Re: [PATCH -next V5 3/7] riscv: ftrace: Reduce the detour code size to half

[Guo Ren]

On Tue, Dec 27, 2022 at 4:50 PM Evgenii Shatokhin <e.shatokhin@xxxxxxxxx> wrote:
>
> Hi,
>
> On 08.12.2022 12:12, 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
> > 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.
>
> Your patch fixes the issue, and such "function replacement" starts
> working, which is great.
Thx for the comment and test.

>
> > Link: https://lore.kernel.org/linux-riscv/20221122075440.1165172-1-suagrfillet@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>
> > ---
> > Changes in v4:
> >   - Add Sob of Song Shuai
> >
> > Changes in v3:
> >   - Fixup ftrace_modify_call without "caller = rec->ip + 4". [Song Shuai]
> >   - Optimize .macro make_call_ra & make_call_t0
> > ---
> >   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 36cc609c5d03..d60939e2c596 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..61b24d767e2e 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, 8))
>
> A small nit: how about using sizeof(call) or MCOUNT_INSN_SIZE here
> rather than 8?
Yes, I shouldn't use 8 here.

>
> Besides, adding BUILD_BUG_ON(sizeof(call) != MCOUNT_INSN_SIZE) would
> help catch the errors if someone changes the sizes in the future.
The call[2], nop[2], and replace[2] all need it, and it could be
another enhancement patchset for the future.

>
> >                  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..64bc79816f5e 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)
> > @@ -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)
> > +       REG_L   a1, PT_RA(sp)
>
> ra contains the original return address at this point, so, I think, you
> can load it into a1 directly, like you do in ftrace_caller:
>
>         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