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SubjectRe: [PATCH] show_task() and thread_saved_pc() fix for x86


On Sat, 11 Nov 2000, Linus Torvalds wrote:

> On Fri, 10 Nov 2000, Alexander Viro wrote:
> > diff -urN rc11-2/include/asm-i386/processor.h rc11-2-show_task/include/asm-i386/processor.h
> > --- rc11-2/include/asm-i386/processor.h Fri Nov 10 09:14:04 2000
> > +++ rc11-2-show_task/include/asm-i386/processor.h Fri Nov 10 16:08:15 2000
> > @@ -412,7 +412,7 @@
> > */
> > extern inline unsigned long thread_saved_pc(struct thread_struct *t)
> > {
> > - return ((unsigned long *)t->esp)[3];
> > + return ((unsigned long **)t->esp)[0][1];
> > }
>
> The above needs to get verified: it should be something like
>
> unsigned long *ebp = *((unsigned long **)t->esp);
>
> if ((void *) ebp < (void *) t)
> return 0;
> if ((void *) ebp >= (void *) t + 2*PAGE_SIZE)
> return 0;
> if (3 & (unsigned long)ebp)
> return 0;
> return *ebp;
>
> because otherwise I guarantee that we'll eventually have a bug with a
> invalid pointer reference in the debugging code and that would be bad.

I would probably turn it into
unsigned long *ebp = *((unsigned long **)t->esp);

/* Bits 0,1 and 13..31 must be shared with the stack base */
if (((unsigned long)ebp ^ (unsigned long)t) & ~(2*PAGE_SIZE-4))
return 0;

return *ebp;

Comments? Alternative variant: just let schedule() store its return address
in the task_struct. Yeah, it's couple of tacts per schedule(). And much saner
code, without second-guessing the compiler. OTOH, the value is used only
by Alt-SysRq-T, so... Hell knows.
Cheers,
Al

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