Re: [PATCH -next V6 3/7] riscv: ftrace: Reduce the detour code size to half

From: Guo Ren
Date: Wed Jan 11 2023 - 05:02:50 EST

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In reply to: Evgenii Shatokhin: "Re: [PATCH -next V6 3/7] riscv: ftrace: Reduce the detour code size to half"
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On Wed, Jan 11, 2023 at 1:13 AM Evgenii Shatokhin <e.shatokhin@xxxxxxxxx> wrote:
>
> On 07.01.2023 16:35, guoren@xxxxxxxxxx wrote:
> >
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > Use a temporary register to reduce the size of detour code from 16 bytes to
> > 8 bytes. The previous implementation is from 'commit afc76b8b8011 ("riscv:
> > Using PATCHABLE_FUNCTION_ENTRY instead of MCOUNT")'.
> >
> > Before the patch:
> > <func_prolog>:
> > 0: REG_S ra, -SZREG(sp)
> > 4: auipc ra, ?
> > 8: jalr ?(ra)
> > 12: REG_L ra, -SZREG(sp)
> > (func_boddy)
> >
> > After the patch:
> > <func_prolog>:
> > 0: auipc t0, ?
> > 4: jalr t0, ?(t0)
> > (func_boddy)
> >
> > This patch not just reduces the size of detour code, but also fixes an
> > important issue:
> >
> > An Ftrace callback registered with FTRACE_OPS_FL_IPMODIFY flag can
> > actually change the instruction pointer, e.g. to "replace" the given
> > kernel function with a new one, which is needed for livepatching, etc.
> >
> > In this case, the trampoline (ftrace_regs_caller) would not return to
> > <func_prolog+12> but would rather jump to the new function. So, "REG_L
> > ra, -SZREG(sp)" would not run and the original return address would not
> > be restored. The kernel is likely to hang or crash as a result.
> >
> > This can be easily demonstrated if one tries to "replace", say,
> > cmdline_proc_show() with a new function with the same signature using
> > instruction_pointer_set(&fregs->regs, new_func_addr) in the Ftrace
> > callback.
> >
> > Link: https://lore.kernel.org/linux-riscv/20221122075440.1165172-1-suagrfillet@xxxxxxxxx/
> > Link: https://lore.kernel.org/linux-riscv/d7d5730b-ebef-68e5-5046-e763e1ee6164@xxxxxxxxx/
> > Co-developed-by: Song Shuai <suagrfillet@xxxxxxxxx>
> > Signed-off-by: Song Shuai <suagrfillet@xxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > Cc: Evgenii Shatokhin <e.shatokhin@xxxxxxxxx>
> > ---
> > arch/riscv/Makefile | 4 +-
> > arch/riscv/include/asm/ftrace.h | 50 +++++++++++++++++++------
> > arch/riscv/kernel/ftrace.c | 65 ++++++++++-----------------------
> > arch/riscv/kernel/mcount-dyn.S | 43 +++++++++-------------
> > 4 files changed, 76 insertions(+), 86 deletions(-)
> >
> > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > index ea5a91da6897..3c9aaf67ed79 100644
> > --- a/arch/riscv/Makefile
> > +++ b/arch/riscv/Makefile
> > @@ -12,9 +12,9 @@ ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
> > LDFLAGS_vmlinux := --no-relax
> > KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
> > ifeq ($(CONFIG_RISCV_ISA_C),y)
> > - CC_FLAGS_FTRACE := -fpatchable-function-entry=8
> > -else
> > CC_FLAGS_FTRACE := -fpatchable-function-entry=4
> > +else
> > + CC_FLAGS_FTRACE := -fpatchable-function-entry=2
> > endif
> > endif
> >
> > diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> > index 04dad3380041..9e73922e1e2e 100644
