Re: [PATCH v3 4/6] xen: Switch to byteswap

[Andrew Cooper <amc96@xxxxxxxx>]

On 10/05/2022 11:51, Julien Grall wrote:
> On 10/05/2022 11:15, Lin Liu wrote:
>> diff --git a/xen/common/device_tree.c b/xen/common/device_tree.c
>> index 4aae281e89..70d3be3be6 100644
>> --- a/xen/common/device_tree.c
>> +++ b/xen/common/device_tree.c
>> @@ -171,7 +171,7 @@ bool_t dt_property_read_u32(const struct
>> dt_device_node *np,
>>       if ( !val || len < sizeof(*out_value) )
>>           return 0;
>>   -    *out_value = be32_to_cpup(val);
>> +    *out_value = be32_to_cpu(*val);
>
> This code has been taken from Linux and I would rather prefer to keep
> the *cpup* helpers to avoid any changes when backporting.

I specifically requested that this be de-obfuscated.  Hiding indirection
is a fantastic way to introduce bugs, and we've had XSAs in the past
because of it (admittedly in libxl, but still...).

This file is already Xen style, not Linux, so won't be taking backports
directly, and the resulting compiler diagnostic will make it obvious
what is going on. be32_to_cpu(*val) works fine on older versions of Xen too.

In this case, the cost of changing is well worth the improvements and
simplifications gained.  See the 0/6 diffstat and see that the compiler
can make better optimisations when it can see the builtin.

>
>> diff --git a/xen/include/xen/unaligned.h b/xen/include/xen/unaligned.h
>> index 0a2b16d05d..16b2e6f5f0 100644
>> --- a/xen/include/xen/unaligned.h
>> +++ b/xen/include/xen/unaligned.h
>> @@ -20,62 +20,62 @@
>>     static inline uint16_t get_unaligned_be16(const void *p)
>>   {
>> -    return be16_to_cpup(p);
>> +    return be16_to_cpu(*(const uint16_t *)p)
>
> I haven't checked the existing implementation of be16_to_cpup().

It's a plain dereference, just like this.  AFAICT, it wasn't unaligned
safe before, either.

It should be reasonably easy to fix in a followup patch.  Just memcpy()
to/from the void pointer to a stack variable of the appropriate type.

~Andrew