Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

[Steven Sistare]

On 7/30/2020 1:49 PM, Matthew Wilcox wrote:
> On Thu, Jul 30, 2020 at 01:35:51PM -0400, Steven Sistare wrote:
>> mshare + VA reservation is another possible solution.
>>
>> Or MADV_DOEXEC alone, which is ready now.  I hope we can get back to reviewing that.
> 
> We are.  This is the part of the review process where we explore other
> solutions to the problem.
> 
>>>> Also, we need to support updating legacy processes that already created anon segments.
>>>> We inject code that calls MADV_DOEXEC for such segments.
>>>
>>> Yes, I was assuming you'd inject code that called mshare().
>>
>> OK, mshare works on existing memory and builds a new vma.
> 
> Actually, reparents an existing VMA, and reuses the existing page tables.
> 
>>> Actually, since you're injecting code, why do you need the kernel to
>>> be involved?  You can mmap the new executable and any libraries it depends
>>> upon, set up a new stack and jump to the main() entry point, all without
>>> calling exec().  I appreciate it'd be a fair amount of code, but it'd all
>>> be in userspace and you can probably steal / reuse code from ld.so (I'm
>>> not familiar with the details of how setting up an executable is done).
>>
>> Duplicating all the work that the kernel and loader do to exec a process would
>> be error prone, require ongoing maintenance, and be redundant.  Better to define 
>> a small kernel extension and leave exec to the kernel.
> 
> Either this is a one-off kind of thing, in which case it doesn't need
> ongoing maintenance, or it's something with broad applicability, in
> which case it can live as its own userspace project.  It could even
> start off life as part of qemu and then fork into its own project.

exec will be enhanced over time in the kernel.  A separate user space implementation
would need to track that.

Reimplementing exec in userland would be a big gross mess.  Not a good solution when
we have simple and concise ways of solving the problem.

> The idea of tagging an ELF executable to say "I can cope with having
> chunks of my address space provided to me by my executor" is ... odd.

I don't disagree.  But it is useful.  We already pass a block of data containing
environment variables and arguments from one process to the next.  Preserving 
additional segments is not a big leap from there.

- Steve