[Gllug] Strange memory mapping

[Nix]

On Thu, 9 Jun 2005, Richard Jones murmured woefully:
> 
> Any idea what this memory mapping is?  It appears at the end of all my
> process memory maps:
> 
> ffffe000-fffff000 ---p 00000000 00:00 0 

ldd will show you e.g.

	linux-gate.so.1 =>  (0xffffe000)
[...]

Recognise it?

It's the vsyscall area, a fake `shared library' exported to all
processes by the kernel and filled with suitable instructions to execute
each syscall, and with suitable DWARF2 exception-unwinding stuff.

It allows applications (in practice, glibc) to call syscalls by means of
an ordinary (cross-shared-object) function call, and the fastest
available method fo the platform the binary is *running* on will be
used, because that `function' is implemented by the kernel.

(This is significant because the 686 and above have a means of calling
syscalls via SYSENTER which is very much faster than the old INT 80
method.)

It's also possible for read-only data which benefits from very high
access speeds to be mapped there, e.g. data used by gettimeofday():
but I'm not aware of anything doing so yet.

See e.g. linux/arch/i386/kernel/vsyscall* for the implementation on
the kernel side.

-- 
`It's as bizarre an intrusion as, I don't know, the hobbits coming home
 to find that the Shire has been taken over by gangsta rappers.'
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