Re: [x86-64 psABI] RFC: Extend x86-64 PLT entry to support MPX

["Jan Beulich" <JBeulich at suse dot com>]

>>> On 09.08.13 at 19:03, "H.J. Lu" <hjl.tools@gmail.com> wrote:
> On Fri, Aug 9, 2013 at 12:08 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>> On 08.08.13 at 18:01, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>>> On Thu, Aug 8, 2013 at 12:19 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>>> On 08.08.13 at 02:33, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>>>>> We use the .gnu_attribute directive to record an object attribute:
>>>>>
>>>>> enum
>>>>> {
>>>>>   Tag_GNU_X86_EXTERN_BRANCH = 4,
>>>>> };
>>>>>
>>>>> for the types of external branch instructions in relocatable files.
>>>>>
>>>>> enum
>>>>> {
>>>>>   /* All external branch instructions are legacy.  */
>>>>>   Val_GNU_X86_EXTERN_BRANCH_LEGACY = 0,
>>>>>   /* There is at lease one external branch instruction with BND prefix.  */
>>>>>   Val_GNU_X86_EXTERN_BRANCH_BND = 1,
>>>>> };
>>>>>
>>>>> An x86 feature note section, .note.x86-feature, is used to indicate
>>>>> features in executables and shared library. The contents of this note
>>>>> section are:
>>>>>
>>>>>     .section        .note.x86-feature
>>>>>     .align          4
>>>>>     .long           .L1 - .L0
>>>>>     .long           .L3 - .L2
>>>>>     .long           1
>>>>> .L0:
>>>>>     .asciz         "x86 feature"
>>>>> .L1:
>>>>>     .align          4
>>>>> .L2:
>>>>>     .long        FeatureFlag (Feature flag)
>>>>> .L3:
>>>>>
>>>>> The current valid bits in FeatureFlag are
>>>>>
>>>>> #define NT_X86_FEATURE_PLT_BND    (0x1 << 0)
>>>>>
>>>>> It should be set if PLT entry has BND prefix to preserve bound registers.
>>>>>
>>>>> The remaining bits in FeatureFlag are reserved.
>>>>>
>>>>> When merging Tag_GNU_X86_EXTERN_BRANCH, if any input relocatable
>>>>> file has Tag_GNU_X86_EXTERN_BRANCH set to Val_GNU_X86_EXTERN_BRANCH_BND,
>>>>> the resulting Tag_GNU_X86_EXTERN_BRANCH value should be
>>>>> Val_GNU_X86_EXTERN_BRANCH_BND.
>>>>>
>>>>> When generating executable or shared library, if PLT is needed and
>>>>> Tag_GNU_X86_EXTERN_BRANCH value is Val_GNU_X86_EXTERN_BRANCH_BND,
>>>>> the 32-byte PLT entry should be used and the feature note section should
>>>>> be generated with the NT_X86_FEATURE_PLT_BND bit set to 1 and the feature
>>>>> note section should be included in PT_NOTE segment. The benefit of the
>>>>> note section is it is backward compatible with existing run-time and tools.
>>>>
>>>> While I can see the purpose of the attribute section, I don't see
>>>> what the note section is for: You don't mention at all what it's
>>>> consumed for, and I also can't see how it validly would be for
>>>> anything. That's because iirc note section contents, if not
>>>> understood by the consumer, is required to not have any effect
>>>> on the correctness of the program. Hence if loaded on a system
>>>> that MPX capable, has an MPX aware kernel, but no MPX aware
>>>> user space (apart from this one executable or shared library, or
>>>> a set thereof), it ought to still work correctly. Which - afaict - it
>>>> won't if the dynamic loader itself isn't MPX aware.
>>>>
>>>
>>> The note section isn't required for correctness.  But it can be used
>>> by ld.so to select an alternate MPX aware shared library in a different
>>> directory, instead of a legacy one.
>>
>> Okay, that clarifies your intentions with the note section. However,
>> then you need something else to make sure an MPX aware app can't
>> load on an MPX enabled kernel without MPX-enabled ld.so.
> 
> The MPX enabled app will still run correctly.  ld.so will clear the bound
> registers (that makes unlimited bound) for the first call with lazy binding.

Only if those registers are used for their primary purpose. The
documentation specifically says that this isn't a requirement.
But anyway, I see we're once again not going to get anywhere
with this...

Jan