Re: [5/5] powerpc/livepatch: Add live patching support on ppc64le

[Michael Ellerman]

On Wed, 2016-13-04 at 12:53:23 UTC, Michael Ellerman wrote:
> Add the kconfig logic & assembly support for handling live patched
> functions. This depends on DYNAMIC_FTRACE_WITH_REGS, which in turn
> depends on the new -mprofile-kernel ftrace ABI, which is only supported
> currently on ppc64le.
...
> 
> Signed-off-by: Michael Ellerman 
> Reviewed-by: Torsten Duwe 
> Reviewed-by: Balbir Singh 

Applied to powerpc next.

https://git.kernel.org/powerpc/c/85baa095497f3e590df9f6c893

cheers
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[PATCH 5/5] powerpc/livepatch: Add live patching support on ppc64le

[Michael Ellerman]

Add the kconfig logic & assembly support for handling live patched
functions. This depends on DYNAMIC_FTRACE_WITH_REGS, which in turn
depends on the new -mprofile-kernel ftrace ABI, which is only supported
currently on ppc64le.

Live patching is handled by a special ftrace handler. This means it runs
from ftrace_caller(). The live patch handler modifies the NIP so as to
redirect the return from ftrace_caller() to the new patched function.

However there is one particularly tricky case we need to handle.

If a function A calls another function B, and it is known at link time
that they share the same TOC, then A will not save or restore its TOC,
and will call the local entry point of B.

When we live patch B, we replace it with a new function C, which may
not have the same TOC as A. At live patch time it's too late to modify A
to do the TOC save/restore, so the live patching code must interpose
itself between A and C, and do the TOC save/restore that A omitted.

An additionaly complication is that the livepatch code can not create a
stack frame in order to save the TOC. That is because if C takes > 8
arguments, or is varargs, A will have written the arguments for C in
A's stack frame.

To solve this, we introduce a "livepatch stack" which grows upward from
the base of the regular stack, and is used to store the TOC & LR when
calling a live patched function.

When the patched function returns, we retrieve the real LR & TOC from
the livepatch stack, restore them, and pop the livepatch "stack frame".

Signed-off-by: Michael Ellerman 
Reviewed-by: Torsten Duwe 
Reviewed-by: Balbir Singh 
---
 arch/powerpc/Kconfig  |  3 ++
 arch/powerpc/kernel/asm-offsets.c |  4 ++
 arch/powerpc/kernel/entry_64.S| 97 +++
 3 files changed, 104 insertions(+)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 7cd32c038286..ed0603102442 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -160,6 +160,7 @@ config PPC
select HAVE_ARCH_SECCOMP_FILTER
select ARCH_HAS_UBSAN_SANITIZE_ALL
select ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT
+   select HAVE_LIVEPATCH if HAVE_DYNAMIC_FTRACE_WITH_REGS
 
 config GENERIC_CSUM
def_bool CPU_LITTLE_ENDIAN
@@ -1107,3 +1108,5 @@ config PPC_LIB_RHEAP
bool
 
 source "arch/powerpc/kvm/Kconfig"
+
+source "kernel/livepatch/Kconfig"
diff --git a/arch/powerpc/kernel/asm-offsets.c 
b/arch/powerpc/kernel/asm-offsets.c
index 0d0183d3180a..c9370d4e36bd 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -86,6 +86,10 @@ int main(void)
DEFINE(KSP_LIMIT, offsetof(struct thread_struct, ksp_limit));
 #endif /* CONFIG_PPC64 */
 
+#ifdef CONFIG_LIVEPATCH
+   DEFINE(TI_livepatch_sp, offsetof(struct thread_info, livepatch_sp));
+#endif
+
DEFINE(KSP, offsetof(struct thread_struct, ksp));
DEFINE(PT_REGS, offsetof(struct thread_struct, regs));
 #ifdef CONFIG_BOOKE
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 9916d150b28c..39a79c89a4b6 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -20,6 +20,7 @@
 
 #include 
 #include 
+#include 
 #include 
 #include 
 #include 
@@ -1248,6 +1249,9 @@ _GLOBAL(ftrace_caller)
addir3,r3,function_trace_op@toc@l
ld  r5,0(r3)
 
+#ifdef CONFIG_LIVEPATCH
+   mr  r14,r7  /* remember old NIP */
+#endif
/* Calculate ip from nip-4 into r3 for call below */
subir3, r7, MCOUNT_INSN_SIZE
 
@@ -1272,6 +1276,9 @@ ftrace_call:
/* Load ctr with the possibly modified NIP */
ld  r3, _NIP(r1)
mtctr   r3
+#ifdef CONFIG_LIVEPATCH
+   cmpdr14,r3  /* has NIP been altered? */
+#endif
 
/* Restore gprs */
REST_8GPRS(0,r1)
@@ -1289,6 +1296,11 @@ ftrace_call:
ld  r0, LRSAVE(r1)
mtlrr0
 
+#ifdef CONFIG_LIVEPATCH
+/* Based on the cmpd above, if the NIP was altered handle livepatch */
+   bne-livepatch_handler
+#endif
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
stdur1, -112(r1)
 .globl ftrace_graph_call
@@ -1305,6 +1317,91 @@ _GLOBAL(ftrace_graph_stub)
 
 _GLOBAL(ftrace_stub)
blr
+
+#ifdef CONFIG_LIVEPATCH
+   /*
+* This function runs in the mcount context, between two functions. As
+* such it can only clobber registers which are volatile and used in
+* function linkage.
+*
+* We get here when a function A, calls another function B, but B has
+* been live patched with a new function C.
+*
+* On entry:
+*  - we have no stack frame and can not allocate one
+*  - LR points back to the original caller (in A)
+*  - CTR holds the new NIP in C
+*  - r0 & r12 are free
+*
+* r0 can't be used as the base register for a DS-form load or