[PATCH] arm64: Fix early single-stepping
Will Deacon
will at kernel.org
Mon Oct 26 13:38:37 EDT 2020
On Mon, Oct 26, 2020 at 06:29:09PM +0100, Jean-Philippe Brucker 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.
Hmm, does this mean we end up setting MDSCR_EL1.KDE before we've reset the
hardware breakpoint/watchpoint registers? Why do we need kprobes so early?
Will
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