structure access and tapsets

[Hien Nguyen]

As I recalled, Vara pointed out before that we need to have support for 
struct data type in the language.
Accessing struct data in function arguments should be as easy as

$target->fieldname

Looks like #3 try to do the samething.

Hien
Martin Hunt wrote:

>Accessing Structures
>
>The current aux_syscalls.stp has some functions to access structures
>from userspace. For example
>
>function __uget_ts_m:long(u_addr:long,member:long)
>%{
>   struct timespec ts;
>   char *ptr = (char *)(unsigned long)THIS->u_addr;
>   size_t sz = sizeof(struct timespec);
>
>   if(_stp_copy_from_user((char *)&ts,ptr,sz))
>      THIS->__retvalue = -EFAULT;
>   else if(THIS->member == 0)
>      THIS->__retvalue = ts.tv_sec;
>   else
>      THIS->__retvalue = ts.tv_nsec;
>%}
>
>This copies a timespec from userspace and returns either the seconds or
>nanoseconds field.  It isn't very convenient for the syscall tapset. It
>is also badly named. __uget_ts_m() means nothing to me.
>
>So I wrote the following, which is very useful for building argstr in
>the syscall tapset:
>
>function _utimespec_str(uaddr:long)
>%{
>	struct timespec ts;
>	char *ptr = (char *)(unsigned long)THIS->uaddr;
>
>	if (ptr == NULL)
>		strlcpy (THIS->__retvalue, "NULL", MAXSTRINGLEN);
>	else {
>		if(_stp_copy_from_user((char *)&ts,ptr,sizeof(struct timespec)))  {
>			strlcpy (THIS->__retvalue, "UNKNOWN", MAXSTRINGLEN);
>		} else
>			snprintf(THIS->__retvalue, MAXSTRINGLEN, "[%ld.%09ld]", 
>				(unsigned long)ts.tv_sec, (unsigned long)ts.tv_nsec);
>	}
>%}
>
>So the questions I am considering are:
>
>1. Is there any reason to make arguments passed in a structure available
>to tapset callers? For example, in sys_futex(), if a timespec is passed
>in, should the tapset set a variable "secs" and "nsecs" with the seconds
>and nanoseconds pulled out of the struct?
>
>2. If we provide a bunch of functions to decode common system
>structures, how should they be named?  This is important; I already have
>60 functions in aux_syscalls and they mostly decode flags and return
>strings.  Add in a bunch that do optional userspace copies then decode
>structs and return either longs or strings.
>
>Maybe
>_struct_timespec(addr, num) - returns element num
>_struct_timespec_str(addr) - returns formatted string
>_struct_utimespec_str(uaddr) - same as above but takes userspace
>address.
>
>3. How does http://sources.redhat.com/bugzilla/show_bug.cgi?id=2049
>affect this? From the brief description, I can't be sure, but it looks
>like eliminate all the need for creating ways to access fields.
>Functions to print a struct as a neatly-formatted string (as in
>_utimespec_str above) seem like they would be still useful.
>
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