Re: [PATCH v2] rslib: Remove VLAs by setting upper bound on nroots

[Thomas Gleixner]

On Tue, 27 Mar 2018, Kees Cook wrote:
> On Tue, Mar 27, 2018 at 4:45 PM, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Mon, 26 Mar 2018 16:17:57 -0700 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> >> On Fri, Mar 16, 2018 at 11:25 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >> > On Fri, Mar 16, 2018 at 3:59 PM, Andrew Morton
> >> > <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >> >> On Thu, 15 Mar 2018 15:59:19 -0700 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >> >>
> >> >>> Avoid stack VLAs[1] by always allocating the upper bound of stack space
> >> >>> needed. The existing users of rslib appear to max out at 24 roots[2],
> >> >>> so use that as the upper bound until we have a reason to change it.
> >> >>>
> >> >>> Alternative considered: make init_rs() a true caller-instance and
> >> >>> pre-allocate the workspaces. This would possibly need locking and
> >> >>> a refactoring of the returned structure.
> >> >>>
> >> >>> Using kmalloc in this path doesn't look great, especially since at
> >> >>> least one caller (pstore) is sensitive to allocations during rslib
> >> >>> usage (it expects to run it during an Oops, for example).
> >> >>
> >> >> Oh.
> >> >>
> >> >> Could we allocate the storage during init_rs(), attach it to `struct
> >> >> rs_control'?
> >> >
> >> > No, because they're modified during decode, and struct rs_control is
> >> > shared between users. :(
> >> >
> >> > Doing those changes is possible, but it requires a rather extensive
> >> > analysis of callers, etc.
> >> >
> >> > Hence, the 24 ultimately.
> >>
> >> Can this land in -mm, or does this need further discussion?
> >
> > Grumble.  That share-the-rs_control-if-there's-already-a-matching-one
> > thing looks like premature optimization to me :(

That was done back then in the days when the first NAND chips required Reed
solomon error correction and we were still running on rather small devices.

> > I guess if we put this storage into the rs_control (rather than on the
> > stack) then we'd have to worry about concurrent uses of it.  It looks
> > like all the other fields are immutable once it's set up so there might
> > be such users.  In fact, I suspect there are...
> 
> Exactly. :( This is the same conclusion tglx and I came to.

I think we can lift that and just let all users set up a new rs_control
from scratch. Takes some time to init the tables, but ....

Thanks,

	tglx