Re: [PATCH] ARM: kprobes: move __kretprobe_trampoline to out of line assembler

From: Ard Biesheuvel
Date: Wed Oct 05 2022 - 19:00:02 EST

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Hi NIck,

On Mon, 26 Sept 2022 at 20:37, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> commit 1069c1dd20a3 ("ARM: 9231/1: Recover kretprobes return address for
> EABI stack unwinder")
> tickled a bug in clang's integrated assembler where the .save and .pad
> directives must have corresponding .fnstart directives. The integrated
> assembler is unaware that the compiler will be generating the .fnstart
> directive.
>
> arch/arm/probes/kprobes/core.c:409:30: error: .fnstart must precede
> .save or .vsave directives
> <inline asm>:3:2: note: instantiated into assembly here
> .save {sp, lr, pc}
> ^
> arch/arm/probes/kprobes/core.c:412:29: error: .fnstart must precede
> .pad directive
> <inline asm>:6:2: note: instantiated into assembly here
> .pad #52
> ^
>
> __kretprobe_trampoline's definition is already entirely inline asm. Move
> it to out-of-line asm to avoid breaking the build.
>

I think this is the right approach. I don't think is is even specified
what exactly attribute((naked)) entails in the context of unwind
information

Some nits below, but regardless:

Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

> Link: https://github.com/llvm/llvm-project/issues/57993
> Link: https://github.com/ClangBuiltLinux/linux/issues/1718
> Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
> Suggested-by: Logan Chien <loganchien@xxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> Note: I wasn't quite sure if a Fixes tag against 1069c1dd20a3 was
> appropriate here? Either way, if 1069c1dd20a3 gets picked up for stable
> without this, it will break clang builds.
>
> arch/arm/probes/kprobes/Makefile | 1 +
> arch/arm/probes/kprobes/core.c | 54 ++----------------
> .../arm/probes/kprobes/kretprobe-trampoline.S | 55 +++++++++++++++++++
> include/asm-generic/kprobes.h | 13 +++--
> 4 files changed, 69 insertions(+), 54 deletions(-)
> create mode 100644 arch/arm/probes/kprobes/kretprobe-trampoline.S
>
> diff --git a/arch/arm/probes/kprobes/Makefile b/arch/arm/probes/kprobes/Makefile
> index 6159010dac4a..cdbe9dd99e28 100644
> --- a/arch/arm/probes/kprobes/Makefile
> +++ b/arch/arm/probes/kprobes/Makefile
> @@ -3,6 +3,7 @@ KASAN_SANITIZE_actions-common.o := n
> KASAN_SANITIZE_actions-arm.o := n
> KASAN_SANITIZE_actions-thumb.o := n
> obj-$(CONFIG_KPROBES) += core.o actions-common.o checkers-common.o
> +obj-$(CONFIG_KPROBES) += kretprobe-trampoline.o
> obj-$(CONFIG_ARM_KPROBES_TEST) += test-kprobes.o
> test-kprobes-objs := test-core.o
>
> diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
> index 1435b508aa36..17d7e0259e63 100644
> --- a/arch/arm/probes/kprobes/core.c
> +++ b/arch/arm/probes/kprobes/core.c
> @@ -375,58 +375,10 @@ int __kprobes kprobe_exceptions_notify(struct notifier_block *self,
> return NOTIFY_DONE;
> }
>
> -/*
> - * When a retprobed function returns, trampoline_handler() is called,
> - * calling the kretprobe's handler. We construct a struct pt_regs to
> - * give a view of registers r0-r11, sp, lr, and pc to the user
