Re: [PATCH 2/2] exec: increase BINPRM_BUF_SIZE to 256

[Oleg Nesterov <oleg@redhat.com>]

On 11/12, Andrew Morton wrote:

> On Mon, 12 Nov 2018 17:09:56 +0100 Oleg Nesterov <oleg@redhat.com> wrote:
>
> >  /* sizeof(linux_binprm->buf) */
> > -#define BINPRM_BUF_SIZE 128
> > +#define BINPRM_BUF_SIZE 256
> >
> >  #endif /* _UAPI_LINUX_BINFMTS_H */
>
> It does seem a rather silly restriction, and it's tempting to suggest
> reworking the code so that linux_binprm.buf is dynamically sized to
> accommodate even ludicrously large strings.

I actually tried to do this ;)

Of course this is possible, but we need some limits anyway, we need to
read the file until we find '\n' or '\0' in kmalloc/vmalloc'ed buffer,
then split and copy the strings to bprm->vma in reverse order.

So I decided to make the trivial change for now and (hopefully) forget
about this problem.

> However it would be basically cost-free to increase
> BINPRM_BUF_SIZE up to the point where sizeof(struct linux_binprm) ==
> PAGE_SIZE?

I don't think we should take sizeof(struct linux_binprm) into account, the
new members can come at any time and we can never decrease BINPRM_BUF_SIZE.

Oleg.