[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 2/4] hvm/dmop: Implement copy_{to, from}_guest_buf() in terms of raw accessors
On 21/04/2017 09:54, Andrew Cooper wrote: > On 21/04/2017 08:27, Jan Beulich wrote: >>>>> On 20.04.17 at 19:59, <jennifer.herbert@xxxxxxxxxx> wrote: >>> From: Jennifer Herbert <Jennifer.Herbert@xxxxxxxxxx> >> Is this correct, considering that iirc the patch was new in v5 and ... >> >>> This also allows the usual cases to be simplified, by omitting an >>> unnecessary >>> buf parameters, and because the macros can appropriately size the object. >>> >>> This makes copying to or from a buf that isn't big enough an error. >>> If the buffer isnt big enough, trying to carry on regardless >>> can only cause trouble later on. >>> >>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> >>> Signed-off-by: Jennifer Herbert <Jennifer.Herbert@xxxxxxxxxx> >> ... this sequence of S-o-b-s? >> >>> --- a/xen/arch/x86/hvm/dm.c >>> +++ b/xen/arch/x86/hvm/dm.c >>> @@ -32,36 +32,47 @@ struct dmop_args { >>> struct xen_dm_op_buf buf[2]; >>> }; >>> >>> -static bool copy_buf_from_guest(const xen_dm_op_buf_t bufs[], >>> - unsigned int nr_bufs, void *dst, >>> - unsigned int idx, size_t dst_size) >>> +static bool _raw_copy_from_guest_buf(void *dst, >>> + const struct dmop_args *args, >>> + unsigned int buf_idx, >>> + size_t dst_bytes) >>> { >>> - size_t size; >>> + size_t buf_bytes; >>> >>> - if ( idx >= nr_bufs ) >>> + if ( buf_idx >= args->nr_bufs ) >>> return false; >>> >>> - memset(dst, 0, dst_size); >>> + buf_bytes = args->buf[buf_idx].size; >>> >>> - size = min_t(size_t, dst_size, bufs[idx].size); >>> + if ( dst_bytes > buf_bytes ) >>> + return false; >> While this behavioral change is now being mentioned in the >> description, I'm not sure I buy the argument of basically being >> guaranteed to cause trouble down the road. Did you consider the >> forward compatibility aspect here, allowing us to extend interface >> structures by adding fields to their ends without breaking old >> callers? Paul, what are your thoughts here? > DMOP is a stable ABI. There is no legal extending of any objects. > > The previous semantics are guaranteed to break the ABI with future > subops, which is why I removed it. In the case that the guest > originally passed an overly long buffer, and someone tried be "clever" > here passing the same object here expecting _raw_copy_from_guest_buf() > to DTRT, the function will end up copying too much data from the guest, > and you will end up with something the guest wasn't intending to be part > of the structure replacing the zero extension. > > New subops which take a longer object should use a brand new object. > > $MAGIC compatibility logic like you want has no business living in the > copy helper. Had I spotted the intention during the original dmop > series, I would have rejected it during review. Actually, the existing behaviour is already broken. If a guest passes an overly short buf[0], the dmop logic won't get a failure, and instead get a truncated structure which has been zero extended. This is very definitely the wrong thing to do, because such a truncated structure might actually contain some legitimate operations. I reiterate my statement that $MAGIC like this has no place living in the copy helpers. All it does is introduce bugs. ~Andrew _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel
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