GNU indirect function

GNU indirect function (ifunc) is a mechanism making a direct function call resolve to an implementation picked by a resolver. It is mainly used in glibc but has adoption in FreeBSD.

For some performance critical functions, e.g. memcpy/memset/strcpy, glibc provides multiple implementations optimized for different architecture levels. The application just uses memcpy(...) which compiles to call memcpy. The linker will create a PLT for memcpy and produce an associated special dynamic relocation referencing the resolver symbol/address. During relocation resolving at runtime, the return value of the resolver will be placed in the GOT entry and the PLT entry will load the address.

On Mach-O, there is similar feature, N_SYMBOL_RESOLVER.

Representation

ifunc has a dedicated symbol type STT_GNU_IFUNC to mark it different from a regular function (STT_FUNC). The value 10 is in the OS-specific range (10~12). readelf -s tell you that the symbol is ifunc if OSABI is ELFOSABI_GNU or ELFOSABI_FREEBSD.

On Linux, by default GNU as uses ELFOSABI_NONE (0). If ifunc is used, the OSABI will be changed to ELFOSABI_GNU. Similarly, GNU ld sets the OSABI to ELFOSABI_GNU if ifunc is used. gold does not do this PR17735.

Things are loose in LLVM. The integrated assembler and LLD do not set ELFOSABI_GNU. Currently the only problem I know is the readelf -s display. Everything else works fine.

Assembler behavior

In assembly, you can assign the type STT_GNU_IFUNC to a symbol via .type foo, @gnu_indirect_function. An ifunc symbol is typically STB_GLOBAL.

In the object file, st_shndx and st_value of an STT_GNU_IFUNC symbol indicate the resolver. After linking, if the symbol is still STT_GNU_IFUNC, its st_value field indicates the resolver address in the linked image.

Assemblers usually convert relocations referencing a local symbol to reference the section symbol, but this behavior needs to be inhibited for STT_GNU_IFUNC.

Example

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cat > b.s <<e
.global ifunc
.type ifunc, @gnu_indirect_function
.set ifunc, resolver

resolver:
leaq impl(%rip), %rax
ret

impl:
movq $42, %rax
ret
e

cat > a.c <<e
int ifunc(void);
int main() { return ifunc(); }
e

cc a.c b.s
./a.out # exit code 42

GNU as makes transitive aliases to an STT_GNU_IFUNC ifunc as well.

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.type foo,@gnu_indirect_function
.set foo, foo_resolver

.set foo2, foo
.set foo3, foo2

GCC and Clang support a function attribute which emits .type ifunc, @gnu_indirect_function; .set ifunc, resolver:

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static int impl(void) { return 42; }
static void *resolver(void) { return impl; }
void *ifunc(void) __attribute__((ifunc("resolver")));

Preemptible ifunc

A preemptible ifunc call is no different from a regular function call from the linker perspective.

The linker creates a PLT entry, reserves an associated GOT entry, and emits an R_*_JUMP_SLOT relocation resolving the address into the GOT entry. The PLT code sequence is the same as a regular PLT for STT_FUNC.

If the ifunc is defined within the module, the symbol type in the linked image is STT_GNU_IFUNC, otherwise (defined in a DSO), the symbol type is STT_FUNC.

The difference resides in the loader.

At runtime, the relocation resolver checks whether the R_*_JUMP_SLOT relocation refers to an ifunc. If it does, instead of filling the GOT entry with the target address, the resolver calls the target address as an indirect function, with ABI specified additional parameters (hwcap related), and places the return value into the GOT entry.

Non-preemptible ifunc

The non-preemptible ifunc case is where all sorts of complexity come from.

First, the R_*_JUMP_SLOT relocation type cannot be used in some cases:

  • A non-preemptible ifunc may not have a dynamic symbol table entry. It can be local. It can be defined in the executable without the need to export.
  • A non-local STV_DEFAULT symbol defined in a shared object is by default preemptible. Using R_*_JUMP_SLOT for such a case will make the ifunc look like preemptible.

Therefore a new relocation type R_*_IRELATIVE was introduced. There is no associated symbol and the address indicates the resolver.