> - * return-handler. This is not a complete pt_regs structure, but that
> - * should be enough for stacktrace from the return handler with or
> - * without pt_regs.
> - */
> -void __naked __kprobes __kretprobe_trampoline(void)
> -{
> - __asm__ __volatile__ (
> - "ldr lr, =__kretprobe_trampoline \n\t"
> -#ifdef CONFIG_FRAME_POINTER
> - /* __kretprobe_trampoline makes a framepointer on pt_regs. */
> -#ifdef CONFIG_CC_IS_CLANG
> - "stmdb sp, {sp, lr, pc} \n\t"
> - "sub sp, sp, #12 \n\t"
> - /* In clang case, pt_regs->ip = lr. */
> - "stmdb sp!, {r0 - r11, lr} \n\t"
> - /* fp points regs->r11 (fp) */
> - "add fp, sp, #44 \n\t"
> -#else /* !CONFIG_CC_IS_CLANG */
> - /* In gcc case, pt_regs->ip = fp. */
> - "stmdb sp, {fp, sp, lr, pc} \n\t"
> - "sub sp, sp, #16 \n\t"
> - "stmdb sp!, {r0 - r11} \n\t"
> - /* fp points regs->r15 (pc) */
> - "add fp, sp, #60 \n\t"
> -#endif /* CONFIG_CC_IS_CLANG */
> -#else /* !CONFIG_FRAME_POINTER */
> - /* store SP, LR on stack and add EABI unwind hint */
> - "stmdb sp, {sp, lr, pc} \n\t"
> - ".save {sp, lr, pc} \n\t"
> - "sub sp, sp, #16 \n\t"
> - "stmdb sp!, {r0 - r11} \n\t"
> - ".pad #52 \n\t"
> -#endif /* CONFIG_FRAME_POINTER */
> - "mov r0, sp \n\t"
> - "bl trampoline_handler \n\t"
> - "mov lr, r0 \n\t"
> - "ldmia sp!, {r0 - r11} \n\t"
> - "add sp, sp, #16 \n\t"
> -#ifdef CONFIG_THUMB2_KERNEL
> - "bx lr \n\t"
> -#else
> - "mov pc, lr \n\t"
> -#endif
> - : : : "memory");
> -}
> +/*void __kretprobe_trampoline(void);*/
>
> /* Called from __kretprobe_trampoline */
> -static __used __kprobes void *trampoline_handler(struct pt_regs *regs)
> +__kprobes void *trampoline_handler(struct pt_regs *regs)
> {
> return (void *)kretprobe_trampoline_handler(regs, (void *)regs->TRAMP_FP);
> }
> @@ -434,6 +386,8 @@ static __used __kprobes void *trampoline_handler(struct pt_regs *regs)
> void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri,
> struct pt_regs *regs)
> {
> + extern void __kretprobe_trampoline(void);
> +
> ri->ret_addr = (kprobe_opcode_t *)regs->ARM_lr;
> ri->fp = (void *)regs->TRAMP_FP;
>
> diff --git a/arch/arm/probes/kprobes/kretprobe-trampoline.S b/arch/arm/probes/kprobes/kretprobe-trampoline.S
> new file mode 100644
> index 000000000000..261c99b8c17f
> --- /dev/null
> +++ b/arch/arm/probes/kprobes/kretprobe-trampoline.S
> @@ -0,0 +1,55 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#include <linux/linkage.h>
> +#include <asm/unwind.h>
> +#include <asm-generic/kprobes.h>
> +
> +/*
> + * When a retprobed function returns, trampoline_handler() is called,
> + * calling the kretprobe's handler. We construct a struct pt_regs to
> + * give a view of registers r0-r11, sp, lr, and pc to the user
> + * return-handler. This is not a complete pt_regs structure, but that
> + * should be enough for stacktrace from the return handler with or
> + * without pt_regs.
> + */
> +__KPROBE
> +SYM_FUNC_START(__kretprobe_trampoline)
> +UNWIND(.fnstart)
> + ldr lr, =__kretprobe_trampoline

Nit: adr lr, __kretprobe_trampoline

Now that you made a clean spot, might as well clean this up a bit
(although perhaps not in the same patch). I'd merge the
!CONFIG_FRAME_POINTER and CC_IS_CLANG cases, and just put the UNWIND()
directives in there.

