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

[Nadav Amit]

> On Jun 5, 2019, at 6:08 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Julia Cartwright <julia@xxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Jason Baron <jbaron@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Jiri Kosina <jkosina@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: David Laight <David.Laight@xxxxxxxxxx>
> Cc: Jessica Yu <jeyu@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> 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 donât know if you want to be more gentle handling of this case (or perhaps
modify can_probe() to prevent such a case).