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SubjectRe: [PATCH v9 0/5] Add the ability to do BPF directed error injection
On Wed, Dec 13, 2017 at 10:07:32AM -0800, Darrick J. Wong wrote:
> On Wed, Dec 13, 2017 at 01:03:57PM -0500, Josef Bacik wrote:
> > On Tue, Dec 12, 2017 at 03:11:50PM -0800, Darrick J. Wong wrote:
> > > On Mon, Dec 11, 2017 at 11:36:45AM -0500, Josef Bacik wrote:
> > > > This is the same as v8, just rebased onto the bpf tree.
> > > >
> > > > v8->v9:
> > > > - rebased onto the bpf tree.
> > > >
> > > > v7->v8:
> > > > - removed the _ASM_KPROBE_ERROR_INJECT since it was not needed.
> > > >
> > > > v6->v7:
> > > > - moved the opt-in macro to bpf.h out of kprobes.h.
> > > >
> > > > v5->v6:
> > > > - add BPF_ALLOW_ERROR_INJECTION() tagging for functions that will support this
> > > > feature. This way only functions that opt-in will be allowed to be
> > > > overridden.
> > > > - added a btrfs patch to allow error injection for open_ctree() so that the bpf
> > > > sample actually works.
> > > >
> > > > v4->v5:
> > > > - disallow kprobe_override programs from being put in the prog map array so we
> > > > don't tail call into something we didn't check. This allows us to make the
> > > > normal path still fast without a bunch of percpu operations.
> > > >
> > > > v3->v4:
> > > > - fix a build error found by kbuild test bot (I didn't wait long enough
> > > > apparently.)
> > > > - Added a warning message as per Daniels suggestion.
> > > >
> > > > v2->v3:
> > > > - added a ->kprobe_override flag to bpf_prog.
> > > > - added some sanity checks to disallow attaching bpf progs that have
> > > > ->kprobe_override set that aren't for ftrace kprobes.
> > > > - added the trace_kprobe_ftrace helper to check if the trace_event_call is a
> > > > ftrace kprobe.
> > > > - renamed bpf_kprobe_state to bpf_kprobe_override, fixed it so we only read this
> > > > value in the kprobe path, and thus only write to it if we're overriding or
> > > > clearing the override.
> > > >
> > > > v1->v2:
> > > > - moved things around to make sure that bpf_override_return could really only be
> > > > used for an ftrace kprobe.
> > > > - killed the special return values from trace_call_bpf.
> > > > - renamed pc_modified to bpf_kprobe_state so bpf_override_return could tell if
> > > > it was being called from an ftrace kprobe context.
> > > > - reworked the logic in kprobe_perf_func to take advantage of bpf_kprobe_state.
> > > > - updated the test as per Alexei's review.
> > > >
> > > > - Original message -
> > > >
> > > > A lot of our error paths are not well tested because we have no good way of
> > > > injecting errors generically. Some subystems (block, memory) have ways to
> > > > inject errors, but they are random so it's hard to get reproduceable results.
> > > >
> > > > With BPF we can add determinism to our error injection. We can use kprobes and
> > > > other things to verify we are injecting errors at the exact case we are trying
> > > > to test. This patch gives us the tool to actual do the error injection part.
> > > > It is very simple, we just set the return value of the pt_regs we're given to
> > > > whatever we provide, and then override the PC with a dummy function that simply
> > > > returns.
> > >
> > > Heh, this looks cool. I decided to try it to see what happens, and saw
> > > a bunch of dmesg pasted in below. Is that supposed to happen? Or am I
> > > the only fs developer still running with lockdep enabled? :)
> > >
> > > It looks like bpf_override_return has some sort of side effect such that
> > > we get the splat, since commenting it out makes the symptom go away.
