Re: [PATCH] ftrace/module: Allow ftrace to make only loaded module text read-write

[Peter Zijlstra]

On Thu, Oct 10, 2019 at 09:31:21AM +0200, Peter Zijlstra wrote:
> On Wed, Oct 09, 2019 at 10:36:38PM -0400, Steven Rostedt wrote:
> > From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> > 
> > In the process of using text_poke_bp() for ftrace on x86, when
> > performing the following action:
> > 
> >  # rmmod snd_hda_codec_hdmi
> >  # echo function > /sys/kernel/tracing/current_tracer
> >  # modprobe snd_hda_codec_hdmi
> > 
> > It triggered this:
> > 
> >  BUG: unable to handle page fault for address: ffffffffa03d0000
> >  #PF: supervisor write access in kernel mode
> >  #PF: error_code(0x0003) - permissions violation
> >  PGD 2a12067 P4D 2a12067 PUD 2a13063 PMD c42bc067 PTE c58a0061
> >  Oops: 0003 [#1] PREEMPT SMP KASAN PTI
> >  CPU: 1 PID: 1182 Comm: modprobe Not tainted 5.4.0-rc2-test+ #50
> >  Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
> >  RIP: 0010:memcpy_erms+0x6/0x10
> >  Code: 90 90 90 90 eb 1e 0f 1f 00 48 89 f8 48 89 d1 48 c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 c3 66 0f 1f 44 00 00 48 89 f8 48 89 d1 <f3> a4 c3 0f 1f 80 00 00 00 00 48 89 f8 48 83 fa 20 72 7e 40 38 fe
> >  RSP: 0018:ffff8880a10479e0 EFLAGS: 00010246
> >  RAX: ffffffffa03d0000 RBX: ffffffffa03d0000 RCX: 0000000000000005
> >  RDX: 0000000000000005 RSI: ffffffff8363e160 RDI: ffffffffa03d0000
> >  RBP: ffff88807e9ec000 R08: fffffbfff407a001 R09: fffffbfff407a001
> >  R10: fffffbfff407a000 R11: ffffffffa03d0004 R12: ffffffff8221f160
> >  R13: ffffffffa03d0000 R14: ffff88807e9ec000 R15: ffffffffa0481640
> >  FS:  00007eff92e28280(0000) GS:ffff8880d4840000(0000) knlGS:0000000000000000
> >  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >  CR2: ffffffffa03d0000 CR3: 00000000a1048001 CR4: 00000000001606e0
> >  Call Trace:
> >   ftrace_make_call+0x76/0x90
> >   ftrace_module_enable+0x493/0x4f0
> >   load_module+0x3a31/0x3e10
> >   ? ring_buffer_read+0x70/0x70
> >   ? module_frob_arch_sections+0x20/0x20
> >   ? rb_commit+0xee/0x600
> >   ? tracing_generic_entry_update+0xe1/0xf0
> >   ? ring_buffer_unlock_commit+0xfb/0x220
> >   ? 0xffffffffa0000061
> >   ? __do_sys_finit_module+0x11a/0x1b0
> >   __do_sys_finit_module+0x11a/0x1b0
> >   ? __ia32_sys_init_module+0x40/0x40
> >   ? ring_buffer_unlock_commit+0xfb/0x220
> >   ? function_trace_call+0x179/0x260
> >   ? __do_sys_finit_module+0x1b0/0x1b0
> >   ? __do_sys_finit_module+0x1b0/0x1b0
> >   ? do_syscall_64+0x58/0x1a0
> >   do_syscall_64+0x68/0x1a0
> >   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >  RIP: 0033:0x7eff92f42efd
> > 
> > The reason is that ftrace_module_enable() is called after the module
> > has set its text to read-only. There's subtle reasons that this needs
> > to be called afterward, and we need to continue to do so.
> 
> Please explain.

Also, no. If you _have_ to modify code after, then you get to use
text_poke_bp() instead of text_poke_early().

But first we need to document why this needs to be late. That's the
crucial point and you forgot to explain.