[PATCH] arm64: Fix early single-stepping

[Jean-Philippe Brucker]

On Tue, Oct 27, 2020 at 07:42:58PM +0900, Masami Hiramatsu wrote:
> On Tue, 27 Oct 2020 19:13:18 +0900
> Masami Hiramatsu <mhiramat at kernel.org> wrote:
> 
> > On Mon, 26 Oct 2020 18:29:09 +0100
> > Jean-Philippe Brucker <jean-philippe at linaro.org> wrote:
> > 
> > > To use debug features such as single-step, the OS lock must be unlocked
> > > in the debug registers. Currently this is done in postcore_initcall
> > > which is now too late.
> > > 
> > > Commit 36dadef23fcc ("kprobes: Init kprobes in early_initcall") enabled
> > > using kprobes from early_initcall, when OS lock is still locked. So when
> > > kprobe attempts to single-step a patched instruction, instead of
> > > trapping, execution continues until it throws an undef exception:
> > > 
> > > [    0.064233] Kprobe smoke test: started
> > > [    0.151133] ------------[ cut here ]------------
> > > [    0.151458] kernel BUG at arch/arm64/kernel/traps.c:406!
> > > [    0.151812] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
> > >               ...
> > > [    0.162689] Call trace:
> > > [    0.163014]  do_undefinstr+0x1d4/0x1f4
> > > [    0.163336]  el1_sync_handler+0xbc/0x140
> > > [    0.163839]  el1_sync+0x80/0x100
> > > [    0.164154]  0xffffffc01001d004
> > > [    0.164527]  init_kprobes+0x13c/0x154
> > > [    0.164968]  do_one_initcall+0x54/0x2e0
> > > [    0.165322]  kernel_init_freeable+0xf4/0x258
> > > [    0.165783]  kernel_init+0x20/0x12c
> > > [    0.166117]  ret_from_fork+0x10/0x30
> > > [    0.166595] Code: 97ffff53 a9425bf5 17ffff9b f9001bf7 (d4210000)
> > > [    0.167084] ---[ end trace 36778fdf576e9a79 ]---
> > > 
> > > To fix this, unlock the OS lock as early as possible. Do it in
> > > traps_init() for CPU0, since KGDB wants to use single-step from that
> > > point on according to commit b322c65f8ca3 ("arm64: Call
> > > debug_traps_init() from trap_init() to help early kgdb").
> > > For secondary CPUs, setup the CPU hotplug handler at early_initcall.
> > > 
> > > Fixes: 36dadef23fcc ("kprobes: Init kprobes in early_initcall")
> > > Signed-off-by: Jean-Philippe Brucker <jean-philippe at linaro.org>
> > 
> > Hi Jean,
> > 
> > How have you confirmed this fixes the issue?
> > On my environment, this doesn't fix the issue.
> 
> Oops, it was my mistake. I missed to boot up with Xen. (so I find another bug...)
> Anyway this works for me too.

No worries :)  Although now I've been wondering whether it would be better
to just disable the OS lock lazily, on the first call to
enable_debug_monitors(). It might add a tiny performance penalty but would
avoid this problem reappearing if one of the debugger needs to start even
earlier in the future.

> Reviewed-by: Masami Hiramatsu <mhiramat at kernel.org>
> Tested-by: Masami Hiramatsu <mhiramat at kernel.org>

Thanks!
Jean