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    SubjectRe: [PATCH 08/12] add trace events for each syscall entry/exit
    On Mon, Aug 10, 2009 at 04:52:47PM -0400, Jason Baron wrote:
    > +void ftrace_syscall_enter(struct pt_regs *regs, long id)
    > {
    > struct syscall_trace_enter *entry;
    > struct syscall_metadata *sys_data;
    > @@ -150,6 +105,8 @@ void ftrace_syscall_enter(struct pt_regs *regs)
    > int syscall_nr;
    >
    > syscall_nr = syscall_get_nr(current, regs);
    > + if (!test_bit(syscall_nr, enabled_enter_syscalls))
    > + return;
    >
    > sys_data = syscall_nr_to_meta(syscall_nr);
    > if (!sys_data)

    > +void ftrace_syscall_exit(struct pt_regs *regs, long ret)
    > {
    > struct syscall_trace_exit *entry;
    > struct syscall_metadata *sys_data;
    > @@ -178,6 +135,8 @@ void ftrace_syscall_exit(struct pt_regs *regs)
    > int syscall_nr;
    >
    > syscall_nr = syscall_get_nr(current, regs);
    > + if (!test_bit(syscall_nr, enabled_exit_syscalls))
    > + return;
    Most arch syscall_get_nr() implementations returns -1 if the syscall
    number is not valid. Accessing the bit field without a check might
    result in a kernel oops (at least I saw it on s390 for ftrace selftest).

    Before this change, this problem did not occur, because the invalid
    syscall number (-1) caused syscall_nr_to_meta() to return NULL.

    There are at least two scenarios where syscall_get_nr() can return -1:

    1. For example, ptrace stores an invalid syscall number, and thus,
    tracing code resets it.
    (see do_syscall_trace_enter in arch/s390/kernel/ptrace.c)

    2. The syscall_regfunc() (kernel/tracepoint.c) sets the TIF_SYSCALL_FTRACE
    (now: TIF_SYSCALL_TRACEPOINT) flag for all threads which includes
    kernel threads.
    However, the ftrace selftest triggers a kernel oops when testing syscall
    trace points:
    - The kernel thread is started as ususal (do_fork()),
    - tracing code sets TIF_SYSCALL_FTRACE,
    - the ret_from_fork() function is triggered and starts
    ftrace_syscall_exit() with an invalid syscall number.

    To avoid these scenarios, I suggest to check the syscall_nr.

    For instance, the ftrace selftest fails for s390 (with config option
    CONFIG_FTRACE_SYSCALLS set) and produces the following kernel oops.

    Unable to handle kernel pointer dereference at virtual kernel address 2000000000

    Oops: 0038 [#1] PREEMPT SMP
    Modules linked in:
    CPU: 0 Not tainted 2.6.31-rc6-next-20090819-dirty #18
    Process kthreadd (pid: 818, task: 000000003ea207e8, ksp: 000000003e813eb8)
    Krnl PSW : 0704100180000000 00000000000ea54c (ftrace_syscall_exit+0x58/0xdc)
    R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:0 CC:1 PM:0 EA:3
    Krnl GPRS: 0000000000000000 00000000000e0000 ffffffffffffffff 20000000008c2650
    0000000000000007 0000000000000000 0000000000000000 0000000000000000
    0000000000000000 0000000000000000 ffffffffffffffff 000000003e813d78
    000000003e813f58 0000000000505ba8 000000003e813e18 000000003e813d78
    Krnl Code: 00000000000ea540: e330d0000008 ag %r3,0(%r13)
    00000000000ea546: a7480007 lhi %r4,7
    00000000000ea54a: 1442 nr %r4,%r2
    >00000000000ea54c: e31030000090 llgc %r1,0(%r3)
    00000000000ea552: 5410d008 n %r1,8(%r13)
    00000000000ea556: 8a104000 sra %r1,0(%r4)
    00000000000ea55a: 5410d00c n %r1,12(%r13)
    00000000000ea55e: 1211 ltr %r1,%r1
    Call Trace:
    ([<0000000000000000>] 0x0)
    [<000000000001fa22>] do_syscall_trace_exit+0x132/0x18c
    [<000000000002d0c4>] sysc_return+0x0/0x8
    [<000000000001c738>] kernel_thread_starter+0x0/0xc
    Last Breaking-Event-Address:
    [<00000000000ea51e>] ftrace_syscall_exit+0x2a/0xdc

    Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
    ---
    kernel/trace/trace_syscalls.c | 4 ++++
    1 file changed, 4 insertions(+)

    --- a/kernel/trace/trace_syscalls.c
    +++ b/kernel/trace/trace_syscalls.c
    @@ -224,6 +224,8 @@ void ftrace_syscall_enter(struct pt_regs
    int syscall_nr;

    syscall_nr = syscall_get_nr(current, regs);
    + if (syscall_nr < 0)
    + return;
    if (!test_bit(syscall_nr, enabled_enter_syscalls))
    return;

    @@ -254,6 +256,8 @@ void ftrace_syscall_exit(struct pt_regs
    int syscall_nr;

    syscall_nr = syscall_get_nr(current, regs);
    + if (syscall_nr < 0)
    + return;
    if (!test_bit(syscall_nr, enabled_exit_syscalls))
    return;


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