Re: [PATCH v3 3/3] linux-user/syscall.c: do_ppoll: eliminate large alloca

[Michael Tokarev]

14.09.2023 11:18, Daniel P. Berrangé wrote:
> On Thu, Sep 14, 2023 at 10:43:37AM +0300, Michael Tokarev wrote:
> > do_ppoll() in linux-user/syscall.c uses alloca() to allocate
> > an array of struct pullfds on the stack.  The only upper
> > boundary for number of entries for this array is so that
> > whole thing fits in INT_MAX.  This is definitely too much
> > for stack allocation.
> >
> > Use heap allocation when large number of entries is requested
> > (currently 32, arbitrary), and continue to use alloca() for
>
> Typo ? The code uses 64 rather than 32.

Yeah, it's a typo, after a few iterations trying to split this
all into pieces and editing in the process.


> > -    struct pollfd *pfd = NULL;
> > +    struct pollfd *pfd = NULL, *heap_pfd = NULL;
>
> g_autofree struct pollfd *heap_pdf = NULL;
...
> >   
> >   out:
> > +    g_free(heap_pfd);
>
> This can be dropped with g_autofree usage

Yes, I know this, - this was deliberate choice.
Personally I'm just too used to old-school explicit resource deallocations.
Here, there's a single place where everything gets freed, so there's little
reason to use fancy modern automatic deallocations. To my taste anyway.
Maybe some future modifications adding some future ppoll3.. :)

Sure thing I can drop that and change it to autofree.

Thanks,

/mjt