> > >
> > > <shrug>
> > >
> > > --D
> > >
> > > [ 1847.769183] BTRFS error (device (null)): open_ctree failed
> > > [ 1847.770130] BUG: sleeping function called from invalid context at /storage/home/djwong/cdev/work/linux-xfs/kernel/locking/rwsem.c:69
> > > [ 1847.771976] in_atomic(): 1, irqs_disabled(): 0, pid: 1524, name: mount
> > > [ 1847.773016] 1 lock held by mount/1524:
> > > [ 1847.773530] #0: (&type->s_umount_key#34/1){+.+.}, at: [<00000000653a9bb4>] sget_userns+0x302/0x4f0
> > > [ 1847.774731] Preemption disabled at:
> > > [ 1847.774735] [< (null)>] (null)
> > > [ 1847.777009] CPU: 2 PID: 1524 Comm: mount Tainted: G W 4.15.0-rc3-xfsx #3
> > > [ 1847.778800] Call Trace:
> > > [ 1847.779047] dump_stack+0x7c/0xbe
> > > [ 1847.779361] ___might_sleep+0x1f7/0x260
> > > [ 1847.779720] down_write+0x29/0xb0
> > > [ 1847.780046] unregister_shrinker+0x15/0x70
> > > [ 1847.780427] deactivate_locked_super+0x2e/0x60
> > > [ 1847.780935] btrfs_mount+0xbb6/0x1000 [btrfs]
> > > [ 1847.781353] ? __lockdep_init_map+0x5c/0x1d0
> > > [ 1847.781750] ? mount_fs+0xf/0x80
> > > [ 1847.782065] ? alloc_vfsmnt+0x1a1/0x230
> > > [ 1847.782429] mount_fs+0xf/0x80
> > > [ 1847.782733] vfs_kern_mount+0x62/0x160
> > > [ 1847.783128] btrfs_mount+0x3d3/0x1000 [btrfs]
> > > [ 1847.783493] ? __lockdep_init_map+0x5c/0x1d0
> > > [ 1847.783849] ? __lockdep_init_map+0x5c/0x1d0
> > > [ 1847.784207] ? mount_fs+0xf/0x80
> > > [ 1847.784502] mount_fs+0xf/0x80
> > > [ 1847.784835] vfs_kern_mount+0x62/0x160
> > > [ 1847.785235] do_mount+0x1b1/0xd50
> > > [ 1847.785594] ? _copy_from_user+0x5b/0x90
> > > [ 1847.786028] ? memdup_user+0x4b/0x70
> > > [ 1847.786501] SyS_mount+0x85/0xd0
> > > [ 1847.786835] entry_SYSCALL_64_fastpath+0x1f/0x96
> > > [ 1847.787311] RIP: 0033:0x7f6ebecc1b5a
> > > [ 1847.787691] RSP: 002b:00007ffc7bd1c958 EFLAGS: 00000202 ORIG_RAX: 00000000000000a5
> > > [ 1847.788383] RAX: ffffffffffffffda RBX: 00007f6ebefba63a RCX: 00007f6ebecc1b5a
> > > [ 1847.789106] RDX: 0000000000bfd010 RSI: 0000000000bfa230 RDI: 0000000000bfa210
> > > [ 1847.789807] RBP: 0000000000bfa0f0 R08: 0000000000000000 R09: 0000000000000014
> > > [ 1847.790511] R10: 00000000c0ed0000 R11: 0000000000000202 R12: 00007f6ebf1ca83c
> > > [ 1847.791211] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000001
> > > [ 1847.792029] BUG: scheduling while atomic: mount/1524/0x00000002
> > > [ 1847.792680] 1 lock held by mount/1524:
> > > [ 1847.793087] #0: (rcu_preempt_state.exp_mutex){+.+.}, at: [<00000000a6c536a9>] _synchronize_rcu_expedited+0x1ce/0x400
> > > [ 1847.794129] Modules linked in: xfs libcrc32c btrfs xor zstd_decompress zstd_compress xxhash lzo_compress lzo_decompress zlib_deflate raid6_pq dax_pmem device_dax nd_pmem sch_fq_codel af_packet [last unloaded: xfs]
> > > [ 1847.795949] Preemption disabled at:
> > > [ 1847.795951] [< (null)>] (null)
> > > [ 1847.796844] CPU: 2 PID: 1524 Comm: mount Tainted: G W 4.15.0-rc3-xfsx #3
> > > [ 1847.797621] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.10.2-1ubuntu1djwong0 04/01/2014
> > > [ 1847.798510] Call Trace:
> > > [ 1847.798786] dump_stack+0x7c/0xbe
> > > [ 1847.799134] __schedule_bug+0x88/0xe0
> > > [ 1847.799517] __schedule+0x78c/0xb20
> > > [ 1847.799890] ? trace_hardirqs_on_caller+0x119/0x180
> > > [ 1847.800391] schedule+0x40/0x90
