On Thu, Aug 20, 2020 at 05:21:11PM +0200, pet...@infradead.org wrote: > On Thu, Aug 20, 2020 at 10:16:59AM -0500, Josh Poimboeuf wrote: > > On Thu, Aug 20, 2020 at 05:08:41PM +0200, pet...@infradead.org wrote: > > > On Thu, Aug 20, 2020 at 10:45:12AM -0400, Brian Gerst wrote: > > > > On Thu, Aug 20, 2020 at 6:53 AM Peter Zijlstra <pet...@infradead.org> > > > > wrote: > > > > > > > > > > > > > > > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > > > > > --- > > > > > arch/x86/kernel/traps.c | 24 ++++++++++++------------ > > > > > 1 file changed, 12 insertions(+), 12 deletions(-) > > > > > > > > > > --- a/arch/x86/kernel/traps.c > > > > > +++ b/arch/x86/kernel/traps.c > > > > > @@ -820,18 +820,6 @@ static void handle_debug(struct pt_regs > > > > > goto out; > > > > > } > > > > > > > > > > - if (WARN_ON_ONCE((dr6 & DR_STEP) && !user_mode(regs))) { > > > > > - /* > > > > > - * Historical junk that used to handle SYSENTER > > > > > single-stepping. > > > > > - * This should be unreachable now. If we survive for > > > > > a while > > > > > - * without anyone hitting this warning, we'll turn > > > > > this into > > > > > - * an oops. > > > > > - */ > > > > > - tsk->thread.debugreg6 &= ~DR_STEP; > > > > > - set_tsk_thread_flag(tsk, TIF_SINGLESTEP); > > > > > - regs->flags &= ~X86_EFLAGS_TF; > > > > > - } > > > > > - > > > > > si_code = get_si_code(tsk->thread.debugreg6); > > > > > if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || > > > > > user_icebp) > > > > > send_sigtrap(regs, 0, si_code); > > > > > @@ -874,6 +862,18 @@ static __always_inline void exc_debug_ke > > > > > if (kprobe_debug_handler(regs)) > > > > > goto out; > > > > > > > > > > + if (WARN_ON_ONCE(dr6 & DR_STEP)) { > > > > > + /* > > > > > + * Historical junk that used to handle SYSENTER > > > > > single-stepping. > > > > > + * This should be unreachable now. If we survive for > > > > > a while > > > > > + * without anyone hitting this warning, we'll turn > > > > > this into > > > > > + * an oops. > > > > > + */ > > > > > + dr6 &= ~DR_STEP; > > > > > + set_thread_flag(TIF_SINGLESTEP); > > > > > + regs->flags &= ~X86_EFLAGS_TF; > > > > > + } > > > > > + > > > > > handle_debug(regs, dr6, false); > > > > > > > > > > out: > > > > > > > > Can this be removed or changed to a BUG()? The warning has been there > > > > since 2016 and nobody has apparently complained about it. > > > > > > Something like: > > > > > > /* > > > * The kernel doesn't do TF outside of Kprobes, so if we get > > > * here, something's fishy. > > > */ > > > BUG_ON(dr6 & DR_STEP); > > > > > > ? > > > > What about kernel debuggers like kgdb and qemu gdb? > > qemu-gdb stub should eat the event before it lands in the guest
Are we sure about that? I triggered the warning just now, stepping through the debug handler. [ 34.577903] ------------[ cut here ]------------ [ 34.578410] WARNING: CPU: 0 PID: 950 at arch/x86/kernel/traps.c:843 handle_debug+0x10e/0x140 [ 34.579152] Modules linked in: [ 34.579439] ip6t_rpfilter [ 34.579690] ip6t_REJECT [ 34.579945] nf_reject_ipv6 [ 34.580216] xt_conntrack [ 34.580467] ebtable_nat [ 34.580715] ebtable_broute [ 34.581003] ip6table_raw [ 34.581258] ip6table_mangle [ 34.581534] ip6table_nat [ 34.581801] ip6table_security [ 34.582116] iptable_raw [ 34.582361] iptable_mangle [ 34.582642] iptable_nat [ 34.582916] nf_nat [ 34.583136] nf_conntrack [ 34.583394] nf_defrag_ipv6 [ 34.583665] nf_defrag_ipv4 [ 34.583937] iptable_security [ 34.584246] ebtable_filter [ 34.584504] ebtables [ 34.584735] ip6table_filter [ 34.585038] ip6_tables [ 34.585262] rfkill [ 34.585453] snd_hda_codec_generic [ 34.585792] sunrpc [ 34.586006] crct10dif_pclmul [ 34.586285] crc32_pclmul [ 34.586549] ghash_clmulni_intel [ 34.586892] snd_hda_intel [ 34.587172] snd_intel_dspcfg [ 34.587456] snd_hda_codec [ 34.587705] snd_hwdep [ 34.587953] snd_hda_core [ 34.588222] snd_pcm [ 34.588435] virtio_balloon [ 34.588706] snd_timer [ 34.588946] snd [ 34.589131] soundcore [ 34.589354] i2c_piix4 [ 34.589889] xfs [ 34.590381] libcrc32c [ 34.590904] virtio_net [ 34.591460] net_failover [ 34.592004] virtio_blk [ 34.592544] failover [ 34.593035] virtio_console [ 34.593514] crc32c_intel [ 34.593967] qxl [ 34.594359] drm_ttm_helper [ 34.594831] ttm [ 34.595219] serio_raw [ 34.595648] drm_kms_helper [ 34.596126] virtio_pci [ 34.596568] virtio_ring [ 34.597026] drm [ 34.597418] virtio [ 34.597831] ata_generic [ 34.598304] pata_acpi [ 34.598757] CPU: 0 PID: 950 Comm: a.out Not tainted 5.9.0-rc1+ #275 [ 34.599555] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014 [ 34.601250] RIP: 0010:handle_debug+0x10e/0x140 [ 34.601916] Code: 91 00 00 00 02 0f 84 55 ff ff ff fa 66 0f 1f 44 00 00 e8 35 f7 20 00 e9 44 ff ff ff e8 1b f6 20 00 fb 66 0f 1f 44 00 00 cc 87 <0f> 0b 80 e4 bf 49 89 85 18 28 00 00 3e 41 80 4d 00 10 48 81 a3 90 [ 34.604087] RSP: 0000:fffffe000000dee8 EFLAGS: 00010246 [ 34.604870] RAX: 0000000000004000 RBX: fffffe000000df58 RCX: 00000000ffffffff [ 34.605722] RDX: ffff8881366c0000 RSI: ffffffff81b136f3 RDI: ffffffff81037b65 [ 34.606557] RBP: fffffe000000df10 R08: 0000000000000001 R09: 0000000000000001 [ 34.607770] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 [ 34.608667] R13: ffff8881366c0000 R14: 0000000000000200 R15: 0000000000004000 [ 34.609579] FS: 00007f3503d09700(0000) GS:ffff88813b600000(0000) knlGS:0000000000000000 [ 34.610578] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 34.611399] CR2: 00007f3503822ae0 CR3: 0000000137cd4003 CR4: 00000000001706f0 [ 34.612345] DR0: 00000000004008cf DR1: 0000000000000000 DR2: 0000000000000000 [ 34.613368] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400 [ 34.614437] Call Trace: [ 34.615098] <#DB> -- Josh