powerpc/ftrace: Ignore ftrace locations in exit text sections

powerpc/ftrace: Ignore ftrace locations in exit text sections

Michael reported that we are seeing an ftrace bug on bootup when KASAN
is enabled and we are using -fpatchable-function-entry:

  ftrace: allocating 47780 entries in 18 pages
  ftrace-powerpc: 0xc0000000020b3d5c: No module provided for non-kernel address
  ------------[ ftrace bug ]------------
  ftrace faulted on modifying
  [<c0000000020b3d5c>] 0xc0000000020b3d5c
  Initializing ftrace call sites
  ftrace record flags: 0
   (0)
   expected tramp: c00000000008cef4
  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 0 at kernel/trace/ftrace.c:2180 ftrace_bug+0x3c0/0x424
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper Not tainted 6.5.0-rc3-00120-g0f71dcfb4aef #860
  Hardware name: IBM pSeries (emulated by qemu) POWER9 (raw) 0x4e1202 0xf000005 of:SLOF,HEAD hv:linux,kvm pSeries
  NIP:  c0000000003aa81c LR: c0000000003aa818 CTR: 0000000000000000
  REGS: c0000000033cfab0 TRAP: 0700   Not tainted  (6.5.0-rc3-00120-g0f71dcfb4aef)
  MSR:  8000000002021033 <SF,VEC,ME,IR,DR,RI,LE>  CR: 28028240  XER: 00000000
  CFAR: c0000000002781a8 IRQMASK: 3
  ...
  NIP [c0000000003aa81c] ftrace_bug+0x3c0/0x424
  LR [c0000000003aa818] ftrace_bug+0x3bc/0x424
  Call Trace:
   ftrace_bug+0x3bc/0x424 (unreliable)
   ftrace_process_locs+0x5f4/0x8a0
   ftrace_init+0xc0/0x1d0
   start_kernel+0x1d8/0x484

With CONFIG_FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY=y and
CONFIG_KASAN=y, compiler emits nops in functions that it generates for
registering and unregistering global variables (unlike with -pg and
-mprofile-kernel where calls to _mcount() are not generated in those
functions). Those functions then end up in INIT_TEXT and EXIT_TEXT
respectively. We don't expect to see any profiled functions in
EXIT_TEXT, so ftrace_init_nop() assumes that all addresses that aren't
in the core kernel text belongs to a module. Since these functions do
not match that criteria, we see the above bug.

Address this by having ftrace ignore all locations in the text exit
sections of vmlinux.

Fixes: 0f71dcfb4aef ("powerpc/ftrace: Add support for -fpatchable-function-entry")
Cc: stable@vger.kernel.org # v6.6+
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Naveen N Rao <naveen@kernel.org>
Reviewed-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240213175410.1091313-1-naveen@kernel.org