Re: [User-mode-linux-devel] Performance enhancement idea / security issue...

[Chris L. <sa...@sk...>]

Oh, I see... No, I wasn't talking about doing it as preprocessing step,
nor eliminating the tracing thread.  I was talking about replacing int
80's as they are found, with ud2 (ud2 is a 2 byte opcode that generates an
illegal instruction exception [SIGILL]).  That way any int 80s would still
be trapped.

Thanks for clearing up why you change the syscall to getpid... that makes
much more sense now.  :)

-Chris


On Fri, 7 Jul 2000, Jeff Dike wrote:

> sa...@sk... said:
> > How would security be reduced by having an alternate system call entry
> > point?
> 
> It's reduced if the alternate system call entry point can be turned off by the 
> process.  One of the proposals was to replace "int 0x80" with a direct call 
> into the kernel.  If that is done by a preprocessor, and there is also no 
> system call tracing (this bit wasn't explicitly proposed by anyone), then the 
> direct call can be rewritten by the process back into a system call into the 
> host kernel.
> 
> > If so, using 'ud2' to get into the kernel would be the same as using a
> > tracing thread... tracing is disabled on entrance, because we 'know'
> > we are in safe code...
> 
> What is ud2, anyway?  Never heard of it...
> 
> And there's one thing I forgot to mention:
> 
> jd...@ka... said:
> > My long-term plan for system calls is to eliminate the tracing thread
> > altogether.  There are two aspects to this - making threads able to
> > PTRACE_CONT and PTRACE_SYSCALL themselves, and allowing threads to
> > intercept  their own system calls. 
> 
> <snip>
> 
> And that is that plan for eliminating the tracing thread isn't exactly set in 
> stone.  If anyone has better ideas, I'll happily drop my own crappy plans, and 
> go with them.
> 
> 				Jeff
> 
> 
> 
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