or "not the result of a valid invocation of ERR_PTR". But
yes, feel free to read "not an ERR_PTR" as "something for which IS_ERR
is false".

Can you expand on why you think %pe, -ptr  would leak the value of ptr?

>> If you want to print the pointer value just do %px. No need for silly
>> games.
> 
> There's no silly game here.  %pe would either print a string or a value.

A hashed value, that is, never the raw value.

> It already does that in 2 cases.

Yes, if you pass it ERR_PTR(-1234) (where no E symbol exists) or
ERR_PTR(-EINVAL) but CONFIG_SYMBOLIC_ERRNAME=n, it prints the value in
decimal, because people will probably recognize "-22" and values in that
range don't reveal anything about the kernel image. Anything outside
[-4095,0] or so is hashed.

Rasmus