Re: hardcoded SIGSEGV in __die() ?

[Joakim Tjernlund <Joakim.Tjernlund@infinera.com>]

On Thu, 2020-03-26 at 11:28 +1100, Michael Ellerman wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.
> 
> 
> 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%7Caa316058f9e34dd758c808d7d11ca391%7C285643de5f5b4b03a1530ae2dc8aaf77%7C1%7C0%7C637207793252449714&amp;sdata=LBzRMxHWJzNEztnnG0UzJb7PHvaDGVswQD%2B8WpY9YX8%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%7Caa316058f9e34dd758c808d7d11ca391%7C285643de5f5b4b03a1530ae2dc8aaf77%7C1%7C0%7C637207793252449714&amp;sdata=Dh%2BUTRgG85oVSgC3SCR1B7izQH4HofT4ppOMiy9xvDA%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.

I had a look at opal_machine_check and friends and came up with:

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 0381242920d9..12715d24141c 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -621,6 +621,11 @@ int machine_check_e500mc(struct pt_regs *regs)
                       reason & MCSR_MEA ? "Effective" : "Physical", addr);
        }
 
+       if ((user_mode(regs))) {
+               _exception(SIGBUS, regs, reason, regs->nip);
+               recoverable = 1;
+       }
+
 silent_out:
        mtspr(SPRN_MCSR, mcsr);
        return mfspr(SPRN_MCSR) == 0 && recoverable;
@@ -665,6 +670,10 @@ int machine_check_e500(struct pt_regs *regs)
        if (reason & MCSR_BUS_RPERR)
                printk("Bus - Read Parity Error\n");
 
+       if ((user_mode(regs))) {
+               _exception(SIGBUS, regs, reason, regs->nip);
+               return 1;
+       }
        return 0;
 }
 
@@ -695,6 +704,10 @@ int machine_check_e200(struct pt_regs *regs)
        if (reason & MCSR_BUS_WRERR)
                printk("Bus - Write Bus Error on buffered store or cache line push\n");
 
+       if ((user_mode(regs))) {
+               _exception(SIGBUS, regs, reason, regs->nip);
+               return 1;
+       }
        return 0;
 }
 #elif defined(CONFIG_PPC32)
@@ -731,6 +744,10 @@ int machine_check_generic(struct pt_regs *regs)
        default:
                printk("Unknown values in msr\n");
        }
+       if ((user_mode(regs))) {
+               _exception(SIGBUS, regs, reason, regs->nip);
+               return 1;
+       }
        return 0;
 }
 #endif /* everything else */

I don't really know what I am doing, does the above make sense to you?

     Jocke