[U-Boot] [PATCH v5 2/2] dlmalloc: fix malloc range at end of ram

[Simon Goldschmidt]

On Fri, Apr 26, 2019 at 11:56 AM Marek Vasut <marek.vasut at gmail.com> wrote:
>
> On 4/26/19 11:36 AM, Simon Goldschmidt wrote:
> > On Fri, Apr 26, 2019 at 11:32 AM Marek Vasut <marek.vasut at gmail.com> wrote:
> >>
> >> On 4/26/19 8:19 AM, Simon Goldschmidt wrote:
> >>> Marek Vasut <marek.vasut at gmail.com> schrieb am Fr., 26. Apr. 2019, 00:22:
> >>>
> >>>> On 4/25/19 9:22 PM, Simon Goldschmidt wrote:
> >>>>> If the malloc range passed to mem_malloc_init() is at the end of address
> >>>>> range and 'start + size' overflows to 0, following allocations fail as
> >>>>> mem_malloc_end is zero (which looks like uninitialized).
> >>>>>
> >>>>> Fix this by subtracting 1 of 'start + size' overflows to zero.
> >>>>>
> >>>>> Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt at gmail.com>
> >>>>> ---
> >>>>>
> >>>>> Changes in v5:
> >>>>> - this patch was 1/2 in v4 but is now 2/2 as the 2nd patch of v4 has
> >>>>>   already been accepted
> >>>>> - rearrange the code to make it only 8 bytes plus in code size for arm
> >>>>>   (which fixes smartweb SPL overflowing)
> >>>>>
> >>>>>  common/dlmalloc.c | 6 +++++-
> >>>>>  1 file changed, 5 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/common/dlmalloc.c b/common/dlmalloc.c
> >>>>> index 6f12a18d54..38859ecbd4 100644
> >>>>> --- a/common/dlmalloc.c
> >>>>> +++ b/common/dlmalloc.c
> >>>>> @@ -601,8 +601,12 @@ void *sbrk(ptrdiff_t increment)
> >>>>>  void mem_malloc_init(ulong start, ulong size)
> >>>>>  {
> >>>>>       mem_malloc_start = start;
> >>>>> -     mem_malloc_end = start + size;
> >>>>>       mem_malloc_brk = start;
> >>>>> +     mem_malloc_end = start + size;
> >>>>> +     if (size > mem_malloc_end) {
> >>>>> +             /* overflow: malloc area is at end of address range */
> >>>>> +             mem_malloc_end--;
> >>>>
> >>>> Does this mean a memory wrap-around happened ?
> >>>> I don't think decrementing malloc area size by 1 is a proper solution.
> >>>> You can have it overflow by 2 and decrementing by 1 won't help.
> >>>>
> >>>
> >>> No, not a real overflow. Instead, as I tried to described in the commit
> >>> message, mem_malloc_end gets 0 if the range is at the end of addr range,
> >>> e.g. malloc start is 0xffff0000 and malloc size is 0x10000. Subtracting 1
> >>> will be enough here. It reduces the available mall of aize, but I don't
> >>> think that should be a problem.
> >>
> >> That's a wrap-around . What happens with your example if malloc_size is
> >> 0x10001 ? Hint: It fails ...
> >
> > Yes it fails. But in contrast, that's an invalid configuration, while
> > my patch makes
> > a valid configuration work. I don't know if we want to fix all invalid
> > configurations.
>
> Yes ? Should be easy, just clamp() size to (size, (BIT(32) - 1) -
> mem_malloc_start) or similar for 64bit systems.

I'm not convinced we should. This range is normally generated using
something like:
SIZE=2048
START=RAM_END - SIZE

I don't want to be overprotective here. I don't think there's much point
in fixing the out-of-ram-range check if it produces an overflow but not
fix it if it's in the middle of an address space.

Again, this patch simply fixes the case for something like this:
RAM_SIZE=0x10000
RAM_START=0xFFFF0000
so RAM_END=0

We can use clamp as you suggested, but what would it be good for
if it only fixes an out-of-range heap if an overflow occurs?

>
> > You could as well enter a range without RAM, that would fail as well.
>
> That info is available in gd , but I wonder whether this is the right
> place to check for it.

Indeed, that would seem misplaced here.

Regards,
Simon

>
> > A different approach to fix my valid end-of-ram configuration would be to set
> > the end to "start + size - 1" and to change all the checks using it. But that
> > would probably lead to more code size problems in various SPL...
> >
> > Regards,
> > Simon
> >
> >>
> >>> I got this when experimenting with full heap in socfpga. Due to other
> >>> patches not being accepted, this is not an issue currebtly, but can easily
> >>> become one on the future.
> >>>
> >>> Regrds,
> >>> Simon
> >>>
> >>>
> >>>>> +     }
> >>>>>
> >>>>>       debug("using memory %#lx-%#lx for malloc()\n", mem_malloc_start,
> >>>>>             mem_malloc_end);
> >>>>>
> >>>>
> >>>>
> >>>> --
> >>>> Best regards,
> >>>> Marek Vasut
> >>>>
> >>>
> >>
> >>
> >> --
> >> Best regards,
> >> Marek Vasut
>
>
> --
> Best regards,
> Marek Vasut