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From: Peter Maydell <peter.maydell@linaro.org>
Date: Wed, 12 Sep 2018 13:54:50 +0100
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Subject: Re: [Qemu-devel] [PATCH] memory region: check the old.mmio.read
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To: Li Qiang <liq3ea@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, Eduardo Habkost <ehabkost@redhat.com>, =?UTF-8?B?TWFyYy1BbmRyw6kgTHVyZWF1?= <marcandre.lureau@gmail.com>, Laszlo Ersek <lersek@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, P J P <ppandit@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>

On 12 September 2018 at 13:32, Li Qiang <liq3ea@gmail.com> wrote:
> To avoid NULL-deref for the devices without read callbacks
>
> Signed-off-by: Li Qiang <liq3ea@gmail.com>
> ---
>  memory.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/memory.c b/memory.c
> index 9b73892768..48d025426b 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -406,6 +406,10 @@ static MemTxResult memory_region_oldmmio_read_accessor(MemoryRegion *mr,
>  {
>      uint64_t tmp;
>
> +    if (!mr->ops->old_mmio.read[ctz32(size)]) {
> +        return MEMTX_DECODE_ERROR;
> +    }
> +
>      tmp = mr->ops->old_mmio.read[ctz32(size)](mr->opaque, addr);
>      if (mr->subpage) {
>          trace_memory_region_subpage_read(get_cpu_index(), mr, addr, tmp, size);
> --
> 2.11.0
>

There's patches on-list which drop the old_mmio field from the MemoryRegion
struct entirely, so I think this patch as it stands is obsolete.

Currently our semantics are "you must provide both read and write, even
if one of them just always returns 0 / does nothing / returns an error".
We could probably reasonably assert this at the point when the
MemoryRegionOps is registered.

thanks
-- PMM