For some reason old binutils genertate larger headers so increase the text
offset of the vdso to avoid linker errors.
Roland McGrath explains:
"There are extra symbols in the '.dynsym' section that are responsible
for the size difference (They also cause corresponding inflation in
'.gnu.version')
Older ld's wrongly generated these unneeded symbols in .dynsym. This
was fixed not all that long ago (2006); binutils-2.17.50.0.6 might be
the first fixed version, but I have not verified for sure where the
cutoff was.
The unneeded symbols et al from old ld add almost 700 bytes excess.
This limits fairly tightly the amount by which the actual text and
data in the vDSO can grow in the future without pushing the whole
file over 4kb. If it does grow later on, we should consider changing
the layout with a config option or something to pack it better
without that padding, when building the kernel with newer binutils."
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Roland McGrath <roland@redhat.com>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This implements new vDSO for x86-64. The concept is similar
to the existing vDSOs on i386 and PPC. x86-64 has had static
vsyscalls before, but these are not flexible enough anymore.
A vDSO is a ELF shared library supplied by the kernel that is mapped into
user address space. The vDSO mapping is randomized for each process
for security reasons.
Doing this was needed for clock_gettime, because clock_gettime
always needs a syscall fallback and having one at a fixed
address would have made buffer overflow exploits too easy to write.
The vdso can be disabled with vdso=0
It currently includes a new gettimeofday implemention and optimized
clock_gettime(). The gettimeofday implementation is slightly faster
than the one in the old vsyscall. clock_gettime is significantly faster
than the syscall for CLOCK_MONOTONIC and CLOCK_REALTIME.
The new calls are generally faster than the old vsyscall.
Advantages over the old x86-64 vsyscalls:
- Extensible
- Randomized
- Cleaner
- Easier to virtualize (the old static address range previously causes
overhead e.g. for Xen because it has to create special page tables for it)
Weak points:
- glibc support still to be written
The VM interface is partly based on Ingo Molnar's i386 version.
Includes compile fix from Joachim Deguara
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Andi Kleen