Re: [PATCH] x86/unwind/fp: Fix FP unwinding in ret_from_fork

[peterz]

On Mon, Sep 14, 2020 at 12:04:22PM -0500, Josh Poimboeuf wrote:
> There have been some reports of "bad bp value" warnings printed by the
> frame pointer unwinder:
> 
>   WARNING: kernel stack regs at 000000005bac7112 in sh:1014 has bad 'bp' value 0000000000000000
> 
> This warning happens when unwinding from an interrupt in
> ret_from_fork().  If entry code gets interrupted, the state of the frame
> pointer (rbp) may be undefined, which can confuse the unwinder,
> resulting in warnings like the above.
> 
> There's an in_entry_code() check which normally silences such warnings
> for entry code.  But in this case, ret_from_fork() is getting
> interrupted.  It recently got moved out of .entry.text, so the
> in_entry_code() check no longer works.
> 
> We could move it back into .entry.text, but that would break the noinstr
> validation because of the call to schedule_tail().
> 
> Instead, initialize each new task's RBP to point to the task's entry
> regs via an encoded frame pointer.  That will allow the unwinder to
> reach the end of the stack gracefully.
> 
> Fixes: b9f6976bfb94 ("x86/entry/64: Move non entry code into .text section")
> Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
> Reported-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>