Re: [PATCH for-7.2 01/10] hw/arm/boot.c: do not free machine->fdt in arm_load_dtb()

[BALATON Zoltan <balaton@eik.bme.hu>]

On Fri, 22 Jul 2022, 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 | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/hw/arm/boot.c b/hw/arm/boot.c
> index ada2717f76..1d9c6047b1 100644
> --- a/hw/arm/boot.c
> +++ b/hw/arm/boot.c
> @@ -684,7 +684,8 @@ 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 ms->fdt pointer */
> +    ms->fdt = fdt;

Not sure this comment is useful as it just states what the assignment does 
so provides no further info.

Regards,
BALATON Zoltan

>
>     return size;
>
>