[PATCH v3 3/7] riscv: ftrace: prepare ftrace for atomic code patching

Andy Chiu
2024-11-27T17:30:04Z

From: Andy Chiu <andy.chiu@sifive.com>

We use an AUIPC+JALR pair to jump into a ftrace trampoline. Since
instruction fetch can break down to 4 byte at a time, it is impossible
to update two instructions without a race. In order to mitigate it, we
initialize the patchable entry to AUIPC + NOP4. Then, the run-time code
patching can change NOP4 to JALR to eable/disable ftrcae from a
function. This limits the reach of each ftrace entry to +-2KB displacing
from ftrace_caller.

Starting from the trampoline, we add a level of indirection for it to
reach ftrace caller target. Now, it loads the target address from a
memory location, then perform the jump. This enable the kernel to update
the target atomically.

The ordering of reading/updating the targert address should be guarded
by generic ftrace code, where it sends smp_rmb ipi.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
---
arch/riscv/include/asm/ftrace.h | 4 ++
arch/riscv/kernel/ftrace.c | 80 +++++++++++++++++++++------------
arch/riscv/kernel/mcount-dyn.S | 9 ++--
3 files changed, 62 insertions(+), 31 deletions(-)

diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
index 4ca7ce7f34d7..36734d285aad 100644
--- a/arch/riscv/include/asm/ftrace.h
+++ b/arch/riscv/include/asm/ftrace.h
@@ -80,6 +80,7 @@ struct dyn_arch_ftrace {
#define JALR_T0 (0x000282e7)
#define AUIPC_T0 (0x00000297)
#define NOP4 (0x00000013)
+#define JALR_RANGE (JALR_SIGN_MASK - 1)

#define to_jalr_t0(offset) \
(((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_T0)
@@ -117,6 +118,9 @@ do { \
* Let auipc+jalr be the basic *mcount unit*, so we make it 8 bytes here.
*/
#define MCOUNT_INSN_SIZE 8
+#define MCOUNT_AUIPC_SIZE 4
+#define MCOUNT_JALR_SIZE 4
+#define MCOUNT_NOP4_SIZE 4

#ifndef __ASSEMBLY__
struct dyn_ftrace;
diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
index 4b95c574fd04..5ebe412280ef 100644
--- a/arch/riscv/kernel/ftrace.c
+++ b/arch/riscv/kernel/ftrace.c
@@ -64,42 +64,64 @@ static int ftrace_check_current_call(unsigned long hook_pos,
return 0;
}

-static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
- bool enable, bool ra)
+static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target, bool validate)
{
unsigned int call[2];
- unsigned int nops[2] = {NOP4, NOP4};
+ unsigned int replaced[2];
+
+ make_call_t0(hook_pos, target, call);

- if (ra)
- make_call_ra(hook_pos, target, call);
- else
- make_call_t0(hook_pos, target, call);
+ if (validate) {
+ /*
+ * Read the text we want to modify;
+ * return must be -EFAULT on read error
+ */
+ if (copy_from_kernel_nofault(replaced, (void *)hook_pos,
+ MCOUNT_INSN_SIZE))
+ return -EFAULT;
+
+ if (replaced[0] != call[0]) {
+ pr_err("%p: expected (%08x) but got (%08x)\n",
+ (void *)hook_pos, call[0], replaced[0]);
+ return -EINVAL;
+ }
+ }

- /* Replace the auipc-jalr pair at once. Return -EPERM on write error. */
- if (patch_insn_write((void *)hook_pos, enable ? call : nops, MCOUNT_INSN_SIZE))
+ /* Replace the jalr at once. Return -EPERM on write error. */
+ if (patch_insn_write((void *)(hook_pos + MCOUNT_AUIPC_SIZE), call + 1, MCOUNT_JALR_SIZE))
return -EPERM;

return 0;
}

-int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
+static int __ftrace_modify_call_site(ftrace_func_t *hook_pos, ftrace_func_t target, bool enable)
{
- unsigned int call[2];
+ ftrace_func_t call = target;
+ ftrace_func_t nops = &ftrace_stub;

