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    Subject[2.6.20.16 review 26/28] i386: fix infinite loop with singlestep int80 syscalls
    The commit 635cf99a80f4ebee59d70eb64bb85ce829e4591f introduced a
    regression. Executing a ptrace single step after certain int80
    accesses will infinitely loop and never advance the PC.

    The TIF_SINGLESTEP check should be done on the return from the syscall
    and not before it.

    The new test case is below:

    /* Test whether singlestep through an int80 syscall works.
    */
    #define _GNU_SOURCE
    #include <stdio.h>
    #include <unistd.h>
    #include <fcntl.h>
    #include <sys/ptrace.h>
    #include <sys/wait.h>
    #include <sys/mman.h>
    #include <asm/user.h>
    #include <string.h>

    static int child, status;
    static struct user_regs_struct regs;

    static void do_child()
    {
    char str[80] = "child: int80 test
    ";

    ptrace(PTRACE_TRACEME, 0, 0, 0);
    kill(getpid(), SIGUSR1);
    write(fileno(stdout),str,strlen(str));
    asm ("int $0x80" : : "a" (20)); /* getpid */
    }

    static void do_parent()
    {
    unsigned long eip, expected = 0;
    again:
    waitpid(child, &status, 0);
    if (WIFEXITED(status) || WIFSIGNALED(status))
    return;

    if (WIFSTOPPED(status)) {
    ptrace(PTRACE_GETREGS, child, 0, &regs);
    eip = regs.eip;
    if (expected)
    fprintf(stderr, "child stop @ %08lx, expected %08lx %s
    ",
    eip, expected,
    eip == expected ? "" : " <== ERROR");

    if (*(unsigned short *)eip == 0x80cd) {
    fprintf(stderr, "int 0x80 at %08x
    ", (unsigned int)eip);
    expected = eip + 2;
    } else
    expected = 0;

    ptrace(PTRACE_SINGLESTEP, child, NULL, NULL);
    }
    goto again;
    }

    int main(int argc, char * const argv[])
    {
    child = fork();
    if (child)
    do_parent();
    else
    do_child();
    return 0;
    }

    Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
    Cc: Jeremy Fitzhardinge <jeremy@goop.org>
    Cc: Chuck Ebbert <cebbert@redhat.com>
    Signed-off-by: Chris Wright <chrisw@sous-sol.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
    ---
    arch/i386/kernel/entry.S | 8 ++++----
    1 files changed, 4 insertions(+), 4 deletions(-)

    diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S
    index 5e47683..9bf056e 100644
    --- a/arch/i386/kernel/entry.S
    +++ b/arch/i386/kernel/entry.S
    @@ -367,10 +367,6 @@ ENTRY(system_call)
    CFI_ADJUST_CFA_OFFSET 4
    SAVE_ALL
    GET_THREAD_INFO(%ebp)
    - testl $TF_MASK,PT_EFLAGS(%esp)
    - jz no_singlestep
    - orl $_TIF_SINGLESTEP,TI_flags(%ebp)
    -no_singlestep:
    # system call tracing in operation / emulation
    /* Note, _TIF_SECCOMP is bit number 8, and so it needs testw and not testb */
    testw $(_TIF_SYSCALL_EMU|_TIF_SYSCALL_TRACE|_TIF_SECCOMP|_TIF_SYSCALL_AUDIT),TI_flags(%ebp)
    @@ -385,6 +381,10 @@ syscall_exit:
    # setting need_resched or sigpending
    # between sampling and the iret
    TRACE_IRQS_OFF
    + testl $TF_MASK,PT_EFLAGS(%esp) # If tracing set singlestep flag on exit
    + jz no_singlestep
    + orl $_TIF_SINGLESTEP,TI_flags(%ebp)
    +no_singlestep:
    movl TI_flags(%ebp), %ecx
    testw $_TIF_ALLWORK_MASK, %cx # current->work
    jne syscall_exit_work
    --
    1.5.2.4
    --
    -
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