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R_*_RELATIVE: B + A
R_*_IRELATIVE: call (B + A) as a function
R_*_JUMP_SLOT: S

When an R_*_JUMP_SLOT can be used, there is a trade-off between R_*_JUMP_SLOT and R_*_IRELATIVE: an R_*_JUMP_SLOT can be lazily resolved but needs a symbol lookup. Currently powerpc can use R_PPC64_JMP_SLOT in some cases PR27203.

A PLT entry is needed for two reasons:

  • The call sites emit instructions like call foo. We need to forward them to a place to perform the indirection. Text relocations are usually not an option (exception: {ifunc-noplt}).
  • If the ifunc is exported, we need a place to mark its canonical address.

Such PLT entries are sometimes referred to as IPLT. They are placed in the synthetic section .iplt. In GNU ld, .iplt will be placed in the output section .plt. In LLD, I decided that .iplt is better https://reviews.llvm.org/D71520.

On many architectures (e.g. AArch64/PowerPC/x86), the PLT code sequence is the same as a regular PLT, but it could be different.

On x86-64, the code sequence is:

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jmp *got(%rip)
pushq $0
jmp .plt

Since there is no lazy binding, pushq $0; jmp .plt are not needed. However, to make all PLT entries of the same shape to simplify linker implementations and facilitate analyzers, it is find to keep it this way.

PowerPC32 -msecure-plt IPLT

As a design to work around the lack of PC-relative instructions, PowerPC32 uses multiple GOT sections, one per file in .got2. To support multiple GOT pointers, the addend on each R_PPC_PLTREL24 reloc will have the offset within .got2.

-msecure-plt has small/large PIC differences.

  • -fpic/-fpie: R_PPC_PLTREL24 r_addend=0. The call stub loads an address relative to _GLOBAL_OFFSET_TABLE_.
  • -fPIC/-fPIE: R_PPC_PLTREL24 r_addend=0x8000. (A partial linked object file may have an addend larger than 0x8000.) The call stub loads an address relative to .got2+0x8000.

If a non-preemptible ifunc is referenced in two object files, in -pie/-shared mode, the two object files cannot share the same IPLT entry. When I added non-preemptible ifunc support for PowerPC32 to LLD https://reviews.llvm.org/D71621, I did not handle this case.

.rela.dyn vs .rela.plt

LLD placed R_*_IRELATIVE in the .rela.plt section because many ports of GNU ld behaved this way. While implementing ifunc for PowerPC, I noticed that GNU ld powerpc actually places R_*_IRELATIVE in .rela.dyn and glibc powerpc does not actually support R_*_IRELATIVE in .rela.plt. This makes a lot of sense to me because .rela.plt normally just contains R_*_JUMP_SLOT which can be lazily resolved. ifunc relocations need to be eagerly resolved so .rela.plt was a misplace. Therefore I changed LLD to use .rela.dyn in https://reviews.llvm.org/D65651.

__rela_iplt_start and __rela_iplt_end

A statically linked position dependent executable traditionally had no dynamic relocations.

With ifunc, these R_*_IRELATIVE relocations must be resolved at runtime. Such relocations are in a magic array delimitered by __rela_iplt_start and __rela_iplt_end. In glibc, csu/libc-start.c has special code processing the relocation range.

GNU ld and gold define __rela_iplt_start in -no-pie mode, but not in -pie mode. LLD defines __rela_iplt_start regardless of -no-pie, -pie or -shared.

In glibc, static pie uses self-relocation (_dl_relocate_static_pie) to take care of R_*_IRELATIVE. The above magic array code is executed by static pie as well. If __rela_iplt_start/__rela_iplt_end are defined, we will get 0 < __rela_iplt_start < __rela_iplt_end in csu/libc-start.c. ARCH_SETUP_IREL will crash when resolving the first relocation which has been processed.

I think the difference in the diff -u =(ld.bfd --verbose) =(ld.bfd -pie --verbose) output is unneeded. https://sourceware.org/pipermail/libc-alpha/2021-January/121755.html

Address significance

A non-GOT-generating non-PLT-generating relocation referencing a STT_GNU_IFUNC indicates a potential address-taken operation.

