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    SubjectRe: [GIT pull] x86 vdso updates
    On Sun, May 29, 2011 at 11:10 AM, richard -rw- weinberger
    <> wrote:
    > On Sun, May 29, 2011 at 4:57 PM, Andrew Lutomirski <> wrote:
    >> On Sun, May 29, 2011 at 5:51 AM, richard -rw- weinberger
    >> <> wrote:
    >>> Yesterday I had a closer look at 64bit UML.
    >>> Glibc is always using vsyscalls because 64bit UML does not support the vDSO.
    >>> On 32bit UML simply scans the ELF auxiliary vector provided by the host to
    >>> get the address of the vDSO.
    >>> How can I get this address on a 64bit host?
    >> I believe it's exactly the same.  There's an auxv entry that points to the vDSO.
    > I don't think so.
    > See:
    > Section "Address space randomization".
    > The demo program finds the vDSO only on x86.
    > UML uses quite the same method to find it.
    > arch/um/os-Linux/elf_aux.c

    The attached program works for me.

    I don't know what this is supposed to mean, though:

    /* See if the page is under TASK_SIZE */
    if (vsyscall_ehdr < (unsigned long) envp)
    vsyscall_ehdr = 0;

    First, envp != TASK_SIZE.

    Second, the vDSO can be wherever it wants. On current kernels at
    least it is *always* mapped below TASK_SIZE (in the unsigned sense)
    because it's mapped into user address space.

    #include <unistd.h>
    #include <string.h>
    #include <stdio.h>
    #include <elf.h>
    #include <stdlib.h>

    extern char **environ;

    int __captured_argc;
    char **__captured_argv;
    char **__captured_envp;
    Elf64_auxv_t *__captured_auxv;

    void _capture_args(int argc, char **argv, char **envp)
    __captured_argc = argc;
    __captured_argv = argv;
    __captured_envp = envp;


    __captured_auxv = (Elf64_auxv_t*)(envp + 1);

    static void * initarray_entry __attribute__((section(".preinit_array"), used)) =

    int main()
    printf("envp = %p\n", __captured_envp);

    for (int i = 0; __captured_auxv[i].a_type != AT_NULL; i++)
    char *desc = "";
    if (__captured_auxv[i].a_type == AT_SYSINFO)
    desc = "AT_SYSINFO";
    else if (__captured_auxv[i].a_type == AT_SYSINFO_EHDR)
    desc = "AT_SYSINFO_EHDR";
    printf("aux type %02u = 0x%016lX %s\n",
    __captured_auxv[i].a_type, (long)__captured_auxv[i].a_un.a_val,
    return 0;
     \ /
      Last update: 2011-05-29 17:31    [W:0.025 / U:2.536 seconds]
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