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? Thanks. -- H.J.
