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Re: [x86-64 psABI] RFC: Extend x86-64 PLT entry to support MPX
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Jan Beulich <JBeulich at suse dot com>
- Cc: GCC Development <gcc at gcc dot gnu dot org>, Ian Lance Taylor <iant at google dot com>, Roland McGrath <roland at hack dot frob dot com>, David L Kreitzer <david dot l dot kreitzer at intel dot com>, Milind Girkar <milind dot girkar at intel dot com>, Binutils <binutils at sourceware dot org>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Thu, 8 Aug 2013 09:01:44 -0700
- Subject: Re: [x86-64 psABI] RFC: Extend x86-64 PLT entry to support MPX
- References: <CAMe9rOp=1v38F_aV-pbv50YOGSEr_ju+byZP1L_G_h4bm5Ad3w at mail dot gmail dot com> <20130724233621 dot DA6942C08C at topped-with-meat dot com> <CAKOQZ8zn2KHayUrdKPOB0ys0Y794c5-t8Zw6hefeB5NGXTXKRw at mail dot gmail dot com> <CAMe9rOrbU--x0vZWB0mdjwJo6U5CsOJ6h_7tnwaeya8VHcujGw at mail dot gmail dot com> <CAMe9rOq6jBhmJeuYJBbf3yBd6xB8_8YZC=hJBLkKr702v0iJag at mail dot gmail dot com> <5203628F02000078000EA1EE at nat28 dot tlf dot novell dot com>
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" <email@example.com> wrote:
>> We use the .gnu_attribute directive to record an object attribute:
>> Tag_GNU_X86_EXTERN_BRANCH = 4,
>> for the types of external branch instructions in relocatable files.
>> /* 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
>> .asciz "x86 feature"
>> .align 4
>> .long FeatureFlag (Feature flag)
>> 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
>> 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.
There is another way to encode this information in the first entry
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.