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SubjectRe: [PATCH 12/15] x86/static_call: Add out-of-line static call implementation
Date
> 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).

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