Re: [PATCH 12/15] x86/static_call: Add out-of-line static call implementation

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 7 Jun 2019 06:13:58 +0000
Nadav Amit <namit@vmware.com> wrote:

> > On Jun 5, 2019, at 6:08 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> > From: Josh Poimboeuf <jpoimboe@redhat.com>
> > 
> > Add the x86 out-of-line static call implementation.  For each key, a
> > permanent trampoline is created which is the destination for all static
> > calls for the given key.  The trampoline has a direct jump which gets
> > patched by static_call_update() when the destination function changes.
> > 
> > Cc: x86@kernel.org
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Julia Cartwright <julia@ni.com>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > Cc: Jason Baron <jbaron@akamai.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Jiri Kosina <jkosina@suse.cz>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Masami Hiramatsu <mhiramat@kernel.org>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: David Laight <David.Laight@ACULAB.COM>
> > Cc: Jessica Yu <jeyu@kernel.org>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Link: https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F00b08f2194e80241decbf206624b6580b9b8855b.1543200841.git.jpoimboe%40redhat.com&amp;data=02%7C01%7Cnamit%40vmware.com%7C13bc03381930464a018e08d6e9b8f90e%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636953378007810030&amp;sdata=UnHEUYEYV3FBSZj667lZYzGKRov%2B1PdAjAnM%2BqOz3Ns%3D&amp;reserved=0
> > ---
> > arch/x86/Kconfig                   |    1 
> > arch/x86/include/asm/static_call.h |   28 +++++++++++++++++++++++++++
> > arch/x86/kernel/Makefile           |    1 
> > arch/x86/kernel/static_call.c      |   38 +++++++++++++++++++++++++++++++++++++
> > 4 files changed, 68 insertions(+)
> > create mode 100644 arch/x86/include/asm/static_call.h
> > create mode 100644 arch/x86/kernel/static_call.c
> > 
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -198,6 +198,7 @@ config X86
> > 	select HAVE_FUNCTION_ARG_ACCESS_API
> > 	select HAVE_STACKPROTECTOR		if CC_HAS_SANE_STACKPROTECTOR
> > 	select HAVE_STACK_VALIDATION		if X86_64
> > +	select HAVE_STATIC_CALL
> > 	select HAVE_RSEQ
> > 	select HAVE_SYSCALL_TRACEPOINTS
> > 	select HAVE_UNSTABLE_SCHED_CLOCK
> > --- /dev/null
> > +++ b/arch/x86/include/asm/static_call.h
> > @@ -0,0 +1,28 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _ASM_STATIC_CALL_H
> > +#define _ASM_STATIC_CALL_H
> > +
> > +/*
> > + * Manually construct a 5-byte direct JMP to prevent the assembler from
> > + * optimizing it into a 2-byte JMP.
> > + */
> > +#define __ARCH_STATIC_CALL_JMP_LABEL(key) ".L" __stringify(key ## _after_jmp)
> > +#define __ARCH_STATIC_CALL_TRAMP_JMP(key, func)				\
> > +	".byte 0xe9						\n"	\
> > +	".long " #func " - " __ARCH_STATIC_CALL_JMP_LABEL(key) "\n"	\
> > +	__ARCH_STATIC_CALL_JMP_LABEL(key) ":"
> > +
> > +/*
> > + * This is a permanent trampoline which does a direct jump to the function.
> > + * The direct jump get patched by static_call_update().
> > + */
> > +#define ARCH_DEFINE_STATIC_CALL_TRAMP(key, func)			\
> > +	asm(".pushsection .text, \"ax\"				\n"	\
> > +	    ".align 4						\n"	\
> > +	    ".globl " STATIC_CALL_TRAMP_STR(key) "		\n"	\
> > +	    ".type " STATIC_CALL_TRAMP_STR(key) ", @function	\n"	\
> > +	    STATIC_CALL_TRAMP_STR(key) ":			\n"	\
> > +	    __ARCH_STATIC_CALL_TRAMP_JMP(key, func) "           \n"	\
> > +	    ".popsection					\n")
> > +
> > +#endif /* _ASM_STATIC_CALL_H */
> > --- a/arch/x86/kernel/Makefile
> > +++ b/arch/x86/kernel/Makefile
> > @@ -63,6 +63,7 @@ obj-y			+= tsc.o tsc_msr.o io_delay.o rt
> > obj-y			+= pci-iommu_table.o
> > obj-y			+= resource.o
> > obj-y			+= irqflags.o
> > +obj-y			+= static_call.o
> > 
> > obj-y				+= process.o
> > obj-y				+= fpu/
> > --- /dev/null
> > +++ b/arch/x86/kernel/static_call.c
> > @@ -0,0 +1,38 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#include <linux/static_call.h>
> > +#include <linux/memory.h>
> > +#include <linux/bug.h>
> > +#include <asm/text-patching.h>
> > +#include <asm/nospec-branch.h>
> > +
> > +#define CALL_INSN_SIZE 5
> > +
> > +void arch_static_call_transform(void *site, void *tramp, void *func)
> > +{
> > +	unsigned char opcodes[CALL_INSN_SIZE];
> > +	unsigned char insn_opcode;
> > +	unsigned long insn;
> > +	s32 dest_relative;
> > +
> > +	mutex_lock(&text_mutex);
> > +
> > +	insn = (unsigned long)tramp;
> > +
> > +	insn_opcode = *(unsigned char *)insn;
> > +	if (insn_opcode != 0xE9) {
> > +		WARN_ONCE(1, "unexpected static call insn opcode 0x%x at %pS",
> > +			  insn_opcode, (void *)insn);
> > +		goto unlock;  
> 
> This might happen if a kprobe is installed on the call, no?
> 
> I don’t know if you want to be more gentle handling of this case (or perhaps
> modify can_probe() to prevent such a case).
> 

Perhaps it is better to block kprobes from attaching to a static call.
Or have it use the static call directly as it does with ftrace. But
that would probably be much more work.

-- Steve