gABI extension proposal: PT_SHMMAP

["H. Peter Anvin" <hpa@zytor.com>]

Hello,

I would like to propose an extension to the ELF gABI, which is analogous
to PT_LOAD except that it requests that the underlying file be
memory-mapped with MAP_SHARED, rather than MAP_PRIVATE as used for
PT_LOAD.  PT_SHARED or PT_SHLOAD would be other possible names.

The main motivation for this is to allow the Linux kernel vDSO to
finally become as close to an "ordinary" ELF DSO as is possible.  The
vDSO depends on having its kernel-provided data page(s) at a specific
offset from the vDSO code; this is currently done by ad hoc memory
areas, which has a number of problems, especially for mixed-mode programs.

The idea is to convert the vdso to a proper file in one of the
kernel-provided filesystems (e.g. /proc or /sys); by defining this type,
an ELF parser would be able to create the appropriate mappings without
any ad hoc code.

This may be usable for other operating systems or perhaps even in
userspace.  However, if there is no such interest then it would be
possible for Linux to use one of the PT_*OS constants; however, I
generally believe it is better to try to be as inclusive as possible.

	-hpa