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RFC: Program Properties
There are cases where linker and run-time loader need more information
about ELF objects beyond what the current gABI provides:
1. Minimum ISAs. Executables and shared objects, which are optimized
specifically to run on a particular processor, will not run on processors
which don't support the same set of ISAs. Since x86 only has EM_IAMCU,
EM_386 and EM_X86_64 ELF machine codes, run-time loader needs additional
information to tell if an executable or a shared object is compatible
with available ISAs.
2. Stack size. Compilers may generate binaries which require larger stack
size than normal. If run-time loader can query the stack size required
by executable or shared object, it can increase stack size as needed.
3. Additional linking command line options embedded in relocatable object
files:
a. Additional libraries should be linked in when creating executable
or shared object.
http://sourceware.org/bugzilla/show_bug.cgi?id=12485
b. Additional compiler command line options are needed to properly
link LTO objects.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41756
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47785
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54231
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53777
4. Relocatable object files compiled against header files from a specific
version of a library must be linked with a compatible library when
creating executable or shared object.
GNU attributes
GNU binutils supports build attribute and run-time platform compatibility
data in relocatable object files. Issues with GNU attributes:
1. Many architectures, including x86, don't support GNU attributes:
a. There are some overlaps between GNU attributes and program
properties.
b. GNU attributes may be extended to embed additional linking command
line options in relocatable object files.
2. On x86, linking a relocatable object full of AVX instructions doesn't
always make the resulting executables or shared libraries to require AVX
to run since AVX functions may be called only via GNU_IFUNC at run-time.
Linker can't set minimum ISAs just from ISAs used by input relocatable
objects.
3. There is no program segment for GNU attributes in executables and
shared objects.
4. Most of attributes aren't applicable to run-time loader.
5. The format of GNU attributes isn't optimal for run-time loader. A
separate string table is used to store string attributes.
gABI support for program properties
To the "Special Sections" section, add:
Name Type Attributes
.note.GNU-properties SHT_NOTE SHF_ALLOC+SHF_GNU_PROPERTIES
#define SHF_GNU_PROPERTIES 0x00100000
Each SHF_GNU_PROPERTIES section starts with a property header and
followed by an array of properties. The property header has the following
structure:
typedef struct {
Elf_Word ph_kind;
unsigned char ph_padding[PH_PADDING];
Elf_SizeWord ph_propsz;
} Elf_Prophdr;
where ph_kind specifies the property kind:
#define NT_GNU_PROPERTIES_HEADER_KIND 0x0
and PH_PADDING, if necessary, aligns ph_propsz to 8 or 4-byte alignment
(depending on whether the file is a 64-bit or 32-bit object). Elf_SizeWord
is Elf32_Word in the 32-bit object or Elf64_Xword in the 64-bit object.
ph_propsz contains the size of the property array. Each array element
represents one property with type, data size and data. In 64-bit objects,
each element is an array of 8-byte words, whose first element is 4-byte
type and data size, in the format of the target processor. In 32-bit
objects, each element is an array of 4-byte words, whose first 2 elements
are 4-byte type and data size, in the format of the target processor. An
array element has the following structure:
typedef struct {
Elf_Word pr_type;
Elf_Word pr_datasz
unsigned char pr_data[PR_DATASZ];
unsigned char pr_padding[PR_PADDING];
} Elf_Prop;
where PR_DATASZ is the data size and PR_PADDING, if necessary, aligns
array element to 8 or 4-byte alignment (depending on whether the file
is a 64-bit or 32-bit object). The array elements are sorted by the
property type. The interpretation of property array depends on both
ph_kind and pr_type.
