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

[Daniel Henrique Barboza <danielhb413@gmail.com>]

On 8/8/22 00:23, David Gibson wrote:
> 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.

Ok! I'll revisit those patches and be sure to return the updated value
of the fdt.


Thanks,


Daniel

>