Re: [PATCH] arm64: dovetail: Fix undefinstr/break trap handling

[Florian Bezdeka <florian.bezdeka@siemens.com>]

On Mon, 2023-08-28 at 16:36 +0200, Philippe Gerum wrote:
> Jan Kiszka <jan.kiszka@siemens.com> writes:
> 
> > On 28.08.23 14:52, Florian Bezdeka wrote:
> > > On Mon, 2023-08-28 at 14:33 +0200, Jan Kiszka wrote:
> > > > On 25.08.23 14:58, Florian Bezdeka wrote:
> > > > > When running an compat RT application on arm64 the break trap is
> > > > > handled via the undefined instruction trap.
> > > > > 
> > > > > A possible call stack looks like this:
> > > > > 
> > > > > Call trace:
> > > > >   handle_inband_event+0x2d0/0x320
> > > > >   inband_event_notify+0x28/0x50
> > > > >   signal_wake_up_state+0x7c/0xa4
> > > > >   complete_signal+0x104/0x2d0
> > > > >   __send_signal_locked+0x1d0/0x3e4
> > > > >   send_signal_locked+0xf0/0x140
> > > > >   force_sig_info_to_task+0xa0/0x164
> > > > >   force_sig_fault+0x64/0x94
> > > > >   arm64_force_sig_fault+0x48/0x80
> > > > >   send_user_sigtrap+0x50/0x8c
> > > > >   aarch32_break_handler+0xac/0x1d0
> > > > >   do_undefinstr+0x6c/0x360
> > > > >   el0_undef+0x4c/0xd0
> > > > >   el0t_32_sync_handler+0xd0/0x140
> > > > >   el0t_32_sync+0x190/0x194
> > > > > 
> > > > > The trap is never reported to the companion core at that stage so
> > > > > running_oob() in do_undefinstr() will always return true. As the
> > > > > following bailout happens before calling the compat breakpoint
> > > > > detection (aarch32_break_handler()) debugging the compat 
> > > > > application does not work.
> > > > > 
> > > > > In addition aarch32_break_handler() has to report the trap entry to the
> > > > > companion core.
> > > > > 
> > > > > Reported-by: Clara Kowalsky <clara.kowalsky@siemens.com>
> > > > > Tested-by: Clara Kowalsky <clara.kowalsky@siemens.com>
> > > > > Signed-off-by: Florian Bezdeka <florian.bezdeka@siemens.com>
> > > > > ---
> > > > >  arch/arm64/kernel/debug-monitors.c | 3 +++
> > > > >  arch/arm64/kernel/traps.c          | 7 -------
> > > > >  2 files changed, 3 insertions(+), 7 deletions(-)
> > > > > 
> > > > > diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> > > > > index 32271ed24ef5..ef7ac042a0a6 100644
> > > > > --- a/arch/arm64/kernel/debug-monitors.c
> > > > > +++ b/arch/arm64/kernel/debug-monitors.c
> > > > > @@ -373,7 +373,10 @@ int aarch32_break_handler(struct pt_regs *regs)
> > > > >  	if (!bp)
> > > > >  		return -EFAULT;
> > > > >  
> > > > > +	mark_trap_entry(ARM64_TRAP_UNDI, regs);
> > > > >  	send_user_sigtrap(TRAP_BRKPT);
> > > > > +	mark_trap_exit(ARM64_TRAP_UNDI, regs);
> > > > > +
> > > > >  	return 0;
> > > > >  }
> > > > >  NOKPROBE_SYMBOL(aarch32_break_handler);
> > > > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > > > > index cc68be400244..9bf2f309248f 100644
> > > > > --- a/arch/arm64/kernel/traps.c
> > > > > +++ b/arch/arm64/kernel/traps.c
> > > > > @@ -489,13 +489,6 @@ void arm64_notify_segfault(unsigned long addr)
> > > > >  
> > > > >  void do_undefinstr(struct pt_regs *regs, unsigned long esr)
> > > > >  {
> > > > > -	/*
> > > > > -	 * If the companion core did not switched us to in-band
> > > > > -	 * context, we may assume that it has handled the trap.
> > > > > -	 */
> > > > > -	if (running_oob())
> > > > > -		return;
> > > > > -
> > > > >  	/* check for AArch32 breakpoint instructions */
> > > > >  	if (!aarch32_break_handler(regs))
> > > > >  		return;
> > > > 
> > > > This is not against v6.5-dovetail-rebase, right? We likely need to start
> > > > from the top, then backport to stable.
> > > 
> > > That applied for 6.1 and all other (lower) versions that are currently
> > > covered by our CI. Might need some lifting for 6.5, didn't check yet.
> > > 
> > > > 
> > > > Also note that this change came in via
> > > > https://source.denx.de/Xenomai/linux-dovetail/-/commit/2b2ccdaeb8116727cf4076960d664a3cedff0ac6,
> > > > so just dropping it will likely cause problems elsewhere. Should we
> > > > rather move that down in the handler?
> > > 
> > > Well, as written in the commit message running_oob() will always return
> > > true for RT tasks so I'm quite sure that the invalid instruction
> > > handling was broken on arm64 for some time now. We bailed out even
> > > before sending the notification to the companion core.
> > > 
> > > Moving it down might fix the compat case but native arm64 would stay
> > > broken. No?
> > > 
> > 
> > Something looks still fishy, either in the original patch that
> > introduced the condition (I still don't get that special case) or now
> > with this change trying to restore things. I agree that also the
> > original change needed the notification to be delivered.
> > 
> > Philippe, can you help clarifying the logic behind
> > do_undefinstr/do_el0_undef?
> > 
> 
> This very much looks like an unfortunate attempt to mimic the arm32
> logic, which does notify the core about the undefined insn trap prior to
> calling do_undefinstr() (und_fault from the asm entry code).
> 
> So Florian is right, this should not apply to the arm64 side because
> unlike arm32, we need to wait until all branches which might be able to
> handle this fault directly from oob are considered
> (e.g. try_emulate_mrs()), before assuming that we might need the core to
> switch us in-band. IOW, do_el0_undef() is broken since it wrongly
> assumes that such switch might already have happened on entry. As a
> matter of fact, it did not for a reason.
> 
> I see another issue hiding in the dark: emulation of the deprecated
> armv8 SWP{B} instruction cannot be done from the oob stage.  So in
> addition to fixing the aarch32 break handler, I would notify the core
> before handling the CONDTEST_PASS case as well (bluntly disabling
> all emulations from the oob stage entirely seems wrong ATM).

So it seems we are on the same page now, that's great.

To sum up: my "backport" is missing the armv8 SWP{B} part and we still
lack a fix for recent dovetail versions.

Who will write the necessary fixes? Philippe, could you jump on that?

Best regards,
Florian

> 
> -- 
> Philippe.