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Re: [Qemu-arm] [PATCH 1/2] hw/arm/boot: Avoid placing the initrd on top
From: |
Peter Maydell |
Subject: |
Re: [Qemu-arm] [PATCH 1/2] hw/arm/boot: Avoid placing the initrd on top of the kernel |
Date: |
Fri, 3 May 2019 18:33:38 +0100 |
On Fri, 3 May 2019 at 18:13, Peter Maydell <address@hidden> 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 a830655e1af..7c978fedde4 100644
> --- a/hw/arm/boot.c
> +++ b/hw/arm/boot.c
> @@ -998,20 +998,6 @@ 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);
> @@ -1056,6 +1042,26 @@ static void arm_setup_direct_kernel_boot(ARMCPU *cpu,
> exit(1);
> }
> info->entry = entry;
> +
> + /*
> + * 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);
> + info->initrd_start = TARGET_PAGE_ALIGN(info->initrd_start);
> +
I belatedly realized that we should probably check here whether
this is off the end of the ram, as otherwise following code
will get an underflow in "info->ram_size - info->initrd_start"
which is unlikely to result in useful behaviour.
thanks
-- PMM