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Commit 23a4d7fd authored by Martin Schwidefsky's avatar Martin Schwidefsky
Browse files

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: f19fbd5e ("s390: introduce execute-trampolines for branches")
Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
parent 97489e06

arch/s390/include/asm/nospec-insn.h

+12 −0
Original line number Diff line number Diff line
@@ -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

arch/s390/kernel/asm-offsets.c

+1 −0
Original line number Diff line number Diff line
@@ -181,6 +181,7 @@ int main(void) 
	OFFSET(__LC_MACHINE_FLAGS, lowcore, machine_flags);

	OFFSET(__LC_PREEMPT_COUNT, lowcore, preempt_count);

	OFFSET(__LC_GMAP, lowcore, gmap);

	OFFSET(__LC_BR_R1, lowcore, br_r1_trampoline);

	/* software defined ABI-relevant lowcore locations 0xe00 - 0xe20 */

	OFFSET(__LC_DUMP_REIPL, lowcore, ipib);

	/* hardware defined lowcore locations 0x1000 - 0x18ff */

arch/s390/kernel/mcount.S

+9 −5
Original line number Diff line number Diff line
@@ -9,13 +9,17 @@ 
#include <linux/linkage.h>

#include <asm/asm-offsets.h>

#include <asm/ftrace.h>

#include <asm/nospec-insn.h>

#include <asm/ptrace.h>

#include <asm/export.h>



	GEN_BR_THUNK %r1

	GEN_BR_THUNK %r14



	.section .kprobes.text, "ax"



ENTRY(ftrace_stub)

	br	%r14

	BR_EX	%r14



#define STACK_FRAME_SIZE  (STACK_FRAME_OVERHEAD + __PT_SIZE)

#define STACK_PTREGS	  (STACK_FRAME_OVERHEAD)
@@ -23,7 +27,7 @@ ENTRY(ftrace_stub) 
#define STACK_PTREGS_PSW  (STACK_PTREGS + __PT_PSW)



ENTRY(_mcount)

	br	%r14

	BR_EX	%r14



EXPORT_SYMBOL(_mcount)
@@ -53,7 +57,7 @@ ENTRY(ftrace_caller) 
#endif

	lgr	%r3,%r14

	la	%r5,STACK_PTREGS(%r15)

	basr	%r14,%r1

	BASR_EX	%r14,%r1

#ifdef CONFIG_FUNCTION_GRAPH_TRACER

# The j instruction gets runtime patched to a nop instruction.

# See ftrace_enable_ftrace_graph_caller.
@@ -68,7 +72,7 @@ ftrace_graph_caller_end: 
#endif

	lg	%r1,(STACK_PTREGS_PSW+8)(%r15)

	lmg	%r2,%r15,(STACK_PTREGS_GPRS+2*8)(%r15)

	br	%r1

	BR_EX	%r1



#ifdef CONFIG_FUNCTION_GRAPH_TRACER
@@ -81,6 +85,6 @@ ENTRY(return_to_handler) 
	aghi	%r15,STACK_FRAME_OVERHEAD

	lgr	%r14,%r2

	lmg	%r2,%r5,32(%r15)

	br	%r14

	BR_EX	%r14



#endif