Re: [RFC 0/5] vmalloc_exec for modules and BPF programs

[Song Liu <songliubraving@fb.com>]

> On Aug 22, 2022, at 11:39 PM, Christophe Leroy <christophe.leroy@csgroup.eu> wrote:
> 
> 
> 
> Le 23/08/2022 à 07:42, Peter Zijlstra a écrit :
>> On Mon, Aug 22, 2022 at 04:56:47PM +0000, Song Liu wrote:
>>> 
>>> 
>>>> On Aug 22, 2022, at 9:34 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>>> 
>>>> On Mon, Aug 22, 2022 at 03:46:38PM +0000, Song Liu wrote:
>>>>> Could you please share your feedback on this?
>>>> 
>>>> I've looked at it all of 5 minutes, so perhaps I've missed something.
>>>> 
>>>> However, I'm a little surprised you went with a second tree instead of
>>>> doing the top-down thing for data. The way you did it makes it hard to
>>>> have guard pages between text and data.
>>> 
>>> I didn't realize the importance of the guard pages. But it is not too
>> 
>> I'm not sure how important it is, just seems like a good idea to trap
>> anybody trying to cross that divide. Also, to me it seems like a good
>> idea to have a single large contiguous text region instead of splintered
>> 2M pages.
>> 
>>> hard to do it with this approach. For each 2MB text page, we can reserve
>>> 4kB on the beginning and end of it. Would this work?
>> 
>> Typically a guard page has different protections (as in none what so
>> ever) so that every access goes *splat*. >
> 
> Text is RO-X, on some architectures even only X. So the only real thing 
> to protect against is bad execution, isn't it ?. So I guess having some 
> areas with invalid or trap instructions would be enough ?

Agreed that filling with trap instructions should be enough. 

Thanks,
Song