Re: [PATCH] ftrace: clean up hash direct_functions on register failures

[Song Liu]

On Thu, May 12, 2022 at 3:08 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> We see the following GPF when register_ftrace_direct fails:
>
> [ ] general protection fault, probably for non-canonical address \
>   0x200000000000010: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI
> [...]
> [ ] RIP: 0010:ftrace_find_rec_direct+0x53/0x70
> [ ] Code: 48 c1 e0 03 48 03 42 08 48 8b 10 31 c0 48 85 d2 74 [...]
> [ ] RSP: 0018:ffffc9000138bc10 EFLAGS: 00010206
> [ ] RAX: 0000000000000000 RBX: ffffffff813e0df0 RCX: 000000000000003b
> [ ] RDX: 0200000000000000 RSI: 000000000000000c RDI: ffffffff813e0df0
> [ ] RBP: ffffffffa00a3000 R08: ffffffff81180ce0 R09: 0000000000000001
> [ ] R10: ffffc9000138bc18 R11: 0000000000000001 R12: ffffffff813e0df0
> [ ] R13: ffffffff813e0df0 R14: ffff888171b56400 R15: 0000000000000000
> [ ] FS:  00007fa9420c7780(0000) GS:ffff888ff6a00000(0000) knlGS:000000000
> [ ] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ ] CR2: 000000000770d000 CR3: 0000000107d50003 CR4: 0000000000370ee0
> [ ] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ ] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ ] Call Trace:
> [ ]  <TASK>
> [ ]  register_ftrace_direct+0x54/0x290
> [ ]  ? render_sigset_t+0xa0/0xa0
> [ ]  bpf_trampoline_update+0x3f5/0x4a0
> [ ]  ? 0xffffffffa00a3000
> [ ]  bpf_trampoline_link_prog+0xa9/0x140
> [ ]  bpf_tracing_prog_attach+0x1dc/0x450
> [ ]  bpf_raw_tracepoint_open+0x9a/0x1e0
> [ ]  ? find_held_lock+0x2d/0x90
> [ ]  ? lock_release+0x150/0x430
> [ ]  __sys_bpf+0xbd6/0x2700
> [ ]  ? lock_is_held_type+0xd8/0x130
> [ ]  __x64_sys_bpf+0x1c/0x20
> [ ]  do_syscall_64+0x3a/0x80
> [ ]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> [ ] RIP: 0033:0x7fa9421defa9
> [ ] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 9 f8 [...]
> [ ] RSP: 002b:00007ffed743bd78 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> [ ] RAX: ffffffffffffffda RBX: 00000000069d2480 RCX: 00007fa9421defa9
> [ ] RDX: 0000000000000078 RSI: 00007ffed743bd80 RDI: 0000000000000011
> [ ] RBP: 00007ffed743be00 R08: 0000000000bb7270 R09: 0000000000000000
> [ ] R10: 00000000069da210 R11: 0000000000000246 R12: 0000000000000001
> [ ] R13: 00007ffed743c4b0 R14: 00000000069d2480 R15: 0000000000000001
> [ ]  </TASK>
> [ ] Modules linked in: klp_vm(OK)
> [ ] ---[ end trace 0000000000000000 ]---
>
> One way to trigger this is:
>   1. load a livepatch that patches kernel function xxx;
>   2. run bpftrace -e 'kfunc:xxx {}', this will fail (expected for now);
>   3. repeat #2 => gpf.
>
> This is because the entry is added to direct_functions, but not removed.
> Fix this by remove the entry from direct_functions when
> register_ftrace_direct fails.
>
> Also remove the last trailing space from ftrace.c, so we don't have to
> worry about it anymore.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 763e34e74bb7 ("ftrace: Add register_ftrace_direct()")
> Signed-off-by: Song Liu <song@xxxxxxxxxx>

Hi Steven,

Could you please share your thoughts on this fix?

Thanks,
Song

> ---
>  kernel/trace/ftrace.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 4f1d2f5e7263..375293c5f3e9 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -4465,7 +4465,7 @@ int ftrace_func_mapper_add_ip(struct ftrace_func_mapper *mapper,
>   * @ip: The instruction pointer address to remove the data from
>   *
>   * Returns the data if it is found, otherwise NULL.
> - * Note, if the data pointer is used as the data itself, (see
> + * Note, if the data pointer is used as the data itself, (see
>   * ftrace_func_mapper_find_ip(), then the return value may be meaningless,
>   * if the data pointer was set to zero.
>   */
> @@ -5200,8 +5200,10 @@ int register_ftrace_direct(unsigned long ip, unsigned long addr)
>
>         if (!ret && !(direct_ops.flags & FTRACE_OPS_FL_ENABLED)) {
>                 ret = register_ftrace_function(&direct_ops);
> -               if (ret)
> +               if (ret) {
>                         ftrace_set_filter_ip(&direct_ops, ip, 1, 0);
> +                       remove_hash_entry(direct_functions, entry);
> +               }
>         }
>
>         if (ret) {
> --
> 2.30.2
>