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Re: [PATCH for-7.2 v2 01/20] hw/arm: do not free machine->fdt in arm_loa

From: David Gibson
Subject: Re: [PATCH for-7.2 v2 01/20] hw/arm: do not free machine->fdt in arm_load_dtb()
Date: Mon, 8 Aug 2022 13:23:44 +1000

On Fri, Aug 05, 2022 at 06:39:29AM -0300, Daniel Henrique Barboza wrote:
> At this moment, arm_load_dtb() can free machine->fdt when
> binfo->dtb_filename is NULL. If there's no 'dtb_filename', 'fdt' will be
> retrieved by binfo->get_dtb(). If get_dtb() returns machine->fdt, as is
> the case of machvirt_dtb() from hw/arm/virt.c, fdt now has a pointer to
> machine->fdt. And, in that case, the existing g_free(fdt) at the end of
> arm_load_dtb() will make machine->fdt point to an invalid memory region.
> This is not an issue right now because there's no code that access
> machine->fdt after arm_load_dtb(), but we're going to add a couple do
> FDT HMP commands that will rely on machine->fdt being valid.
> Instead of freeing 'fdt' at the end of arm_load_dtb(), assign it to
> machine->fdt. This will allow the FDT of ARM machines that relies on
> arm_load_dtb() to be accessed later on.
> Since all ARM machines allocates the FDT only once, we don't need to
> worry about leaking the existing FDT during a machine reset (which is
> something that other machines have to look after, e.g. the ppc64 pSeries
> machine).
> Cc: Peter Maydell <peter.maydell@linaro.org>
> Cc: qemu-arm@nongnu.org
> Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> ---
>  hw/arm/boot.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> diff --git a/hw/arm/boot.c b/hw/arm/boot.c
> index ada2717f76..9f5ceb62d2 100644
> --- a/hw/arm/boot.c
> +++ b/hw/arm/boot.c
> @@ -684,7 +684,13 @@ int arm_load_dtb(hwaddr addr, const struct arm_boot_info 
> *binfo,
>       */
>      rom_add_blob_fixed_as("dtb", fdt, size, addr, as);
> -    g_free(fdt);
> +    /*
> +     * Update the ms->fdt pointer to enable support for 'dumpdtb'
> +     * and 'info fdt' commands. Use fdt_pack() to shrink the blob
> +     * size we're going to store.
> +     */
> +    fdt_pack(fdt);
> +    ms->fdt = fdt;
>      return size;

fdt_pack() could change (reduce) the effective size of the dtb blob,
so returning a 'size' value from above rather than the new value of
fdt_totalsize(fdt) doesn't see right.

I believe some of the other patches in the series have similar concerns.

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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