RFC: Update x86 psABI to support shadow stac

[H.J. Lu]

Any comments?

-- 
H.J.
---
To support shadow stack (SHSTK) in Intel Control-flow Enforcement
Technology (CET) instructions:

https://software.intel.com/sites/default/files/managed/4d/2a/control-flow-enforc
ement-technology-preview.pdf

following changes to x86 psABI are required.

To program properties, add

#define GNU_PROPERTY_X86_FEATURE_1_SHSTK (1U << 1)

to the GNU_PROPERTY_X86_FEATURE_1_AND property to indicate that all
executable sections are compatible with SHSTK where return address
popped from shadow stack always matches return address popped from
normal stack.

GNU_PROPERTY_X86_FEATURE_1_SHSTK is set on output only if it is set on
all relocatable inputs, which means that the C library must be compiled
with SHSTK-enabled compiler.

Load-time processing

On a SHSTK capable processor, the following steps should be taken:

1. When loading an executable, if GNU_PROPERTY_X86_FEATURE_1_SHSTK is
set on the executable as well as all shared objects loaded via the
DT_NEEDED tag, enable SHSTK.
2. After SHSTK is enabled, it is an error to load a shared object
without GNU_PROPERTY_X86_FEATURE_1_SHSTK.
gnu-6:pts/8[8]> cat  shstk.txt                /export/gnu/import/git/properties
Update x86 psABI to support shadow stack

To support shadow stack (SHSTK) in Intel Control-flow Enforcement
Technology (CET) instructions:

https://software.intel.com/sites/default/files/managed/4d/2a/control-flow-enforcement-technology-preview.pdf

following changes to x86 psABI are required.

To program properties, add

#define GNU_PROPERTY_X86_FEATURE_1_SHSTK (1U << 1)

to the GNU_PROPERTY_X86_FEATURE_1_AND property to indicate that all
executable sections are compatible with SHSTK where return address
popped from shadow stack always matches return address popped from
normal stack.

GNU_PROPERTY_X86_FEATURE_1_SHSTK is set on output only if it is set on
all relocatable inputs, which means that the C library must be compiled
with SHSTK-enabled compiler.

Load-time processing

On a SHSTK capable processor, the following steps should be taken:

1. When loading an executable, if GNU_PROPERTY_X86_FEATURE_1_SHSTK is
set on the executable as well as all shared objects loaded via the
DT_NEEDED tag, enable SHSTK.
2. After SHSTK is enabled, it is an error to load a shared object
without GNU_PROPERTY_X86_FEATURE_1_SHSTK.