On Thu, Oct 21, 2021 at 12:56 PM H.J. Lu <hjl.to...@gmail.com> wrote: > > On Wed, Sep 22, 2021 at 7:02 PM H.J. Lu <hjl.to...@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? >
Hi Richard, Jeff, Can you help review this patch for GCC 12: https://gcc.gnu.org/pipermail/gcc-patches/2021-September/580108.html https://gcc.gnu.org/pipermail/gcc-patches/2021-September/580107.html Thanks. -- H.J.
