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

Hi NIck,

On Mon, 26 Sept 2022 at 20:37, Nick Desaulniers ndesaulniers@google.com 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@kernel.org

Link: https://github.com/llvm/llvm-project/issues/57993 Link: https://github.com/ClangBuiltLinux/linux/issues/1718 Reported-by: Nathan Chancellor nathan@kernel.org Reported-by: Linux Kernel Functional Testing lkft@linaro.org Suggested-by: Logan Chien loganchien@google.com Signed-off-by: Nick Desaulniers ndesaulniers@google.com

---

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