Re: System call support in process record and replay

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> Date: Sat, 30 May 2009 13:11:13 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> I have a question about general design of the system call support for
> the record/replay target, for systems whose system calls are entered
> through software interrupts.
> 
> The following excerpt from i386-tdep.c shows the currently-only
> implementation, for Linux system calls entered via INT 80h:
> 
>     case 0xcd:
>       {
> 	int ret;
> 	if (target_read_memory (ir.addr, &tmpu8, 1))
> 	  {
> 	    if (record_debug)
> 	      printf_unfiltered (_("Process record: error reading memory "
> 				   "at addr 0x%s len = 1.\n"),
> 				 paddr_nz (ir.addr));
> 	    return -1;
> 	  }
> 	ir.addr++;
> 	if (tmpu8 != 0x80
> 	    || gdbarch_tdep (gdbarch)->i386_intx80_record == NULL)
> 	  {
> 	    printf_unfiltered (_("Process record doesn't support "
> 				 "instruction int 0x%02x.\n"),
> 			       tmpu8);
> 	    ir.addr -= 2;
> 	    goto no_support;
> 	  }
> 	ret = gdbarch_tdep (gdbarch)->i386_intx80_record (ir.regcache);
> 	if (ret)
> 	  return ret;
>       }
>       break;
> 
> Now, suppose there is another x86 target whose system calls are
> entered through 3 software interrupts: 0x10, 0x21, and 0x31.  Does
> this mean that to support such a target, we will need to define 3
> additional members of `struct gdbarch_tdep', one each for every one of
> the above interrupt numbers, and then tweak the above code to call
> each member whenever the corresponding interrupt number is seen in the
> instruction stream?  And adding support for Windows syscalls means
> that yet another member, for INT 2Eh, should be added?  That seems
> rather inelegant and wasteful to me (since these members will go
> unused on every x86 target that does not use those interrupts), but if
> that's the design we want to follow, I'm okay with it.

Eli, while your concerns are valid, I think that given the fact that
all Open Source OSes that I'm familliar with use the int 0x80 and/or
sysenter the current code is acceptable for now.  IMHO making the code
more complicated for "future flexibility" more often than not leads to
code that will never be used.  I think it is up to whoever adds
support for an OS that doesn't use int 0x80 to come up with a more
general solution.