Re: hardcoded SIGSEGV in __die() ?

From: Michael Ellerman <mpe@ellerman.id.au>
To: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>,
	"christophe.leroy\@c-s.fr" <christophe.leroy@c-s.fr>,
	"linuxppc-dev\@ozlabs.org" <linuxppc-dev@ozlabs.org>
Subject: Re: hardcoded SIGSEGV in __die() ?
Date: Thu, 26 Mar 2020 11:28:42 +1100	[thread overview]
Message-ID: <87lfnovu11.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <4f4f2c97f7393f21f507c58def88514c9f670e0a.camel@infinera.com>

Joakim Tjernlund <Joakim.Tjernlund@infinera.com> writes:
> On Mon, 2020-03-23 at 15:45 +0100, Christophe Leroy wrote:
>> Le 23/03/2020 à 15:43, Christophe Leroy a écrit :
>> > Le 23/03/2020 à 15:17, Joakim Tjernlund a écrit :
>> > > In __die(), see below, there is this call to notify_send() with
>> > > SIGSEGV hardcoded, this seems odd
>> > > to me as the variable "err" holds the true signal(in my case SIGBUS)
>> > > Should not SIGSEGV be replaced with the true signal no.?
>> > 
>> > As far as I can see, comes from
>> > https://nam03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Fpub%2Fscm%2Flinux%2Fkernel%2Fgit%2Ftorvalds%2Flinux.git%2Fcommit%2F%3Fid%3D66fcb1059&amp;data=02%7C01%7CJoakim.Tjernlund%40infinera.com%7C4291ac1b501e4296869a08d7cf38cdb4%7C285643de5f5b4b03a1530ae2dc8aaf77%7C1%7C0%7C637205715189366995&amp;sdata=Z2bFsmDlD2MKhLACQvayk9ejz0dqgMEOlBTlocAmtTg%3D&amp;reserved=0
>> > 
>> 
>> And
>> https://nam03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Fpub%2Fscm%2Flinux%2Fkernel%2Fgit%2Ftorvalds%2Flinux.git%2Fcommit%2F%3Fid%3Dae87221d3ce49d9de1e43756da834fd0bf05a2ad&amp;data=02%7C01%7CJoakim.Tjernlund%40infinera.com%7C4291ac1b501e4296869a08d7cf38cdb4%7C285643de5f5b4b03a1530ae2dc8aaf77%7C1%7C0%7C637205715189366995&amp;sdata=97kyz3Ur88BhDUUYzya5t%2FFQVhXYu6qiHoW8hsEg81s%3D&amp;reserved=0
>> shows it is (was?) similar on x86.
>> 
>
> I tried to follow that chain thinking it would end up sending a signal to user space but I cannot see
> that happens. Seems to be related to debugging.
>
> In short, I cannot see any signal being delivered to user space. If so that would explain why
> our user space process never dies.
> Is there a signal hidden in machine_check handler for SIGBUS I cannot see?

It's platform specific. What platform are you on?

See the ppc_md & cur_cpu_spec calls here:

void machine_check_exception(struct pt_regs *regs)
{
	int recover = 0;
	bool nested = in_nmi();
	if (!nested)
		nmi_enter();

	__this_cpu_inc(irq_stat.mce_exceptions);

	add_taint(TAINT_MACHINE_CHECK, LOCKDEP_NOW_UNRELIABLE);

	/* See if any machine dependent calls. In theory, we would want
	 * to call the CPU first, and call the ppc_md. one if the CPU
	 * one returns a positive number. However there is existing code
	 * that assumes the board gets a first chance, so let's keep it
	 * that way for now and fix things later. --BenH.
	 */
	if (ppc_md.machine_check_exception)
		recover = ppc_md.machine_check_exception(regs);
	else if (cur_cpu_spec->machine_check)
		recover = cur_cpu_spec->machine_check(regs);

	if (recover > 0)
		goto bail;

Either the ppc_md or cpu_spec handlers can send a signal, but after a
bit of grepping I think only the pseries and powernv ones do.

If you get into die() then it's an oops, which is not the same as a
normal signal.

cheers