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

Björn Töpel
2024-12-06T10:02:43Z

Adding Robbin for input, who's doing much more crazy text patching in
JVM, than what we do in the kernel. ;-)

Andy Chiu <andybnac@gmail.com> writes:

> 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.

Let's say we're tracing "f". Previously w/ stop_machine() it was
something like:

f:
1: nop
nop
...
...

ftrace_caller:
...
auipc a2, function_trace_op
ld a2, function_trace_op(a2)
...
2: auipc ra, ftrace_stub
jalr ftrace_stub(ra)

The text was patched by ftrace in 1 and 2.

...and now:
f:
auipc t0, ftrace_caller
A: nop
...
...

ftrace_caller:
...
auipc a2, function_trace_op
ld a2, function_trace_op(a2)
...
auipc ra, ftrace_call_dest
ld ra, ftrace_call_dest(ra)
jalr ra

The text is only patched in A, and the tracer func is loaded via
ftrace_call_dest.

Today, when we enable trace "f" the following is done by ftrace:
Text patch 2: call ftrace_stub -> call arch_ftrace_ops_list_func
Text patch 1: nop,nop -> call ftrace_caller
store function_trace_op
smp_wmb()
IPI: smp_rmb()
Text patch 2: call arch_ftrace_ops_list_func -> call function_trace_call

Disable, would be something like:
Text patch 2: call function_trace_call -> call arch_ftrace_ops_list_func
Text patch 1: call ftrace_caller -> nop,nop
store function_trace_op
smp_wmb()
IPI: smp_rmb()
Text patch 2: call arch_ftrace_ops_list_func -> call ftrace_stub

Now with this change, enable would be:
store ftrace_call_dest (was: Text patch 2: call ftrace_stub -> call arch_ftrace_ops_list_func)
<<ORDERING>>
Text patch A: nop -> jalr ftrace_caller(t0)
store function_trace_op
smp_wmb()
IPI: smp_rmb()
store ftrace_call_dest (was: Text patch 2: call arch_ftrace_ops_list_func -> call function_trace_call)

Seems like we're missing some data ordering in "<<ORDERING>>", wrt to
the text patching A (The arch specific ftrace_update_ftrace_func())? Or
are we OK with reordering there? Maybe add what's done for
function_trace_op?

[...]

> 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

Align please.

>
> #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)

While we're updating this function; Can we rename hook_pos to something
that makes sense from an ftrace perspective?

> {
> 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))

Don't wrap this line.

> + 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;

Confusing to call nops. This is not nops. This is the ftrace_stub. Also
the __ftrace_modify_call_site is not super clear to me. Maybe just ditch
the enable flag, and have two functions? Or just or inline it?

>
> - 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};

It's just one nop, not nops. No biggie, but why array?

>
> - 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)

Not used, please remove.

> - call ftrace_stub
> + REG_L ra, ftrace_call_dest
> + jalr 0(ra)
>
> RESTORE_ABI_REGS
> bnez t1, .Ldirect
> --
> 2.39.3 (Apple Git-145)

Björn