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s390/ftrace: use expoline for indirect branches

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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>
This commit is contained in:
Martin Schwidefsky 2018-04-25 18:35:26 +02:00
parent 97489e0663
commit 23a4d7fd34
3 changed files with 22 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
arch/s390/include/asm/nospec-insn.h
View File

@ -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

1
arch/s390/kernel/asm-offsets.c
View File

@ -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 */

14
arch/s390/kernel/mcount.S
View File

@ -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