Re: Syscall changes registers beyond %eax, on linux-i386

[Richard B. Johnson]

On Thu, 19 Sep 2002, Richard Henderson wrote:

> On Thu, Sep 19, 2002 at 03:40:52PM -0400, Richard B. Johnson wrote:
> > Well it's not modifying those values.
> 
> It's not modifying "a", true, but it _is_ modifying the parameter
> area.  Which is exactly the kernel bug in question.
> 

Yep. This can't be found by the compiler. The parameter area is
writable so it looks like somebody needs to do some 'code inspection'
and some additional testing.

> > It's really bad code because it could have done:
> > 
> > 	incl	$0x04(%esp)
> > 	incl	$0x08(%esp)
> > 	incl	$0x1c(%esp)
> > 	jmp	bar
> 
> Yes, I know.
> 

It's a problem with a 'general purpose' compiler that wants to
be "all things" to all people. If somebody made a gcc-compatible
compiler, tuned to the ix86 characteristics, I think we could
cut the extra instructions by at least 1/2, maybe more.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
The US military has given us many words, FUBAR, SNAFU, now ENRON.
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