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riscv: ftrace: Reduce the detour code size to half
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Use a temporary register to reduce the size of detour code from 16 bytes to
8 bytes. The previous implementation is from 'commit afc76b8 ("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@gmail.com/
Link: https://lore.kernel.org/linux-riscv/d7d5730b-ebef-68e5-5046-e763e1ee6164@yadro.com/
Co-developed-by: Song Shuai <suagrfillet@gmail.com>
Signed-off-by: Song Shuai <suagrfillet@gmail.com>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Evgenii Shatokhin <e.shatokhin@yadro.com>
Reviewed-by: Evgenii Shatokhin <e.shatokhin@yadro.com>
Link: https://lore.kernel.org/r/20230112090603.1295340-4-guoren@kernel.org
Cc: stable@vger.kernel.org
Fixes: 10626c3 ("riscv/ftrace: Add basic support")
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
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Guo Ren authored and Palmer Dabbelt committed Feb 15, 2023
1 parent 409c8fb commit 6724a76
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Showing 4 changed files with 75 additions and 86 deletions.
4 changes: 2 additions & 2 deletions arch/riscv/Makefile
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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

Expand Down
50 changes: 38 additions & 12 deletions arch/riscv/include/asm/ftrace.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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.
*/
Expand All @@ -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.
Expand Down
65 changes: 19 additions & 46 deletions arch/riscv/kernel/ftrace.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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;
Expand All @@ -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
Expand All @@ -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;
Expand All @@ -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

Expand Down Expand Up @@ -203,25 +176,25 @@ 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)
{
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 */
42 changes: 16 additions & 26 deletions arch/riscv/kernel/mcount-dyn.S
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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)
Expand All @@ -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

Expand All @@ -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)
Expand Down Expand Up @@ -100,15 +95,11 @@
.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)
REG_L x5, PT_T0(sp)
REG_L x6, PT_T1(sp)
REG_L x7, PT_T2(sp)
REG_L x8, PT_S0(sp)
Expand Down Expand Up @@ -137,26 +128,25 @@
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:
.global 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
Expand All @@ -166,17 +156,17 @@ 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:
Expand All @@ -196,6 +186,6 @@ ftrace_graph_regs_call:
#endif

RESTORE_ALL
ret
jr t0
ENDPROC(ftrace_regs_caller)
#endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */

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