[Bug kprobes/15675] New: A misusing of function @module_kallsyms_on_each_symbo cause the stap generated module can not resolve the module symbols.

[Bug kprobes/15675] New: A misusing of function @module_kallsyms_on_each_symbo cause the stap generated module can not resolve the module symbols.

[yangwen at huawei dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15675

            Bug ID: 15675
           Summary: A misusing of function @module_kallsyms_on_each_symbo
                    cause the stap generated module can not resolve the
                    module symbols.
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: critical
          Priority: P2
         Component: kprobes
          Assignee: systemtap at sourceware dot org
          Reporter: yangwen at huawei dot com

/* Summary:
 * A misusing of function @module_kallsyms_on_each_symbol
 * cause the stap generated module can not resolve the
 * module symbols.
 * */

/* 1. Supporse we use stap on the following script to probe
 * the 'FuncA' in module named 'ModA'
 * */
probe kprobe.module("ModA").function ("FuncA") {
    printf("Wen Test: cache:readCache called, task:%s pid:%d\n",
           execname(), pid())
}


/* 2. The stap command will generate the following 3 pieces of
 * codes to resolve the symbol referenced above
 * */
stap_dwarfless_probes[] = {
    { .symbol_string="ModA:FuncA", .probe=(&stap_probes[1]), },
};


{
    int p = 0;
    for (i = 0; i < 1; i++) {
        struct stap_dwarfless_probe *sdp = & stap_dwarfless_probes[i];
        if (! sdp->address)
            p++;
    }
    kallsyms_on_each_symbol(kprobe_resolve, &p);
}

static int kprobe_resolve(void *data, const char *name,
                          struct module *owner,
                          unsigned long val) {
    int i;
    int *p = (int *) data;
    for (i=0; i<1 && *p > 0; i++) {
        struct stap_dwarfless_probe *sdp = & stap_dwarfless_probes[i];
        if (! sdp->address)
            if (strcmp(sdp->symbol_string, name) == 0) {
                sdp->address = val;
                (*p)--;
            }
    }
    return (p > 0) ? 0 : -1;
}

/* 3. In the generated function 'kprobe_resolve (a callback function for
 * kallsyms_on_each_symbol), it compares the sdp->symbol_string--in this
 * expamle it's "ModA:FuncA"--with the second parameter @name of
kprobe_resolve.
 *
 * The expr 'if (strcmp(sdp->symbol_string, name) == 0)' NEVER evals TRUE,
because
 * the kernel does NOT merge the module name with the symbol name to give the
parameter
 * @name, the @name parameter always contain ONLY the symbol name. 
 *
 * The kprobe_resolve should compare module name and symbol name respectively.
 * It maybe looks like this:
 * */

static int kprobe_resolve(void *data, const char *name,
                          struct module *owner,
                          unsigned long val) {
    int i;
    int *p = (int *) data;
    for (i=0; i<1 && *p > 0; i++) {
        struct stap_dwarfless_probe *sdp = & stap_dwarfless_probes[i];
        if (! sdp->address) {
            const char * colon;
            if ((colon = strchr(sdp->symbol_string, ':')) && owner) {
                if ((strlen(owner->name) == (colon - sdp->symbol_string)) &&
                    (strncmp(sdp->symbol_string, owner->name, colon -
sdp->symbol_string) == 0) &&
                    (strcmp(colon + 1, name) == 0)) {
                    sdp->address = val;
                    (*p)--;
                }
            } else {
                if (strcmp(sdp->symbol_string, name) == 0) {
                    sdp->address = val;
                    (*p)--;
                }
            }
        }
    }
    return (p > 0) ? 0 : -1;
}

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[Bug kprobes/15675] A misusing of function @kallsyms_on_each_symbo cause the stap generated module can not resolve the module symbols.

[yangwen at huawei dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15675

ywen <yangwen at huawei dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|A misusing of function      |A misusing of function
                   |@module_kallsyms_on_each_sy |@kallsyms_on_each_symbo
                   |mbo cause the stap          |cause the stap generated
                   |generated module can not    |module can not resolve the
                   |resolve the module symbols. |module symbols.

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[Bug kprobes/15675] 'kprobe.module("foo").function("bar")' probes do not work

[dsmith at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15675

David Smith <dsmith at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dsmith at redhat dot com
            Summary|A misusing of function      |'kprobe.module("foo").funct
                   |@kallsyms_on_each_symbo     |ion("bar")' probes do not
                   |cause the stap generated    |work
                   |module can not resolve the  |
                   |module symbols.             |

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[Bug kprobes/15675] 'kprobe.module("foo").function("bar")' probes do not work

[dsmith at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15675

David Smith <dsmith at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #1 from David Smith <dsmith at redhat dot com> ---
This problem is now fixed. I took your suggested code changes and made the
systemtap translator emit code like that in the following commit:

=====
commit 8c2728197442816189a94f3f241186bcf6cb245a
Author: Yang Wen <yangwen@huawei.com>
Date:   Tue Jun 25 13:40:29 2013 -0500

    Fix PR15675 by making 'probe kprobe.module("foo").function("bar") work.

    * tapsets.cxx (kprobe_derived_probe_group::emit_module_decls): Fix emitted
      kprobe_resolve() to lookup "module:function" symbols correctly.
====

I then wondered why we hadn't seen this before. It was because we never tested
it. So, after fixing bug #10395 to fix the kmodule.exp testcase, I added a new
test to that testcase to test probing 'kprobe.module("foo").function("bar")':

====
commit 5039ed9600735c3b2fc23068f5c15282639ad042
Author: David Smith <dsmith@redhat.com>
Date:   Tue Jun 25 13:43:58 2013 -0500

    Add a new test for probing 'kprobe.module("foo").function("bar") (PR15675).

    * testsuite/systemtap.base/kprobe_module.stp: Add new test for
      'kprobe.module("foo").function("bar")'.
    * testsuite/systemtap.base/kmodule.exp: Run new kprobe_module.stp test.
====

Thanks for the code fix.

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