Re: [Qemu-devel] [PATCH v2 3/4] hw/arm/boot: Avoid placing the initrd on top of the kernel

[Peter Maydell]

On Fri, 19 Jul 2019 at 17:47, Mark Rutland <address@hidden> wrote:
>
> Hi Peter,
>
> I've just been testing on QEMU v4.1.0-rc1, and found a case where the
> DTB overlapped the end of the kernel, and I think there's a bug in this
> patch -- explanation below.
>
> On Thu, May 16, 2019 at 03:47:32PM +0100, Peter Maydell wrote:
> > We currently put the initrd at the smaller of:
> >  * 128MB into RAM
> >  * halfway into the RAM
> > (with the dtb following it).
> >
> > However for large kernels this might mean that the kernel
> > overlaps the initrd. For some kinds of kernel (self-decompressing
> > 32-bit kernels, and ELF images with a BSS section at the end)
> > we don't know the exact size, but even there we have a
> > minimum size. Put the initrd at least further into RAM than
> > that. For image formats that can give us an exact kernel size, this
> > will mean that we definitely avoid overlaying kernel and initrd.
> >
> > Signed-off-by: Peter Maydell <address@hidden>
> > ---
> >  hw/arm/boot.c | 34 ++++++++++++++++++++--------------
> >  1 file changed, 20 insertions(+), 14 deletions(-)
> >
> > diff --git a/hw/arm/boot.c b/hw/arm/boot.c
> > index 935be3b92a5..e441393fdf5 100644
> > --- a/hw/arm/boot.c
> > +++ b/hw/arm/boot.c
> > @@ -999,20 +999,6 @@ static void arm_setup_direct_kernel_boot(ARMCPU *cpu,
> >      if (info->nb_cpus == 0)
> >          info->nb_cpus = 1;
> >
> > -    /*
> > -     * We want to put the initrd far enough into RAM that when the
> > -     * kernel is uncompressed it will not clobber the initrd. However
> > -     * on boards without much RAM we must ensure that we still leave
> > -     * enough room for a decent sized initrd, and on boards with large
> > -     * amounts of RAM we must avoid the initrd being so far up in RAM
> > -     * that it is outside lowmem and inaccessible to the kernel.
> > -     * So for boards with less  than 256MB of RAM we put the initrd
> > -     * halfway into RAM, and for boards with 256MB of RAM or more we put
> > -     * the initrd at 128MB.
> > -     */
> > -    info->initrd_start = info->loader_start +
> > -        MIN(info->ram_size / 2, 128 * 1024 * 1024);
> > -
> >      /* Assume that raw images are linux kernels, and ELF images are not.  
> > */
> >      kernel_size = arm_load_elf(info, &elf_entry, &elf_low_addr,
> >                                 &elf_high_addr, elf_machine, as);
> > @@ -1064,6 +1050,26 @@ static void arm_setup_direct_kernel_boot(ARMCPU *cpu,
> >      }
> >
> >      info->entry = entry;
>
> Note: this is the start of the kernel image...

It's the entry point, which isn't quite the same thing as
the start of the image (if we just loaded an ELF file then
'entry' is whatever the ELF file said the entrypoint is, which
could be a long way into the image).

> > +
> > +    /*
> > +     * We want to put the initrd far enough into RAM that when the
> > +     * kernel is uncompressed it will not clobber the initrd. However
> > +     * on boards without much RAM we must ensure that we still leave
> > +     * enough room for a decent sized initrd, and on boards with large
> > +     * amounts of RAM we must avoid the initrd being so far up in RAM
> > +     * that it is outside lowmem and inaccessible to the kernel.
> > +     * So for boards with less  than 256MB of RAM we put the initrd
> > +     * halfway into RAM, and for boards with 256MB of RAM or more we put
> > +     * the initrd at 128MB.
> > +     * We also refuse to put the initrd somewhere that will definitely
> > +     * overlay the kernel we just loaded, though for kernel formats which
> > +     * don't tell us their exact size (eg self-decompressing 32-bit 
> > kernels)
> > +     * we might still make a bad choice here.
> > +     */
> > +    info->initrd_start = info->loader_start +
> > +        MAX(MIN(info->ram_size / 2, 128 * 1024 * 1024), kernel_size);
>
> ... but here we add kernel_size to the start of the loader, which is
> below the kernel. Should that be info->entry?

loader_start here really means "base of RAM". I think what
we want here is something like

  info->initrd_start = info->loader_start + MIN(info->ram_size / 2,
128 * 1024 * 1024);
  info->initrd_start = MAX(info->initrd_start, kernel_end);

where kernel_end is just past whatever the highest addr of the kernel
is, which is not something that's totally trivial to calculate
with the variables we have to hand at this point.

thanks
-- PMM