> > > [ 1847.800729] _synchronize_rcu_expedited+0x36b/0x400
> > > [ 1847.801218] ? rcu_preempt_qs+0xa0/0xa0
> > > [ 1847.801616] ? remove_wait_queue+0x60/0x60
> > > [ 1847.802040] ? rcu_preempt_qs+0xa0/0xa0
> > > [ 1847.802433] ? rcu_exp_wait_wake+0x630/0x630
> > > [ 1847.802872] ? __lock_acquire+0xfb9/0x1120
> > > [ 1847.803302] ? __lock_acquire+0x534/0x1120
> > > [ 1847.803725] ? bdi_unregister+0x57/0x1a0
> > > [ 1847.804135] bdi_unregister+0x5c/0x1a0
> > > [ 1847.804519] bdi_put+0xcb/0xe0
> > > [ 1847.804746] generic_shutdown_super+0xe2/0x110
> > > [ 1847.805066] kill_anon_super+0xe/0x20
> > > [ 1847.805344] btrfs_kill_super+0x12/0xa0 [btrfs]
> > > [ 1847.805664] deactivate_locked_super+0x34/0x60
> > > [ 1847.806111] btrfs_mount+0xbb6/0x1000 [btrfs]
> > > [ 1847.806476] ? __lockdep_init_map+0x5c/0x1d0
> > > [ 1847.806824] ? mount_fs+0xf/0x80
> > > [ 1847.807104] ? alloc_vfsmnt+0x1a1/0x230
> > > [ 1847.807416] mount_fs+0xf/0x80
> > > [ 1847.807712] vfs_kern_mount+0x62/0x160
> > > [ 1847.808112] btrfs_mount+0x3d3/0x1000 [btrfs]
> > > [ 1847.808565] ? __lockdep_init_map+0x5c/0x1d0
> > > [ 1847.809005] ? __lockdep_init_map+0x5c/0x1d0
> > > [ 1847.809425] ? mount_fs+0xf/0x80
> > > [ 1847.809731] mount_fs+0xf/0x80
> > > [ 1847.810070] vfs_kern_mount+0x62/0x160
> > > [ 1847.810469] do_mount+0x1b1/0xd50
> > > [ 1847.810821] ? _copy_from_user+0x5b/0x90
> > > [ 1847.811237] ? memdup_user+0x4b/0x70
> > > [ 1847.811622] SyS_mount+0x85/0xd0
> > > [ 1847.811996] entry_SYSCALL_64_fastpath+0x1f/0x96
> > > [ 1847.812465] RIP: 0033:0x7f6ebecc1b5a
> > > [ 1847.812840] RSP: 002b:00007ffc7bd1c958 EFLAGS: 00000202 ORIG_RAX: 00000000000000a5
> > > [ 1847.813615] RAX: ffffffffffffffda RBX: 00007f6ebefba63a RCX: 00007f6ebecc1b5a
> > > [ 1847.814302] RDX: 0000000000bfd010 RSI: 0000000000bfa230 RDI: 0000000000bfa210
> > > [ 1847.814770] RBP: 0000000000bfa0f0 R08: 0000000000000000 R09: 0000000000000014
> > > [ 1847.815246] R10: 00000000c0ed0000 R11: 0000000000000202 R12: 00007f6ebf1ca83c
> > > [ 1847.815720] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000001
> > i>
> >
> > Looks like this is new, Masami this is happening because of your change here
> >
> > 5bb4fc2d8641 ("kprobes/x86: Disable preemption in ftrace-based jprobes")
> >
> > which makes it not do the preempt_enable() if the handler returns 1. Why is
> > that? Should I be doing preempt_enable_no_resched() from the handler before
> > returning 1? Or is this just an oversight on your part? Thanks,
>
> FWIW I shut up the preemption imbalance warnings with the attached
> coarse bandaid. No idea if that's the correct fix...
>
> --D
>
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 5db8498..fd948e3 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -1215,8 +1215,10 @@ kprobe_perf_func(struct trace_kprobe *tk, struct pt_regs *regs)
> if (__this_cpu_read(bpf_kprobe_override)) {
> __this_cpu_write(bpf_kprobe_override, 0);
> reset_current_kprobe();
> + preempt_enable();
> return 1;
> }
> + preempt_enable();
> if (!ret)
> return 0;
> }

Yeah I'd like to avoid doing this and know why exactly we leave a unpaired
preempt_disable() in kprobe_ftrace_handler() so we don't do something like this
only to have the handler change again in the future and break us again. Thanks,

Josef

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