Types of program properties
#define NT_GNU_PROPERTIES_LOPROC 0xb0000000
#define NT_GNU_PROPERTIES_HIPROC 0xdfffffff
#define NT_GNU_PROPERTIES_LOUSER 0xe0000000
#define NT_GNU_PROPERTIES_HIUSER 0xffffffff
To the "Program Header" section, add a segment type PT_GNU_PROPERTIES
#define PT_GNU_PROPERTIES (PT_LOOS + 0x474e556)
Program header should contain only one PT_GNU_PROPERTIES segment, which
contains at least one .note.GNU-properties section.
Proposed properties
For NT_GNU_PROPERTIES_HEADER_KIND == 0:
#define NT_GNU_PROPERTIES_STACK_SIZE 1
Integer value for minimum stack size.
#define NT_GNU_PROPERTIES_REQUIRED_LIBRARY 2
String of the required library, NAMESPEC:SONAME. NAMESPEC is the string
after the linker -l option. SONAME is DT_SONAME of the required library.
#define NT_GNU_PROPERTIES_PROVIDED_LIBRARY 3
String of the provided library, NAMESPEC:SONAME. NAMESPEC is the string
after the linker -l option. SONAME is DT_SONAME of the provided library.
There is a conflict in linker inputs if
1. Any NT_GNU_PROPERTIES_REQUIRED_LIBRARY or
NT_GNU_PROPERTIES_PROVIDED_LIBRARY have the same NAMESPEC, but different
SONAMEs.
2. NAMESPEC in any NT_GNU_PROPERTIES_REQUIRED_LIBRARY matches an input
libNAMESPEC.so whose DT_SONAME is different from SONAME.
#define NT_GNU_PROPERTIES_GCC_OPTIONS 4
GCC options must be passed to GCC when used with GCC.
#define NT_GNU_PROPERTIES_X86_ISA_1_USED 0xc0000000
The x86 instruction sets indicated by the corresponding bits are used
in program. But their support in the hardware is optional.
#define NT_GNU_PROPERTIES_X86_ISA_1_NEEDED 0xc0000001
The x86 instruction sets indicated by the corresponding bits are used
in program and they must be supported by the hardware. A bit set in
NT_GNU_PROPERTIES_X86_ISA_1_NEEDED must also be set in
NT_GNU_PROPERTIES_X86_ISA_1_USED.
Integer value for the x86 instruction set support.
#define NT_GNU_PROPERTIES_X86_ISA_1_486 (1U << 0)
#define NT_GNU_PROPERTIES_X86_ISA_1_586 (1U << 1)
#define NT_GNU_PROPERTIES_X86_ISA_1_686 (1U << 2)
#define NT_GNU_PROPERTIES_X86_ISA_1_SSE (1U << 3)
#define NT_GNU_PROPERTIES_X86_ISA_1_SSE2 (1U << 4)
#define NT_GNU_PROPERTIES_X86_ISA_1_SSE3 (1U << 5)
#define NT_GNU_PROPERTIES_X86_ISA_1_SSSE3 (1U << 6)
#define NT_GNU_PROPERTIES_X86_ISA_1_SSE4_1 (1U << 7)
#define NT_GNU_PROPERTIES_X86_ISA_1_SSE4_2 (1U << 8)
#define NT_GNU_PROPERTIES_X86_ISA_1_AVX (1U << 9)
#define NT_GNU_PROPERTIES_X86_ISA_1_AVX2 (1U << 10)
#define NT_GNU_PROPERTIES_X86_ISA_1_AVX512F (1U << 11)
#define NT_GNU_PROPERTIES_X86_ISA_1_AVX512CD (1U << 12)
#define NT_GNU_PROPERTIES_X86_ISA_1_AVX512ER (1U << 13)
#define NT_GNU_PROPERTIES_X86_ISA_1_AVX512PF (1U << 14)
#define NT_GNU_PROPERTIES_X86_ISA_1_AVX512VL (1U << 15)
#define NT_GNU_PROPERTIES_X86_ISA_1_AVX512DQ (1U << 16)
#define NT_GNU_PROPERTIES_X86_ISA_1_AVX512BW (1U << 17)