- make_call_t0(rec->ip, addr, call);
-
- if (patch_insn_write((void *)rec->ip, call, MCOUNT_INSN_SIZE))
- return -EPERM;
+ WRITE_ONCE(*hook_pos, enable ? call : nops);

return 0;
}

+int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
+{
+ unsigned long distance, orig_addr;
+
+ orig_addr = (unsigned long)&ftrace_caller;
+ distance = addr > orig_addr ? addr - orig_addr : orig_addr - addr;
+ if (distance > JALR_RANGE)
+ return -EINVAL;
+
+ return __ftrace_modify_call(rec->ip, addr, false);
+}
+
int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
unsigned long addr)
{
- unsigned int nops[2] = {NOP4, NOP4};
+ unsigned int nops[1] = {NOP4};

- if (patch_insn_write((void *)rec->ip, nops, MCOUNT_INSN_SIZE))
+ if (patch_insn_write((void *)(rec->ip + MCOUNT_AUIPC_SIZE), nops, MCOUNT_NOP4_SIZE))
return -EPERM;

return 0;
@@ -114,21 +136,23 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
*/
int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
{
+ unsigned int nops[2];
int out;

+ make_call_t0(rec->ip, &ftrace_caller, nops);
+ nops[1] = NOP4;
+
mutex_lock(&text_mutex);
- out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
+ out = patch_insn_write((void *)rec->ip, nops, MCOUNT_INSN_SIZE);
mutex_unlock(&text_mutex);

return out;
}

+ftrace_func_t ftrace_call_dest = ftrace_stub;
int ftrace_update_ftrace_func(ftrace_func_t func)
{
- int ret = __ftrace_modify_call((unsigned long)&ftrace_call,
- (unsigned long)func, true, true);
-
- return ret;
+ return __ftrace_modify_call_site(&ftrace_call_dest, func, true);
}

struct ftrace_modify_param {
@@ -182,7 +206,7 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
if (ret)
return ret;

- return __ftrace_modify_call(caller, addr, true, false);
+ return __ftrace_modify_call(caller, addr, true);
}
#endif

@@ -217,17 +241,17 @@ void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
prepare_ftrace_return(&fregs->ra, ip, fregs->s0);
}
#else /* CONFIG_DYNAMIC_FTRACE_WITH_ARGS */
-extern void ftrace_graph_call(void);
+ftrace_func_t ftrace_graph_call_dest = ftrace_stub;
int ftrace_enable_ftrace_graph_caller(void)
{
- return __ftrace_modify_call((unsigned long)&ftrace_graph_call,
- (unsigned long)&prepare_ftrace_return, true, true);
+ return __ftrace_modify_call_site(&ftrace_graph_call_dest,
+ &prepare_ftrace_return, true);
}

int ftrace_disable_ftrace_graph_caller(void)
{
- return __ftrace_modify_call((unsigned long)&ftrace_graph_call,
- (unsigned long)&prepare_ftrace_return, false, true);
+ return __ftrace_modify_call_site(&ftrace_graph_call_dest,
+ &prepare_ftrace_return, false);
}
#endif /* CONFIG_DYNAMIC_FTRACE_WITH_ARGS */
#endif /* CONFIG_DYNAMIC_FTRACE */
diff --git a/arch/riscv/kernel/mcount-dyn.S b/arch/riscv/kernel/mcount-dyn.S
index e988bd26b28b..bc06e8ab81cf 100644
--- a/arch/riscv/kernel/mcount-dyn.S
+++ b/arch/riscv/kernel/mcount-dyn.S
@@ -162,7 +162,8 @@ SYM_FUNC_START(ftrace_caller)
mv a3, sp

SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
- call ftrace_stub
+ REG_L ra, ftrace_call_dest
+ jalr 0(ra)

#ifdef CONFIG_FUNCTION_GRAPH_TRACER
addi a0, sp, ABI_RA
@@ -172,7 +173,8 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
mv a2, s0
#endif
SYM_INNER_LABEL(ftrace_graph_call, SYM_L_GLOBAL)
- call ftrace_stub
+ REG_L ra, ftrace_graph_call_dest
+ jalr 0(ra)
#endif
RESTORE_ABI
jr t0
@@ -185,7 +187,8 @@ SYM_FUNC_START(ftrace_caller)
PREPARE_ARGS

SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
- call ftrace_stub
+ REG_L ra, ftrace_call_dest
+ jalr 0(ra)

RESTORE_ABI_REGS
bnez t1, .Ldirect
--
2.39.3 (Apple Git-145)