> > --- a/arch/riscv/include/asm/ftrace.h
> > +++ b/arch/riscv/include/asm/ftrace.h
> > @@ -42,6 +42,14 @@ struct dyn_arch_ftrace {
> > * 2) jalr: setting low-12 offset to ra, jump to ra, and set ra to
> > * return address (original pc + 4)
> > *
> > + *<ftrace enable>:
> > + * 0: auipc t0/ra, 0x?
> > + * 4: jalr t0/ra, ?(t0/ra)
> > + *
> > + *<ftrace disable>:
> > + * 0: nop
> > + * 4: nop
> > + *
> > * Dynamic ftrace generates probes to call sites, so we must deal with
> > * both auipc and jalr at the same time.
> > */
> > @@ -52,25 +60,43 @@ struct dyn_arch_ftrace {
> > #define AUIPC_OFFSET_MASK (0xfffff000)
> > #define AUIPC_PAD (0x00001000)
> > #define JALR_SHIFT 20
> > -#define JALR_BASIC (0x000080e7)
> > -#define AUIPC_BASIC (0x00000097)
> > +#define JALR_RA (0x000080e7)
> > +#define AUIPC_RA (0x00000097)
> > +#define JALR_T0 (0x000282e7)
> > +#define AUIPC_T0 (0x00000297)
> > #define NOP4 (0x00000013)
> >
> > -#define make_call(caller, callee, call) \
> > +#define to_jalr_t0(offset) \
> > + (((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_T0)
> > +
> > +#define to_auipc_t0(offset) \
> > + ((offset & JALR_SIGN_MASK) ? \
> > + (((offset & AUIPC_OFFSET_MASK) + AUIPC_PAD) | AUIPC_T0) : \
> > + ((offset & AUIPC_OFFSET_MASK) | AUIPC_T0))
> > +
> > +#define make_call_t0(caller, callee, call) \
> > do { \
> > - call[0] = to_auipc_insn((unsigned int)((unsigned long)callee - \
> > - (unsigned long)caller)); \
> > - call[1] = to_jalr_insn((unsigned int)((unsigned long)callee - \
> > - (unsigned long)caller)); \
> > + unsigned int offset = \
> > + (unsigned long) callee - (unsigned long) caller; \
> > + call[0] = to_auipc_t0(offset); \
> > + call[1] = to_jalr_t0(offset); \
> > } while (0)
> >
> > -#define to_jalr_insn(offset) \
> > - (((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_BASIC)
> > +#define to_jalr_ra(offset) \
> > + (((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_RA)
> >
> > -#define to_auipc_insn(offset) \
> > +#define to_auipc_ra(offset) \
> > ((offset & JALR_SIGN_MASK) ? \
> > - (((offset & AUIPC_OFFSET_MASK) + AUIPC_PAD) | AUIPC_BASIC) : \
> > - ((offset & AUIPC_OFFSET_MASK) | AUIPC_BASIC))
> > + (((offset & AUIPC_OFFSET_MASK) + AUIPC_PAD) | AUIPC_RA) : \
> > + ((offset & AUIPC_OFFSET_MASK) | AUIPC_RA))
> > +
> > +#define make_call_ra(caller, callee, call) \
> > +do { \
> > + unsigned int offset = \
> > + (unsigned long) callee - (unsigned long) caller; \
> > + call[0] = to_auipc_ra(offset); \
> > + call[1] = to_jalr_ra(offset); \
> > +} while (0)
> >
> > /*
> > * Let auipc+jalr be the basic *mcount unit*, so we make it 8 bytes here.
> > diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
> > index 2086f6585773..5bff37af4770 100644
> > --- a/arch/riscv/kernel/ftrace.c
> > +++ b/arch/riscv/kernel/ftrace.c
> > @@ -55,12 +55,15 @@ static int ftrace_check_current_call(unsigned long hook_pos,
> > }
> >
> > static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
> > - bool enable)
> > + bool enable, bool ra)
> > {
> > unsigned int call[2];
> > unsigned int nops[2] = {NOP4, NOP4};
> >
> > - make_call(hook_pos, target, call);
> > + if (ra)
> > + make_call_ra(hook_pos, target, call);
> > + else
> > + make_call_t0(hook_pos, target, call);
