PING [PATCH v4 0/2] Implement indirect external access

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

On Wed, Sep 22, 2021 at 7:02 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> Changes in the v4 patch.
>
> 1. Add nodirect_extern_access attribute.
>
> Changes in the v3 patch.
>
> 1. GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS support has been added to
> GNU binutils 2.38.  But the -z indirect-extern-access linker option is
> only available for Linux/x86.  However, the --max-cache-size=SIZE linker
> option was also addded within a day.  --max-cache-size=SIZE is used to
> check for GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS support.
>
> Changes in the v2 patch.
>
> 1. Rename the option to -fdirect-extern-access.
>
> ---
> On systems with copy relocation:
> * A copy in executable is created for the definition in a shared library
> at run-time by ld.so.
> * The copy is referenced by executable and shared libraries.
> * Executable can access the copy directly.
>
> Issues are:
> * Overhead of a copy, time and space, may be visible at run-time.
> * Read-only data in the shared library becomes read-write copy in
> executable at run-time.
> * Local access to data with the STV_PROTECTED visibility in the shared
> library must use GOT.
>
> On systems without function descriptor, function pointers vary depending
> on where and how the functions are defined.
> * If the function is defined in executable, it can be the address of
> function body.
> * If the function, including the function with STV_PROTECTED visibility,
> is defined in the shared library, it can be the address of the PLT entry
> in executable or shared library.
>
> Issues are:
> * The address of function body may not be used as its function pointer.
> * ld.so needs to search loaded shared libraries for the function pointer
> of the function with STV_PROTECTED visibility.
>
> Here is a proposal to remove copy relocation and use canonical function
> pointer:
>
> 1. Accesses, including in PIE and non-PIE, to undefined symbols must
> use GOT.
>   a. Linker may optimize out GOT access if the data is defined in PIE or
>   non-PIE.
> 2. Read-only data in the shared library remain read-only at run-time
> 3. Address of global data with the STV_PROTECTED visibility in the shared
> library is the address of data body.
>   a. Can use IP-relative access.
>   b. May need GOT without IP-relative access.
> 4. For systems without function descriptor,
>   a. All global function pointers of undefined functions in PIE and
>   non-PIE must use GOT.  Linker may optimize out GOT access if the
>   function is defined in PIE or non-PIE.
>   b. Function pointer of functions with the STV_PROTECTED visibility in
>   executable and shared library is the address of function body.
>    i. Can use IP-relative access.
>    ii. May need GOT without IP-relative access.
>    iii. Branches to undefined functions may use PLT.
> 5. Single global definition marker:
>
> Add GNU_PROPERTY_1_NEEDED:
>
> #define GNU_PROPERTY_1_NEEDED GNU_PROPERTY_UINT32_OR_LO
>
> to indicate the needed properties by the object file.
>
> Add GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS:
>
> #define GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS (1U << 0)
>
> to indicate that the object file requires canonical function pointers and
> cannot be used with copy relocation.  This bit should be cleared in
> executable when there are non-GOT or non-PLT relocations in relocatable
> input files without this bit set.
>
>   a. Protected symbol access within the shared library can be treated as
>   local.
>   b. Copy relocation should be disallowed at link-time and run-time.
>   c. GOT function pointer reference is required at link-time and run-time.
>
> The indirect external access marker can be used in the following ways:
>
> 1. Linker can decide the best way to resolve a relocation against a
> protected symbol before seeing all relocations against the symbol.
> 2. Dynamic linker can decide if it is an error to have a copy relocation
> in executable against the protected symbol in a shared library by checking
> if the shared library is built with -fno-direct-extern-access.
>
> Add a compiler option, -fdirect-extern-access. -fdirect-extern-access is
> the default.  With -fno-direct-extern-access:
>
> 1. Always to use GOT to access undefined symbols, including in PIE and
> non-PIE.  This is safe to do and does not break the ABI.
> 2. In executable and shared library, for symbols with the STV_PROTECTED
> visibility:
>   a. The address of data symbol is the address of data body.
>   b. For systems without function descriptor, the function pointer is
>   the address of function body.
> These break the ABI and resulting shared libraries may not be compatible
> with executables which are not compiled with -fno-direct-extern-access.
> 3. Generate an indirect external access marker in relocatable objects if
> supported by linker.
>
> H.J. Lu (2):
>   Add -f[no-]direct-extern-access
>   Add TARGET_ASM_EMIT_GNU_PROPERTY_NOTE
>

Hi,

This has been implemented in binutils 2.38 and glibc 2.35.
What do I need to do to get it into GCC 12?

Thanks.

-- 
H.J.