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

["H.J. Lu" <hjl dot tools at gmail dot com>]

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.

>> There is another way to encode this information in the first entry
>> of PLT:
>>
>>    0:    ff 35 00 00 00 00        pushq  GOT+8(%rip)
>>    6:    f2 ff 25 00 00 00 00     bnd jmpq *GOT+16(%rip)
>>    d:    0f 1f 44 00 00           nopl   0x0(%rax,%rax,1)
>>   12:    0f 1f 80 00 00 00 00     nopl   0x0(%rax)
>>   19:    0f 1f 80 00 00 00 01     nopl   0x1000000(%rax)
>>
>> We can encode PLT property in 10 (4 + 4 + 2) bytes of
>> displacements of 3 nops.  In this example, the first bit
>> of the last byte of PLT0 is 1.
>
> While a nice idea, I think that's worse, because much harder to
> determine from simply dumping information for a given binary.
>

I agree.  That is why a note section is better.


-- 
H.J.