Re: [Xen-devel] [PATCH 3/6] x86/shadow: Use ERR_PTR infrastructure for sh_emulate_map_dest()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/06/17 10:07, Jan Beulich wrote:
>>>> On 22.06.17 at 10:21, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 22/06/2017 09:14, Jan Beulich wrote:
>>>>>> On 21.06.17 at 17:12, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> sh_emulate_map_dest() predates the introduction of the generic ERR_PTR()
>>>> infrasturcture, but take the opportunity to avoid opencoding it.
>>>>
>>>> The chosen error constants require need to be negative to work with 
>> IS_ERR(),
>>>> but no other changes.
>>> Drop one of "require" or "need"?
>>>
>>>> --- a/xen/arch/x86/mm/shadow/private.h
>>>> +++ b/xen/arch/x86/mm/shadow/private.h
>>>> @@ -395,10 +395,9 @@ void shadow_unhook_mappings(struct domain *d, mfn_t 
>> smfn, int user_only);
>>>>  
>>>>  /* Returns a mapped pointer to write to, or one of the following error
>>>>   * indicators. */
>>>> -#define MAPPING_UNHANDLEABLE ((void *)(unsigned long)X86EMUL_UNHANDLEABLE)
>>>> -#define MAPPING_EXCEPTION    ((void *)(unsigned long)X86EMUL_EXCEPTION)
>>>> -#define MAPPING_SILENT_FAIL  ((void *)(unsigned long)X86EMUL_OKAY)
>>>> -#define sh_emulate_map_dest_failed(rc) ((unsigned long)(rc) <= 3)
>>>> +#define MAPPING_UNHANDLEABLE ERR_PTR(~(long)X86EMUL_UNHANDLEABLE)
>>>> +#define MAPPING_EXCEPTION    ERR_PTR(~(long)X86EMUL_EXCEPTION)
>>>> +#define MAPPING_SILENT_FAIL  ERR_PTR(~(long)X86EMUL_OKAY)
>>> While this doesn't change with your patch, this last definition has a
>>> string dependency on X86EMUL_OKAY to be zero, yet there's no
>>> respective BUILD_BUG_ON() anywhere. Would you mind adding
>>> one at once? In any event
>>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>> There is no longer any dependence on X86EMUL_OKAY being 0 (so far as I
>> can see).  All this logic would work fine if OKAY had the value 10 for
>> example.
> The mapping failure is "silent" only if X86EMUL_OKAY is zero, afaict.

It is silent because MAPPING_SILENT_FAIL is considered an error by the
caller (sh_x86_emulate_{write,cmpxchg}()), which unpacks the result
(turning it into X86EMUL_OKAY) and passes it back into the emulation logic.

This doesn't depend on the absolute value of X86EMUL_OKAY.

>
>> The only requirement (now) is that the X86EMUL constants are all within
>> MAX_ERRNO so they count as errors as far as the ERR_PTR logic is
>> concerned, and I suppose those should gain BUILD_BUG_ON()s
> That would be helpful too.

Its sadly not possible, as ERR_PTR() is a static inline and can't be
evaluated in the context of BUILD_BUG_ON().

The best which is possible (without altering the errptr infrastructure)
is BUILD_BUG_ON(IS_ERR_VALUE(~(long)X86EMUL_*)), but this won't catch a
change which redefines the MAPPING_* constants.

~Andrew

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