Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

[Edgecombe, Rick P]

On Wed, 2024-05-15 at 13:35 +0200, Oleg Nesterov wrote:
> Let me repeat I know nothing about shadow stacks, only tried to
> read Documentation/arch/x86/shstk.rst few minutes ago ;)
> 
> On 05/13, Jiri Olsa wrote:
> > 
> > 1) current uretprobe which are not working at the moment and we change
> >     the top value of shadow stack with shstk_push_frame
> > 2) optimized uretprobe which needs to push new frame on shadow stack
> >     with shstk_update_last_frame
> > 
> > I think we should do 1) and have current uretprobe working with shadow
> > stack, which is broken at the moment
> 
> Agreed,
> 
> > I'm ok with not using optimized uretprobe when shadow stack is detected
> > as enabled and we go with current uretprobe in that case
> 
> But how can we detect it? Again, suppose userspace does

the rdssp instruction returns the value of the shadow stack pointer. On non-
shadow stack it is a nop. So you could check if the SSP is non-zero to find if
shadow stack is enabled. This would catch most cases, but I guess there is the
possibility of it getting enabled in a signal that hit between checking and the
rest of operation. Is this uretprobe stuff signal safe in general?