Re: [PATCH 4/9] s390/ftrace: use expoline for indirect branches

[Greg KH <greg@kroah.com>]

On Fri, May 25, 2018 at 03:54:20PM +0200, Greg KH wrote:
> 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@vger.kernel.org # 4.16
> > Fixes: f19fbd5ed6 ("s390: introduce execute-trampolines for branches")
> > Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > ---
> >  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?

Hm, it looks like everything at least boots with just this one patch
removed, so I've done that.  I have no idea if it boots, I'll leave that
to people who have access to those types of boxes :)

thanks,

greg k-h