problem about tapset script

problem about tapset script

[ch huang]

i read tcp.stp script and see the following code ,this is Embedded C
code ,i do not know why each function has THIS->xxx and
CATCH_DEREF_FAULT()
and i can not find the explain in systemtap language reference
document ,any one can help?


%{ /* pure */
        struct sock *sk = (struct sock *)(long) THIS->sock;
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,10)
        struct tcp_opt *tp = tcp_sk(sk);
        THIS->__retvalue = (int64_t) jiffies_to_usecs(kread(&(tp->rto)));
#else
        const struct inet_connection_sock *icsk = inet_csk(sk);
        THIS->__retvalue = (int64_t) jiffies_to_usecs(kread(&(icsk->icsk_rto)));
#endif
        CATCH_DEREF_FAULT();
%}

Re: problem about tapset script

[Adrien Kunysz]

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On Tue, Mar 20, 2012 at 04:07:22PM +0800, ch huang wrote:
> i read tcp.stp script and see the following code ,this is Embedded C
> code ,i do not know why each function has THIS->xxx and
> CATCH_DEREF_FAULT()
> and i can not find the explain in systemtap language reference
> document ,any one can help?

THIS is to access arguments and return value as documented in the
SystemTap Language Reference: http://sourceware.org/systemtap/langref/Components_SystemTap_script.html#SECTION00046000000000000000

The CATCH_DEREF_FAULT macro is defined and documented in runtime/loc2c-runtime.h:

 58 /* The deref and store_deref macros are called to safely access addresses
 59    in the probe context.  These macros are used only for kernel addresses.
 60    The macros must handle bogus addresses here gracefully (as from
 61    corrupted data structures, stale pointers, etc), by doing a "goto
 62    deref_fault".
 ..
979 #define CATCH_DEREF_FAULT()                             \
980   if (0) {                                              \
981 deref_fault: ;                                          \
982   }

> %{ /* pure */
>         struct sock *sk = (struct sock *)(long) THIS->sock;
> #if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,10)
>         struct tcp_opt *tp = tcp_sk(sk);
>         THIS->__retvalue = (int64_t) jiffies_to_usecs(kread(&(tp->rto)));
> #else
>         const struct inet_connection_sock *icsk = inet_csk(sk);
>         THIS->__retvalue = (int64_t) jiffies_to_usecs(kread(&(icsk->icsk_rto)));
> #endif
>         CATCH_DEREF_FAULT();
> %}

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Re: problem about tapset script

[Mark Wielaard]

On Tue, Mar 20, 2012 at 04:07:22PM +0800, ch huang wrote:
> i read tcp.stp script and see the following code ,this is Embedded C
> code ,i do not know why each function has THIS->xxx and
> CATCH_DEREF_FAULT()
> and i can not find the explain in systemtap language reference
> document ,any one can help?

There is some explanation in the language reference guide:
http://sourceware.org/systemtap/langref/Components_SystemTap_script.html#SECTION00045000000000000000

Some more in the tutorial:
http://sourceware.org/systemtap/tutorial/Tapsets.html#SECTION00053000000000000000

And a tiny bit on Embedded C & Safety in the tapset/DEVGUIDE:
http://sourceware.org/git/?p=systemtap.git;a=blob_plain;f=tapset/DEVGUIDE;hb=HEAD

But mainly follow the examples already in the code and in doubt ask
on the list.

Basically whenever you use kread or kwrite in embedded C code you must
add a DEREF_FAULT at the end (defined in runtime/loc2c-runtime.h) to
make sure a failure is reported and causes the script to error out.

Cheers,

Mark