MIPS: ftrace: Fix dynamic tracing of kernel modules

Dynamic tracing of kernel modules is broken on 32-bit MIPS. When modules
are loaded, the kernel crashes when dynamic tracing is enabled with:
 cd /sys/kernel/debug/tracing
 echo > set_ftrace_filter
 echo function > current_tracer

1) arch/mips/kernel/ftrace.c
When the kernel boots, or when a module is initialized, ftrace_make_nop()
modifies every _mcount call site to eliminate the ftrace overhead.
However, when ftrace is later enabled for a call site, ftrace_make_call()
does not currently restore the _mcount call correctly for module call sites.
Added ftrace_modify_code_2r() and modified ftrace_make_call() to fix this.

2) arch/mips/kernel/mcount.S
_mcount assembly routine is supposed to have the caller's _mcount call site
address in register a0. However, a0 is currently not calculated correctly for
module call sites. a0 should be (ra - 20) or (ra - 24), depending on whether
the kernel was built with KBUILD_MCOUNT_RA_ADDRESS or not.

This fix has been tested on Broadcom BMIPS5000 processor. Dynamic tracing
now works for both built-in functions and module functions.

Signed-off-by: Petri Gynther <pgynther@google.com>
Cc: linux-mips@linux-mips.org
Cc: rostedt@goodmis.org
Cc: alcooperx@gmail.com
Cc: cminyard@mvista.com
Patchwork: https://patchwork.linux-mips.org/patch/7476/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/arch/mips/kernel/mcount.S b/arch/mips/kernel/mcount.S
index 539b6294b613..26ceb3c0eca9 100644
--- a/arch/mips/kernel/mcount.S
+++ b/arch/mips/kernel/mcount.S
@@ -84,6 +84,19 @@ _mcount:
 #endif
 
         PTR_SUBU a0, ra, 8      /* arg1: self address */
+        PTR_LA   t1, _stext
+        sltu     t2, a0, t1     /* t2 = (a0 < _stext) */
+        PTR_LA   t1, _etext
+        sltu     t3, t1, a0     /* t3 = (a0 > _etext) */
+        or       t1, t2, t3
+        beqz     t1, ftrace_call
+         nop
+#if defined(KBUILD_MCOUNT_RA_ADDRESS) && defined(CONFIG_32BIT)
+        PTR_SUBU a0, a0, 16     /* arg1: adjust to module's recorded callsite */
+#else
+        PTR_SUBU a0, a0, 12
+#endif
+
         .globl ftrace_call
 ftrace_call:
         nop     /* a placeholder for the call to a real tracing function */