Steven Rostedt wrote:
I expect that if I had a Gentoo system that I compiled for my machine,
this would be different. But I suspect that Debian still wants to run on
my old Pentium 75MHz laptop. How would libc know to use sysenter
instead of int 0x80. It could do a test of the system, but would there
be an if statement for every system call then? I guess that libc needs
to be compiled either to use it or not. Since there are still several
machines out there that don't have this feature, it would be safer to
not use it.
zach-dev2:~ $ ldd /bin/ls
linux-gate.so.1 => (0xffffe000)
This is the vsyscall entry point, which gets linked by ld into all
processes. It is a kernel page which is visible to user space, and is
rewritten to support sysenter if indeed that instruction is available.
Glibc has fixed entry points to this page. Here is a view of the system
call entry point on a machine which supports sysenter:
(gdb) break _init
Breakpoint 1 at 0x8049522
(gdb) run
Starting program: /bin/ls
(no debugging symbols found)...[Thread debugging using libthread_db enabled]
[New Thread 1075283616 (LWP 5328)]
[Switching to Thread 1075283616 (LWP 5328)]
Breakpoint 1, 0x08049522 in _init ()
(gdb) x/10i 0xffffe400
0xffffe400: push %ecx
0xffffe401: push %edx
0xffffe402: push %ebp
0xffffe403: mov %esp,%ebp
0xffffe405: sysenter
0xffffe407: nop
0xffffe408: nop
0xffffe409: nop
0xffffe40a: nop
0xffffe40b: nop
On a machine that does not support sysenter, this will give you:
int $0x80
ret
The int $0x80 system calls are still fully supported by a sysenter
capable kernel, since it must run older binaries and potentially support
syscalls during early boot up before it is known that sysenter is supported.
Zach
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