On Fri, May 3, 2019 at 3:49 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> You are saying that we have a do_int3() for user space int3, and
> do_kernel_int3() for kernel space. That would need to be done in asm
> for both, because having x86_64 call do_int3() for kernel and
> user would be interesting.
The clean/simple way is to just do this
- x86-32 does the special asm for the kernel_do_int3(), case and
calls user_do_int3 otherwise.
- x86-64 doesn't care, and just calls "do_int3()".
We have a trivial helper function like
dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
{
if (user_mode(regs))
user_int3(regs);
else
WARN_ON_ONCE(kernel_int3(regs) != regs);
}
which adds that warning just for debug purposes.
Then we make the rule be that user_int3() does the normal stuff, and
kernel_int3() returns the pt_regs it was passed in.
Easy-peasy, there is absolutely no difference between x86-64 and
x86-32 here except for the trivial case that x86-32 does its thing at
the asm layer, which is what allows "kernel_int3()" to move pt_regs
around by a small amount.
Now, the _real_ difference is when you do the "call_emulate()" case,
which will have to do something like this
static struct pt_regs *emulate_call(struct pt_regs *regs, unsigned
long return, unsigned long target)
{
#ifdef CONFIG_X86_32
/* BIG comment about how we need to move pt_regs to make
room and to update the return 'sp' */
struct pt_regs *new = (void *)regs - 4;
unsigned long *sp = (unsigned long *)(new + 1);
memmove(new, regs, sizeof(*regs));
regs = new;
#else
unsigned long *sp = regs->sp;
regs->sp -= 4;
#endif
*sp = value;
regs->ip = target;
return regs;
}
but look, the above isn't that complicated, is it? And notice how the
subtle pt_regs movement is exactly where it needs to be and nowhere
else.
And what's the cost of all of this? NOTHING. The x86-32 entry code has
to do the test for kernel space anyway, and *all* it does now is to
call "kernel_int3" for the kernel case after having made a bit of
extra room on the stack so that you *can* move pt_regs around (maybe
people want to pop things too? It would work as well).
See what I mean by "localized to the cases the need it"?
Linus