> >
> > /* Replace the auipc-jalr pair at once. Return -EPERM on write error. */
> > if (patch_text_nosync
> > @@ -70,42 +73,13 @@ static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
> > return 0;
> > }
> >
> > -/*
> > - * Put 5 instructions with 16 bytes at the front of function within
> > - * patchable function entry nops' area.
> > - *
> > - * 0: REG_S ra, -SZREG(sp)
> > - * 1: auipc ra, 0x?
> > - * 2: jalr -?(ra)
> > - * 3: REG_L ra, -SZREG(sp)
> > - *
> > - * So the opcodes is:
> > - * 0: 0xfe113c23 (sd)/0xfe112e23 (sw)
> > - * 1: 0x???????? -> auipc
> > - * 2: 0x???????? -> jalr
> > - * 3: 0xff813083 (ld)/0xffc12083 (lw)
> > - */
> > -#if __riscv_xlen == 64
> > -#define INSN0 0xfe113c23
> > -#define INSN3 0xff813083
> > -#elif __riscv_xlen == 32
> > -#define INSN0 0xfe112e23
> > -#define INSN3 0xffc12083
> > -#endif
> > -
> > -#define FUNC_ENTRY_SIZE 16
> > -#define FUNC_ENTRY_JMP 4
> > -
> > int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> > {
> > - unsigned int call[4] = {INSN0, 0, 0, INSN3};
> > - unsigned long target = addr;
> > - unsigned long caller = rec->ip + FUNC_ENTRY_JMP;
> > + unsigned int call[2];
> >
> > - call[1] = to_auipc_insn((unsigned int)(target - caller));
> > - call[2] = to_jalr_insn((unsigned int)(target - caller));
> > + make_call_t0(rec->ip, addr, call);
> >
> > - if (patch_text_nosync((void *)rec->ip, call, FUNC_ENTRY_SIZE))
> > + if (patch_text_nosync((void *)rec->ip, call, MCOUNT_INSN_SIZE))
> > return -EPERM;
> >
> > return 0;
> > @@ -114,15 +88,14 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> > int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
> > unsigned long addr)
> > {
> > - unsigned int nops[4] = {NOP4, NOP4, NOP4, NOP4};
> > + unsigned int nops[2] = {NOP4, NOP4};
> >
> > - if (patch_text_nosync((void *)rec->ip, nops, FUNC_ENTRY_SIZE))
> > + if (patch_text_nosync((void *)rec->ip, nops, MCOUNT_INSN_SIZE))
> > return -EPERM;
> >
> > return 0;
> > }
> >
> > -
> > /*
> > * This is called early on, and isn't wrapped by
> > * ftrace_arch_code_modify_{prepare,post_process}() and therefor doesn't hold
> > @@ -144,10 +117,10 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
> > int ftrace_update_ftrace_func(ftrace_func_t func)
> > {
> > int ret = __ftrace_modify_call((unsigned long)&ftrace_call,
> > - (unsigned long)func, true);
> > + (unsigned long)func, true, true);
> > if (!ret) {
> > ret = __ftrace_modify_call((unsigned long)&ftrace_regs_call,
> > - (unsigned long)func, true);
> > + (unsigned long)func, true, true);
> > }
> >
> > return ret;
> > @@ -159,16 +132,16 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
> > unsigned long addr)
> > {
> > unsigned int call[2];
> > - unsigned long caller = rec->ip + FUNC_ENTRY_JMP;
> > + unsigned long caller = rec->ip;
> > int ret;
> >
> > - make_call(caller, old_addr, call);
> > + make_call_t0(caller, old_addr, call);
> > ret = ftrace_check_current_call(caller, call);
> >
> > if (ret)
> > return ret;
> >
> > - return __ftrace_modify_call(caller, addr, true);
> > + return __ftrace_modify_call(caller, addr, true, false);
> > }
> > #endif
> >
> > @@ -203,12 +176,12 @@ int ftrace_enable_ftrace_graph_caller(void)
> > int ret;
> >
