[RESEND PATCH 4/4] ppc64 ftrace_with_regs recursion protection

From: Torsten Duwe
Date: Thu Jun 18 2015 - 12:25:44 EST

This is an *emergency* parachute to avoid an endless recursion and
consecutively a kernel stack overflow, should any function within some
ftrace framework cause an access fault, which calls _mcount / ftrace_caller
in return and so on. It might also call an ftrace'd function directly.

As Michael Ellerman pointed out, it is a tedious and error-prone task
to maintain a complete list of those functions that _might_ get called
from *_access_fault or any dynamic tracer function. So we'll
concentrate on the most frequent cases to enhance performance later,
while for now sticking with this fill-in. It will later serve as a backup

* arch/powerpc/kernel/entry_64.S:
- test-and-set TRACE_FTRACE_BIT in task_struct's trace_recursion,
do not call the actual tracer function if set, clear flag on return.

Signed-off-by: Torsten Duwe <duwe@xxxxxxx>
arch/powerpc/kernel/asm-offsets.c | 1 +
arch/powerpc/kernel/entry_64.S | 15 +++++++++++++--
2 files changed, 14 insertions(+), 2 deletions(-)
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 4717859..ae10752 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -72,6 +72,7 @@ int main(void)
DEFINE(THREAD, offsetof(struct task_struct, thread));
DEFINE(MM, offsetof(struct task_struct, mm));
DEFINE(MMCONTEXTID, offsetof(struct mm_struct, context.id));
+ DEFINE(TASK_TRACEREC, offsetof(struct task_struct, trace_recursion));
#ifdef CONFIG_PPC64
DEFINE(AUDITCONTEXT, offsetof(struct task_struct, audit_context));
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index a4132ef..4768104 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -1202,7 +1202,13 @@ _GLOBAL(ftrace_caller)

+ ld r3, PACACURRENT(r13)
+ ld r4, TASK_TRACEREC(r3)
+ andi. r5, r4, 0x0010 // ( 1 << TRACE_FTRACE_BIT )
+ ori r4, r4, 0x0010
+ std r4, TASK_TRACEREC(r3)
+ bne- 3f // ftrace in progress - avoid recursion!
ld r5,0(r3)

@@ -1224,9 +1230,14 @@ ftrace_call:
bl ftrace_stub

+ ld r3, PACACURRENT(r13)
+ ld r4, TASK_TRACEREC(r3)
+ andi. r4, r4, 0xffef // ~( 1 << TRACE_FTRACE_BIT )
+ std r4, TASK_TRACEREC(r3)
ld r3, _NIP(r1)
mtlr r3
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