Re: [PATCH] RISC-V: Avoid dereferening NULL regs in die()

[Conor Dooley <conor@kernel.org>]

On Tue, Sep 20, 2022 at 01:00:37PM -0700, Palmer Dabbelt wrote:
> I don't think we can actually die() without a regs pointer, but the
> compiler was warning about a NULL check after a dereference.  It seems
> prudent to just avoid the possibly-NULL dereference, given that when
> die()ing the system is already toast so who knows how we got there.
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

I excitedly tried this in the hopes that I was actually hitting this
somehow with the crash-during-boot-with-no-splat that I am currently
bisecting, but sadly it was not the solution.

Either way it looks like a sane change so we at least can get something
out in an edge failure case.

> ---
>  arch/riscv/kernel/traps.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 635e6ec26938..f3e96d60a2ff 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -33,6 +33,7 @@ void die(struct pt_regs *regs, const char *str)
>  {
>  	static int die_counter;
>  	int ret;
> +	long cause;
>  
>  	oops_enter();
>  
> @@ -42,11 +43,13 @@ void die(struct pt_regs *regs, const char *str)
>  
>  	pr_emerg("%s [#%d]\n", str, ++die_counter);
>  	print_modules();
> -	show_regs(regs);
> +	if (regs)
> +		show_regs(regs);
>  
> -	ret = notify_die(DIE_OOPS, str, regs, 0, regs->cause, SIGSEGV);
> +	cause = regs ? regs->cause : -1;
> +	ret = notify_die(DIE_OOPS, str, regs, 0, cause, SIGSEGV);
>  
> -	if (regs && kexec_should_crash(current))
> +	if (kexec_should_crash(current))
>  		crash_kexec(regs);

Had to go down the rabbit hole to see that there is an if(regs) around
the eventual user of this, so:
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>

>  
>  	bust_spinlocks(0);
> -- 
> 2.34.1
> 
> 
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