> > ret = __ftrace_modify_call((unsigned long)&ftrace_graph_call,
> > - (unsigned long)&prepare_ftrace_return, true);
> > + (unsigned long)&prepare_ftrace_return, true, true);
> > if (ret)
> > return ret;
> >
> > return __ftrace_modify_call((unsigned long)&ftrace_graph_regs_call,
> > - (unsigned long)&prepare_ftrace_return, true);
> > + (unsigned long)&prepare_ftrace_return, true, true);
> > }
> >
> > int ftrace_disable_ftrace_graph_caller(void)
> > @@ -216,12 +189,12 @@ int ftrace_disable_ftrace_graph_caller(void)
> > int ret;
> >
> > ret = __ftrace_modify_call((unsigned long)&ftrace_graph_call,
> > - (unsigned long)&prepare_ftrace_return, false);
> > + (unsigned long)&prepare_ftrace_return, false, true);
> > if (ret)
> > return ret;
> >
> > return __ftrace_modify_call((unsigned long)&ftrace_graph_regs_call,
> > - (unsigned long)&prepare_ftrace_return, false);
> > + (unsigned long)&prepare_ftrace_return, false, true);
> > }
> > #endif /* CONFIG_DYNAMIC_FTRACE */
> > #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> > diff --git a/arch/riscv/kernel/mcount-dyn.S b/arch/riscv/kernel/mcount-dyn.S
> > index d171eca623b6..b75332ced757 100644
> > --- a/arch/riscv/kernel/mcount-dyn.S
> > +++ b/arch/riscv/kernel/mcount-dyn.S
> > @@ -13,8 +13,8 @@
> >
> > .text
> >
> > -#define FENTRY_RA_OFFSET 12
> > -#define ABI_SIZE_ON_STACK 72
> > +#define FENTRY_RA_OFFSET 8
> > +#define ABI_SIZE_ON_STACK 80
> > #define ABI_A0 0
> > #define ABI_A1 8
> > #define ABI_A2 16
> > @@ -23,10 +23,10 @@
> > #define ABI_A5 40
> > #define ABI_A6 48
> > #define ABI_A7 56
> > -#define ABI_RA 64
> > +#define ABI_T0 64
> > +#define ABI_RA 72
> >
> > .macro SAVE_ABI
> > - addi sp, sp, -SZREG
> > addi sp, sp, -ABI_SIZE_ON_STACK
> >
> > REG_S a0, ABI_A0(sp)
> > @@ -37,6 +37,7 @@
> > REG_S a5, ABI_A5(sp)
> > REG_S a6, ABI_A6(sp)
> > REG_S a7, ABI_A7(sp)
> > + REG_S t0, ABI_T0(sp)
> > REG_S ra, ABI_RA(sp)
> > .endm
> >
> > @@ -49,24 +50,18 @@
> > REG_L a5, ABI_A5(sp)
> > REG_L a6, ABI_A6(sp)
> > REG_L a7, ABI_A7(sp)
> > + REG_L t0, ABI_T0(sp)
> > REG_L ra, ABI_RA(sp)
> >
> > addi sp, sp, ABI_SIZE_ON_STACK
> > - addi sp, sp, SZREG
> > .endm
> >
> > #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > .macro SAVE_ALL
> > - addi sp, sp, -SZREG
> > addi sp, sp, -PT_SIZE_ON_STACK
> >
> > - REG_S x1, PT_EPC(sp)
> > - addi sp, sp, PT_SIZE_ON_STACK
> > - REG_L x1, (sp)
> > - addi sp, sp, -PT_SIZE_ON_STACK
> > + REG_S t0, PT_EPC(sp)
> > REG_S x1, PT_RA(sp)
> > - REG_L x1, PT_EPC(sp)
> > -
> > REG_S x2, PT_SP(sp)
> > REG_S x3, PT_GP(sp)
> > REG_S x4, PT_TP(sp)
> > @@ -100,11 +95,8 @@
> > .endm
> >
> > .macro RESTORE_ALL
> > + REG_L t0, PT_EPC(sp)
> > REG_L x1, PT_RA(sp)
> > - addi sp, sp, PT_SIZE_ON_STACK
> > - REG_S x1, (sp)
> > - addi sp, sp, -PT_SIZE_ON_STACK
> > - REG_L x1, PT_EPC(sp)
> > REG_L x2, PT_SP(sp)
> > REG_L x3, PT_GP(sp)
> > REG_L x4, PT_TP(sp)
>
> I did not notice it the last time, but RESTORE_ALL should skip "REG_L
> x5, PT_T0(sp)". Otherwise, the value of t0/x5 loaded from PT_EPC(sp)
> will be overwritten.
>
> With the "ipmodify" use case I mentioned earlier, the Ftrace handler
> might have changed the value in PT_EPC(sp) by that time. Reading t0
> again from PT_T0(sp) here breaks it.

