v2: RFC: Add GNU_PROPERTY_1_GLIBC_2_NEEDED

[H.J. Lu]

Here is the v2 proposal.

-- 
H.J.
--
Motivations:

1. Some binaries which require new ELF features, like DT_RELR, only
work with the new glibc binary.  They crash at run-time with the older
glibc binaries.
2. Somes binaries compiled with the new language features, like C2X
printf specifiers, only generate correct results with the new glibc
binary.  Since we don't add new glibc versions to the printf function
family, they generate incorrect results at run-time with the older
glibc binaries.

Here is a proposal to encode glibc version dependencies in relocatable
objects:

/* The needed glibc 2 minor version property for the object file.  */
#define GNU_PROPERTY_1_GLIBC_2_NEEDED   (GNU_PROPERTY_UINT32_OR_LO + 1)

/* The needed glibc 2 minor base version.  */
#define GNU_PROPERTY_1_GLIBC_2_NEEDED_MINOR_BASE 35

/* Set if the object file requires glibc 2 minor version M.  */
#define GNU_PROPERTY_1_GLIBC_2_NEEDED_MINOR_VERSION(m)  \
  (1U << ((m) - GNU_PROPERTY_1_GLIBC_2_NEEDED_MINOR_BASE))

Linker adds glibc versions in GNU_PROPERTY_1_GLIBC_2_NEEDED to the
.gnu.version_r section and removes GNU_PROPERTY_1_GLIBC_2_NEEDED note
when generating shared libraries and executables.

[hjl@gnu-cfl-2 elfvers-1]$ ./readelf -n x.o

Displaying notes found in: .note.gnu.property
  Owner                Data size Description
  GNU                  0x00000020 NT_GNU_PROPERTY_TYPE_0
      Properties: x86 ISA used:
x86 feature used: x86
[hjl@gnu-cfl-2 elfvers-1]$ ./readelf -n glibc-2-minor-1.o

Displaying notes found in: .note.gnu.property
  Owner                Data size Description
  GNU                  0x00000010 NT_GNU_PROPERTY_TYPE_0
      Properties: 1_glibc_2_needed: 2.35, 2.38
  GNU                  0x00000020 NT_GNU_PROPERTY_TYPE_0
      Properties: x86 ISA used:
x86 feature used: x86
[hjl@gnu-cfl-2 elfvers-1]$ make x
gcc -B./ -o x x.o glibc-2-minor-1.o
[hjl@gnu-cfl-2 elfvers-1]$ ./readelf -n --version-info x
Version symbols section '.gnu.version' contains 4 entries:
 Addr: 0x00000000004004ae  Offset: 0x0004ae  Link: 6 (.dynsym)
  000:   0 (*local*)       2 (GLIBC_2.34)    3 (GLIBC_2.2.5)   1 (*global*)

Version needs section '.gnu.version_r' contains 1 entry:
 Addr: 0x00000000004004b8  Offset: 0x0004b8  Link: 7 (.dynstr)
  000000: Version: 1  File: libc.so.6  Cnt: 4
  0x0010:   Name: GLIBC_2.38  Flags: none  Version: 5
  0x0020:   Name: GLIBC_2.35  Flags: none  Version: 4
  0x0030:   Name: GLIBC_2.2.5  Flags: none  Version: 3
  0x0040:   Name: GLIBC_2.34  Flags: none  Version: 2
...
[hjl@gnu-cfl-2 elfvers-1]$ ./x
./x: /lib64/libc.so.6: version `GLIBC_2.38' not found (required by ./x)
./x: /lib64/libc.so.6: version `GLIBC_2.35' not found (required by ./x)
[hjl@gnu-cfl-2 elfvers-1]$