[U-Boot] [PATCH v3] Fix board init code to use a valid C runtime environment

[Albert ARIBAUD]

Hello Simon,

On Fri, 13 Nov 2015 19:29:21 -0700, Simon Glass <sjg at chromium.org>
wrote:
> Hi Albert,
> 
> On 13 November 2015 at 07:43, Albert ARIBAUD <albert.u.boot at aribaud.net> wrote:
> > board_init_f_mem() alters the C runtime environment's
> > stack it ls actually already using. This is not a valid
> > C runtime environment.
> 
> I think it is doing something like alloca() and reserving space on the
> stack.

AFAICT, board_init_f_mem does *not* "do something like alloca()",
it does something that alloca() precisely does *not* do.

What alloca() does is, it creates a local variable on the stack, only
the size of that local is determined at run time; but that is still a
*local* variable, that will be freed like all locals when the current
function returns. The location of the alloca reservation is determined
by the compiler, which knows how which way the stack works and how much
of it is being used by the calling function already; and the resulting
pointer cannot be accessed beyond the size that was allocated.

What board_init_f_mem does is *completely* different: it wild-guesses
how much of the stack it is using, it assumes which way the stack grows,
and it purposefully accesses space that it was not given access to, and
it expects that allocation to survive upon returning to the calling
context.

Now, if you look at alloca's definition in glibc, you'll see it is
defined to be a gcc built-in, not a C function. There is a reason for
this: stack allocation within a C function requires the compiler's help,
because the compiler must be made aware of the alloca use in order to
properly free the alloca()ed space on every exit point in the function.
Only a built-in can get that level of compiler support.

board_init_f_mem's case is worse yet, as, contrary to alloca() which
only expects /local/ effects, board_init_f_mem expects the allocated
space to survive the return to the calling context. Doing this would
require *more* compiler help than alloca does, because it is not more
a matter of managing the stack pointer; it is a matter of moving
things around so that the allocated space ends up *above* the call
frame for board_init_f.

> It does not actually change the stack pointer in C code.

Non, it does not indeed; it could not, in fact, do this at all without
resorting to the same technique that is resorted to for implementing
alloca(), that is, it would have to be a built-in of gcc, as alloca()
is -- and, as I have just explained, it would require extensive help
from the compiler, as all compiler-generated epilog code would have to
account for that potential allocation.

> It changes data that is below the stack pointer, and therefore by
> definition not part of the stack, I think.

It changes data that it /assumes/ is below the stack pointer and that it
/assumes/ it can write to; but these two assumptions are unfounded, see
below.

> What actually goes wrong here? I read your info in the other thread
> but I'm afraid I still don't understand it.

At least three things are formally wrong here with board_init_f_mem:

- it assumes the stack grows downward.

- it assumes it never uses more than 0x40 bytes of the stack.

- it assumes it can write to the stack beyond what it was allocated for
  local variables, temporaries and arguments.

Each of these assumptions is unfounded in code which is supposed to be
architecture-agnostic. Not all stacks grow downward. There is no way to
ensure that only 0x40 bytes are used by board_init_f_mem. There is no
guarantee that a C function can write to its own stack beyond what it
was allocated -- in fact, there is no guarantee that a C function can
write to its stack anywhere else than its declared or alloca()ed
locals. Such writes are undefined behavior.

These assumptions are just as bad as assuming any pointer-to-long value
is valid when we could ensure it is; and we would fix such a situation,
would we not?

Same here. This patch ensures that the code in board_init_f_mem becomes
compliant with C runtime environment constraints. This requires
splitting the C code in two functions so that the first one does not
access its stack (except the allowed accesses generated by the
compiler) and the second accesses an area which is in fact not the stack
any more, thus is safe to access.

> Regards,
> Simon

Amicalement,
-- 
Albert.