Correct, in kprobe_ftrace_handler, I found:

unsigned long orig_ip = instruction_pointer(regs);

instruction_pointer_set(regs, ip);

So, I must skip "REG_L x5, PT_T0(sp)" to prevent the value from being
overwritten.

>
> The same holds for "[PATCH -next V6 5/7] riscv: ftrace: Make
> ftrace_caller call ftrace_graph_func".
>
> > @@ -137,17 +129,16 @@
> > REG_L x31, PT_T6(sp)
> >
> > addi sp, sp, PT_SIZE_ON_STACK
> > - addi sp, sp, SZREG
> > .endm
> > #endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> >
> > ENTRY(ftrace_caller)
> > SAVE_ABI
> >
> > - addi a0, ra, -FENTRY_RA_OFFSET
> > + addi a0, t0, -FENTRY_RA_OFFSET
> > la a1, function_trace_op
> > REG_L a2, 0(a1)
> > - REG_L a1, ABI_SIZE_ON_STACK(sp)
> > + mv a1, ra
> > mv a3, sp
> >
> > ftrace_call:
> > @@ -155,8 +146,8 @@ ftrace_call:
> > call ftrace_stub
> >
> > #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > - addi a0, sp, ABI_SIZE_ON_STACK
> > - REG_L a1, ABI_RA(sp)
> > + addi a0, sp, ABI_RA
> > + REG_L a1, ABI_T0(sp)
> > addi a1, a1, -FENTRY_RA_OFFSET
> > #ifdef HAVE_FUNCTION_GRAPH_FP_TEST
> > mv a2, s0
> > @@ -166,17 +157,17 @@ ftrace_graph_call:
> > call ftrace_stub
> > #endif
> > RESTORE_ABI
> > - ret
> > + jr t0
> > ENDPROC(ftrace_caller)
> >
> > #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > ENTRY(ftrace_regs_caller)
> > SAVE_ALL
> >
> > - addi a0, ra, -FENTRY_RA_OFFSET
> > + addi a0, t0, -FENTRY_RA_OFFSET
> > la a1, function_trace_op
> > REG_L a2, 0(a1)
> > - REG_L a1, PT_SIZE_ON_STACK(sp)
> > + mv a1, ra
> > mv a3, sp
> >
> > ftrace_regs_call:
> > @@ -185,7 +176,7 @@ ftrace_regs_call:
> >
> > #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > addi a0, sp, PT_RA
> > - REG_L a1, PT_EPC(sp)
> > + REG_L a1, PT_T0(sp)
> > addi a1, a1, -FENTRY_RA_OFFSET
> > #ifdef HAVE_FUNCTION_GRAPH_FP_TEST
> > mv a2, s0
> > @@ -196,6 +187,6 @@ ftrace_graph_regs_call:
> > #endif
> >
> > RESTORE_ALL
> > - ret
> > + jr t0
> > ENDPROC(ftrace_regs_caller)
> > #endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> > --
> > 2.36.1
> >
> >
>
> Regards,
> Evgenii
>

--
Best Regards
Guo Ren

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