s390/ftrace: use expoline for indirect branches
The return from the ftrace_stub, _mcount, ftrace_caller and
return_to_handler functions is done with "br %r14" and "br %r1".
These are indirect branches as well and need to use execute
trampolines for CONFIG_EXPOLINE=y.
The ftrace_caller function is a special case as it returns to the
start of a function and may only use %r0 and %r1. For a pre z10
machine the standard execute trampoline uses a LARL + EX to do
this, but this requires *two* registers in the range %r1..%r15.
To get around this the 'br %r1' located in the lowcore is used,
then the EX instruction does not need an address register.
But the lowcore trick may only be used for pre z14 machines,
with noexec=on the mapping for the first page may not contain
instructions. The solution for that is an ALTERNATIVE in the
expoline THUNK generated by 'GEN_BR_THUNK %r1' to switch to
EXRL, this relies on the fact that a machine that supports
noexec=on has EXRL as well.
Cc: stable@vger.kernel.org # 4.16
Fixes: f19fbd5ed6 ("s390: introduce execute-trampolines for branches")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/include/asm/nospec-insn.h b/arch/s390/include/asm/nospec-insn.h
index 440689c..7d7640e 100644
--- a/arch/s390/include/asm/nospec-insn.h
+++ b/arch/s390/include/asm/nospec-insn.h
@@ -2,12 +2,16 @@
#ifndef _ASM_S390_NOSPEC_ASM_H
#define _ASM_S390_NOSPEC_ASM_H
+#include <asm/alternative-asm.h>
+#include <asm/asm-offsets.h>
#include <asm/dwarf.h>
#ifdef __ASSEMBLY__
#ifdef CONFIG_EXPOLINE
+_LC_BR_R1 = __LC_BR_R1
+
/*
* The expoline macros are used to create thunks in the same format
* as gcc generates them. The 'comdat' section flag makes sure that
@@ -78,13 +82,21 @@
.endm
.macro __THUNK_EX_BR reg,ruse
+ # Be very careful when adding instructions to this macro!
+ # The ALTERNATIVE replacement code has a .+10 which targets
+ # the "br \reg" after the code has been patched.
#ifdef CONFIG_HAVE_MARCH_Z10_FEATURES
exrl 0,555f
j .
#else
+ .ifc \reg,%r1
+ ALTERNATIVE "ex %r0,_LC_BR_R1", ".insn ril,0xc60000000000,0,.+10", 35
+ j .
+ .else
larl \ruse,555f
ex 0,0(\ruse)
j .
+ .endif
#endif
555: br \reg
.endm