> +#ifdef CONFIG_FRAME_POINTER

Drop this

> + /* __kretprobe_trampoline makes a framepointer on pt_regs. */
> +#ifdef CONFIG_CC_IS_CLANG

|| !defined(CONFIG_FRAME_POINTER)

> + stmdb sp, {sp, lr, pc}
> + sub sp, sp, #12

UNWIND(.save {sp, lr, pc})

> + /* In clang case, pt_regs->ip = lr. */

For .S, it is more idiomatic to put the comments in a column on the
right. This reduces visual clutter a lot, so I think it would be worth
the effort.

> + stmdb sp!, {r0 - r11, lr}

UNWIND(.pad #52)

> + /* fp points regs->r11 (fp) */
> + add fp, sp, #44
> +#else /* !CONFIG_CC_IS_CLANG */
> + /* In gcc case, pt_regs->ip = fp. */
> + stmdb sp, {fp, sp, lr, pc}
> + sub sp, sp, #16
> + stmdb sp!, {r0 - r11}
> + /* fp points regs->r15 (pc) */
> + add fp, sp, #60
> +#endif /* CONFIG_CC_IS_CLANG */
> +#else /* !CONFIG_FRAME_POINTER */
> + /* store SP, LR on stack and add EABI unwind hint */
> + stmdb sp, {sp, lr, pc}
> +UNWIND(.save {sp, lr, pc})
> + sub sp, sp, #16
> + stmdb sp!, {r0 - r11}
> +UNWIND(.pad #52)
> +#endif /* CONFIG_FRAME_POINTER */
> + mov r0, sp
> + bl trampoline_handler
> + mov lr, r0
> + ldmia sp!, {r0 - r11}
> + add sp, sp, #16
> +#ifdef CONFIG_THUMB2_KERNEL
> + bx lr
> +#else
> + mov pc, lr
> +#endif

Please use 'ret lr' here. Note that this code does not even compile
for Thumb2, but 'bx lr' should be used at least on v6+ in any case.

> +UNWIND(.fnend)
> +SYM_FUNC_END(__kretprobe_trampoline)
> diff --git a/include/asm-generic/kprobes.h b/include/asm-generic/kprobes.h
> index 060eab094e5a..1509daa281b8 100644
> --- a/include/asm-generic/kprobes.h
> +++ b/include/asm-generic/kprobes.h
> @@ -2,7 +2,11 @@
> #ifndef _ASM_GENERIC_KPROBES_H
> #define _ASM_GENERIC_KPROBES_H
>
> -#if defined(__KERNEL__) && !defined(__ASSEMBLY__)
> +#ifdef __KERNEL__
> +
> +#ifdef __ASSEMBLY__
> +# define __KPROBE .section ".kprobes.text", "ax"
> +#else
> #ifdef CONFIG_KPROBES
> /*
> * Blacklist ganerating macro. Specify functions which is not probed
> @@ -16,11 +20,12 @@ static unsigned long __used \
> /* Use this to forbid a kprobes attach on very low level functions */
> # define __kprobes __section(".kprobes.text")
> # define nokprobe_inline __always_inline
> -#else
> +#else /* !defined(CONFIG_KPROBES) */
> # define NOKPROBE_SYMBOL(fname)
> # define __kprobes
> # define nokprobe_inline inline
> -#endif
> -#endif /* defined(__KERNEL__) && !defined(__ASSEMBLY__) */
> +#endif /* defined(CONFIG_KPROBES) */
> +#endif /* defined(__ASSEMBLY__) */
> +#endif /* defined(__KERNEL__) */
>
> #endif /* _ASM_GENERIC_KPROBES_H */
> --
> 2.37.3.998.g577e59143f-goog
>

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In reply to: kernel test robot: "Re: [PATCH] ARM: kprobes: move __kretprobe_trampoline to out of line assembler"
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