Re: Need help in understanding x86 syscall

From: Steven Rostedt (rostedt_at_goodmis.org)
Date: 08/11/05

  • Next message: linux-os (*** Johnson): "Re: Need help in understanding x86 syscall" 
    To: Zachary Amsden <zach@vmware.com>
Date:	Thu, 11 Aug 2005 14:17:28 -0400

    On Thu, 2005-08-11 at 10:59 -0700, Zachary Amsden wrote:
    >
    > zach-dev2:~ $ ldd /bin/ls
    > linux-gate.so.1 => (0xffffe000)

    OHHH! So THAT is what linux-gate is used for! Thanks, I've been really
    confused by that.

    >
    > 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
    >

    OK, I get the same on my machine.

    > 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.

    Now is the latest glibc using this. Since I put in a ud2 op in my
    sysenter_entry code, which is not triggered, as well as an objdump of
    libc.so shows a bunch of int 0x80 calls.

    -- Steve

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