With a function attribute, the compilers knows that a symbol indicates an ifunc and will avoid generating such relocations. With assembly such relocations may be unavoidable.

In most cases the linker needs to convert the symbol type to STT_FUNC and create a special PLT entry, which is called a "canonical PLT entry" in LLD. References from other modules will resolve to the PLT entry to keep pointer equality: the address taken from the defining module should match the address taken from another module.

This approach has pros and cons:

  • With a canonical PLT entry, the resolver of a symbol is called only once. There is exactly one R_*_IRELATIVE relocation.
  • If the relocation appears in a non-SHF_WRITE section, a text relocation can be avoided.
  • Relocation types which are not valid dynamic relocation types are supported. GNU ld may error relocation R_X86_64_PC32 against STT_GNU_IFUNC symbol `ifunc' isn't supported
  • References will bind to the canonical PLT entry. A function call needs to jump to the PLT, loads the value from the GOT, then does an indirect call.

For a symbolic relocation type (a special case of absolute relocation types where the width matches the word size) like R_X86_64_64, when the addend is 0 and the section has the SHF_WRITE flag, the linker can emit an R_X86_64_IRELATIVE. https://reviews.llvm.org/D65995 dropped the case.

For the following example, GNU ld linked a.out calls fff_resolver three times while LLD calls it once.

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// RUN: split-file %s %t
// RUN: clang -fuse-ld=bfd -fpic %t/dso.c -o %t/dso.so --shared
// RUN: clang -fuse-ld=bfd %t/main.c %t/dso.so -o %t/a.out
// RUN: %t/a.out

//--- dso.c
typedef void fptr(void);
extern void fff(void);

fptr *global_fptr0 = &fff;
fptr *global_fptr1 = &fff;

//--- main.c
#include <stdio.h>

static void fff_impl() { printf("fff_impl()\n"); }
static int z;
void *fff_resolver() { return (char *)&fff_impl + z++; }

__attribute__((ifunc("fff_resolver"))) void fff();
typedef void fptr(void);
fptr *local_fptr = fff;
extern fptr *global_fptr0, *global_fptr1;

int main() {
printf("local %p global0 %p global1 %p\n", local_fptr, global_fptr0, global_fptr1);
return 0;
}

Relocation resolving order

R_*_IRELATIVE relocations are resolved eagerly. In glibc, there used to be a problem where ifunc resolvers ran before GL(dl_hwcap) and GL(dl_hwcap2) were set up https://sourceware.org/bugzilla/show_bug.cgi?id=27072.

For the relocation resolver, the main executable needs to be processed the last to process R_*_COPY. Without ifunc, the resolving order of shared objects can be arbitrary.

For ifunc, if the ifunc is defined in a processed module, it is fine. If the ifunc is defined in an unprocessed module, it may crash.

For an ifunc defined in an executable, calling it from a shared object can be problematic because the executable's relocations haven't been resolved. The issue can be circumvented by converting the non-preemptible ifunc defined in the executable to STT_FUNC. GNU ld's x86 port made the change PR23169.

-z ifunc-noplt

Mark Johnston introduced -z ifunc-noplt for FreeBSD https://reviews.llvm.org/D61613. With this option, all relocations referencing STT_GNU_IFUNC will be emitted as dynamic relocations (if .dynsym is created). The canonical PLT entry will not be used.

Miscellaneous

GNU ld has implemented a diagnostic ("i686 ifunc and non-default symbol visibility") to flag R_386_PC32 referencing non-default visibility ifunc in -pie and -shared links. This diagnostic looks like the most prominent reason blocking my proposal to use R_386_PLT32 for call/jump foo. See Copy relocations, canonical PLT entries and protected visibility for details.

https://sourceware.org/glibc/wiki/GNU_IFUNC misses a lot of information. There are quite a few arch differences. I asked for clarification https://sourceware.org/pipermail/libc-alpha/2021-January/121752.html

Dynamic loader

In glibc, _dl_runtime_resolver needs to save and restore vector and floating point registers. ifunc resolvers add another reason that _dl_runtime_resolver cannot only use integer registers. (The other reasons are that ld.so has string function calls which may use vectors and external calls to libc.so.)