Re: [PATCH v7 20/36] function_graph: Improve push operation for several interrupts

[Steven Rostedt]

On Wed,  7 Feb 2024 00:11:12 +0900
"Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> Improve push and data reserve operation on the shadow stack for
> several sequencial interrupts.
> 
> To push a ret_stack or data entry on the shadow stack, we need to
> prepare an index (offset) entry before updating the stack pointer
> (curr_ret_stack) so that unwinder from interrupts can find the
> next return address from the shadow stack. Currently we do write index,
> update the curr_ret_stack, and rewrite it again. But that is not enough
> for the case if two interrupts happens and the first one breaks it.
> For example,
> 
>  1. write reserved index entry at ret_stack[new_index - 1] and ret addr.
>  2. interrupt comes.
>     2.1. push new index and ret addr on ret_stack.
>     2.2. pop it. (corrupt entries on new_index - 1)
>  3. return from interrupt.
>  4. update curr_ret_stack = new_index
>  5. interrupt comes again.
>     5.1. unwind <------ may not work.

I'm curious if you saw this happen?

That is, did you trigger this or only noticed it by inspection?

This code is already quite complex, I would like to simplify it more before
we try to fix rare race conditions that only affect the unwinder.

Let's hold off on this change.

-- Steve


> 
> To avoid this issue, this introduces a new rsrv_ret_stack stack
> reservation pointer and a new push code (slow path) to commit
> previous reserved code forcibly.
> 
>  0. update rsrv_ret_stack = new_index.
>  1. write reserved index entry at ret_stack[new_index - 1] and ret addr.
>  2. interrupt comes.
>     2.0. if rsrv_ret_stack != curr_ret_stack, add reserved index
>         entry on ret_stack[rsrv_ret_stack - 1] to point the previous
> 	ret_stack pointed by ret_stack[curr_ret_stack - 1]. and
> 	update curr_ret_stack = rsrv_ret_stack.
>     2.1. push new index and ret addr on ret_stack.
>     2.2. pop it. (corrupt entries on new_index - 1)
>  3. return from interrupt.
>  4. update curr_ret_stack = new_index
>  5. interrupt comes again.
>     5.1. unwind works, because curr_ret_stack points the previously
>         saved ret_stack.
>     5.2. this can do push/pop operations too.
> 6. return from interrupt.
> 7. rewrite reserved index entry at ret_stack[new_index] again.
> 
> This maybe a bit heavier but safer.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>