On Wed, May 23, 2018 at 06:21:31PM +0200, Martin Schwidefsky wrote:
> [ Upstream commit 23a4d7fd34856da8218c4cfc23dba7a6ec0a423a ]
>
> 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@xxxxxxxxxxxxxxx # 4.16
> Fixes: f19fbd5ed6 ("s390: introduce execute-trampolines for branches")
> Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> ---
> arch/s390/include/asm/nospec-insn.h | 11 +++++++++++
> arch/s390/kernel/asm-offsets.c | 1 +
> arch/s390/kernel/mcount.S | 14 +++++++++-----
> 3 files changed, 21 insertions(+), 5 deletions(-)
This patch breaks the build for 'allmodconfig' for s390, but defconfig
works fine :(
The error is:
LD init/built-in.o
arch/s390/built-in.o: In function `__s390x_indirect_jump_r1use_r1':
(.text.__s390x_indirect_jump_r1use_r1[__s390x_indirect_jump_r1use_r1]+0x2): undefined reference to `_LC_BR_R1'
make: *** [Makefile:956: vmlinux] Error 1
So I'm going to drop this patch, and the rest in the series, from the
4.4.y tree. Can you please fix this up and resend the needed patches
after I do the next 4.4.y kernel release?
thanks,
greg k-h
