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    Subject[PATCH 1/1] tracing: Introduce "pseudo registers" for FETCH_MTD_reg
    The probe can dump the registers or memory, but it is not possible
    to dump, say, current->pid. This patch adds the pseudo regs table,
    currently it has only two methods to get current/smp_processor_id
    but it can be trivially extended.

    This syntax is %%pseudo-reg-name, I agree in advance with any other
    naming.

    Test-case: 452 == offsetof(task_struct, pid), 39 == __NR_getpid,

    # perf probe 'sys_getpid%return ret=$retval:s32 pid=+452(%%current):s32'
    # perf record -e probe:sys_getpid perl -e 'syscall 39'
    # perf --no-pager script | tail -1
    perl 586 [001] 753.102549: probe:sys_getpid: \
    (ffffffff81052c00 <- ffffffff8134d012) ret=586 pid=586

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    ---
    kernel/trace/trace_probe.c | 57 +++++++++++++++++++++++++++++++++++++++++--
    1 files changed, 54 insertions(+), 3 deletions(-)

    diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
    index 412e959..e167c0a 100644
    --- a/kernel/trace/trace_probe.c
    +++ b/kernel/trace/trace_probe.c
    @@ -109,13 +109,14 @@ DEFINE_FETCH_##method(u64)
    (FETCH_FUNC_NAME(method, string_size) == fn)) \
    && (fn != NULL))

    +static unsigned long probe_get_register(struct pt_regs *, unsigned long);
    +
    /* Data fetch function templates */
    #define DEFINE_FETCH_reg(type) \
    static __kprobes void FETCH_FUNC_NAME(reg, type)(struct pt_regs *regs, \
    void *offset, void *dest) \
    { \
    - *(type *)dest = (type)regs_get_register(regs, \
    - (unsigned int)((unsigned long)offset)); \
    + *(type *)dest = (type)probe_get_register(regs, (long)offset); \
    }
    DEFINE_BASIC_FETCH_FUNCS(reg)
    /* No string on the register */
    @@ -548,6 +549,52 @@ static int parse_probe_vars(char *arg, const struct fetch_type *t,
    return ret;
    }

    +#define PSEUDO_REG_OFFSET 4096 /* arbitrary value > MAX_REG_OFFSET */
    +
    +static unsigned long pseudo_reg_cpu(void)
    +{
    + return (unsigned long)raw_smp_processor_id();
    +}
    +
    +static unsigned long pseudo_reg_current(void)
    +{
    + return (unsigned long)current;
    +}
    +
    +static struct {
    + const char *name;
    + unsigned long (*fetch)(void);
    +}
    +const pseudo_reg_table[] = {
    + {
    + .name = "cpu",
    + .fetch = pseudo_reg_cpu,
    + },
    + {
    + .name = "current",
    + .fetch = pseudo_reg_current,
    + },
    +};
    +
    +static unsigned long probe_get_register(struct pt_regs *regs, unsigned long offset)
    +{
    + if (offset < PSEUDO_REG_OFFSET)
    + return regs_get_register(regs, offset);
    +
    + return pseudo_reg_table[offset - PSEUDO_REG_OFFSET].fetch();
    +}
    +
    +static int pseudo_reg_query_offset(const char *name)
    +{
    + int nr;
    +
    + for (nr = 0; nr < ARRAY_SIZE(pseudo_reg_table); ++nr)
    + if (strcmp(pseudo_reg_table[nr].name, name) == 0)
    + return PSEUDO_REG_OFFSET + nr;
    +
    + return -EINVAL;
    +}
    +
    /* Recursive argument parser */
    static int parse_probe_arg(char *arg, const struct fetch_type *t,
    struct fetch_param *f, bool is_return, bool is_kprobe)
    @@ -569,7 +616,11 @@ static int parse_probe_arg(char *arg, const struct fetch_type *t,
    break;

    case '%': /* named register */
    - ret = regs_query_register_offset(arg + 1);
    + if (arg[1] == '%')
    + ret = pseudo_reg_query_offset(arg + 2);
    + else
    + ret = regs_query_register_offset(arg + 1);
    +
    if (ret >= 0) {
    f->fn = t->fetch[FETCH_MTD_reg];
    f->data = (void *)(unsigned long)ret;
    --
    1.5.5.1



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