[PATCH v4 05/12] riscv: ftrace: prepare ftrace for atomic code patching
Robbin Ehn
rehn at rivosinc.com
Fri Apr 11 06:15:42 PDT 2025
Hi, thanks,
On Tue, 2025-04-08 at 02:08 +0800, Andy Chiu wrote:
> From: Andy Chiu <andy.chiu at 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 new don't-stop-the-world text patching on change only one RISC-V
> instruction:
>
> | -8: &ftrace_ops of the associated tracer function.
> | <ftrace enable>:
> | 0: auipc t0, hi(ftrace_caller)
> | 4: jalr t0, lo(ftrace_caller)
> |
> | -8: &ftrace_nop_ops
> | <ftrace disable>:
> | 0: auipc t0, hi(ftrace_caller)
> | 4: nop
>
> This means that f+0x0 is fixed, and should not be claimed by ftrace,
> e.g. kprobe should be able to put a probe in f+0x0. Thus, we adjust the
> offset and MCOUNT_INSN_SIZE accordingly.
>
> Co-developed-by: Björn Töpel <bjorn at rivosinc.com>
> Signed-off-by: Björn Töpel <bjorn at rivosinc.com>
> Signed-off-by: Andy Chiu <andy.chiu at sifive.com>
> ---
> Changelog v4:
> - Include Björn's fix for kprobe
> - Refactor code for better reading (Robbin, Björn)
> - Remove make_call_ra and friedns (Björn)
> - Update comments to match reality (Björn)
> - Drop code defined by !WITH_ARG
> - Add a synchronization point when updating ftrace_call_dest (Björn)
> ---
> arch/riscv/include/asm/ftrace.h | 49 ++++++------
> arch/riscv/kernel/ftrace.c | 130 ++++++++++++++++----------------
> arch/riscv/kernel/mcount-dyn.S | 9 +--
> 3 files changed, 92 insertions(+), 96 deletions(-)
>
> diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> index d8b2138bd9c6..6a5c0a7fb826 100644
> --- a/arch/riscv/include/asm/ftrace.h
> +++ b/arch/riscv/include/asm/ftrace.h
> @@ -20,10 +20,9 @@ extern void *return_address(unsigned int level);
> #define ftrace_return_address(n) return_address(n)
>
> void _mcount(void);
> -static inline unsigned long ftrace_call_adjust(unsigned long addr)
> -{
> - return addr;
> -}
> +unsigned long ftrace_call_adjust(unsigned long addr);
> +unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip);
> +#define ftrace_get_symaddr(fentry_ip) arch_ftrace_get_symaddr(fentry_ip)
>
> /*
> * Let's do like x86/arm64 and ignore the compat syscalls.
> @@ -57,12 +56,21 @@ struct dyn_arch_ftrace {
> * 2) jalr: setting low-12 offset to ra, jump to ra, and set ra to
> * return address (original pc + 4)
> *
> + * The first 2 instructions for each tracable function is compiled to 2 nop
> + * instructions. Then, the kernel initializes the first instruction to auipc
> at
> + * boot time (<ftrace disable>). The second instruction is patched to jalr to
> + * start the trace.
> + *
> + *<Image>:
> + * 0: nop
> + * 4: nop
> + *
> *<ftrace enable>:
> - * 0: auipc t0/ra, 0x?
> - * 4: jalr t0/ra, ?(t0/ra)
> + * 0: auipc t0, 0x?
> + * 4: jalr t0, ?(t0)
> *
> *<ftrace disable>:
> - * 0: nop
> + * 0: auipc t0, 0x?
> * 4: nop
> *
> * Dynamic ftrace generates probes to call sites, so we must deal with
> @@ -75,10 +83,9 @@ struct dyn_arch_ftrace {
> #define AUIPC_OFFSET_MASK (0xfffff000)
> #define AUIPC_PAD (0x00001000)
> #define JALR_SHIFT 20
> -#define JALR_RA (0x000080e7)
> -#define AUIPC_RA (0x00000097)
> #define JALR_T0 (0x000282e7)
> #define AUIPC_T0 (0x00000297)
> +#define JALR_RANGE (JALR_SIGN_MASK - 1)
>
> #define to_jalr_t0(offset) \
> (((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_T0)
> @@ -96,26 +103,14 @@ do
> { \
> call[1] = to_jalr_t0(offset); \
> } while (0)
>
> -#define to_jalr_ra(offset) \
> - (((offset & JALR_OFFSET_MASK) << JALR_SHIFT) | JALR_RA)
> -
> -#define to_auipc_ra(offset) \
> - ((offset & JALR_SIGN_MASK) ? \
> - (((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.
> + * Only the jalr insn in the auipc+jalr is patched, so we make it 4
> + * bytes here.
> */
> -#define MCOUNT_INSN_SIZE 8
> +#define MCOUNT_INSN_SIZE 4
> +#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 1fd10555c580..cf78eef073a0 100644
> --- a/arch/riscv/kernel/ftrace.c
> +++ b/arch/riscv/kernel/ftrace.c
> @@ -8,10 +8,21 @@
> #include <linux/ftrace.h>
> #include <linux/uaccess.h>
> #include <linux/memory.h>
> +#include <linux/irqflags.h>
> #include <linux/stop_machine.h>
> #include <asm/cacheflush.h>
> #include <asm/text-patching.h>
>
> +unsigned long ftrace_call_adjust(unsigned long addr)
> +{
> + return addr + MCOUNT_AUIPC_SIZE;
> +}
> +
> +unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip)
> +{
> + return fentry_ip - MCOUNT_AUIPC_SIZE;
> +}
> +
> #ifdef CONFIG_DYNAMIC_FTRACE
> void ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
> {
> @@ -32,51 +43,32 @@ void ftrace_arch_code_modify_post_process(void)
> __releases(&text_mutex)
> mutex_unlock(&text_mutex);
> }
>
> -static int ftrace_check_current_call(unsigned long hook_pos,
> - unsigned int *expected)
> +static int __ftrace_modify_call(unsigned long source, unsigned long target,
> bool validate)
> {
> + unsigned int call[2], offset;
There is a mix of unsigned int and u32.
It doesn't seem to serve any purpose?
> unsigned int replaced[2];
> - unsigned int nops[2] = {RISCV_INSN_NOP4, RISCV_INSN_NOP4};
>
> - /* we expect nops at the hook position */
> - if (!expected)
> - expected = nops;
> + offset = target - source;
> + call[1] = to_jalr_t0(offset);
>
> - /*
> - * 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;
> -
> - /*
> - * Make sure it is what we expect it to be;
> - * return must be -EINVAL on failed comparison
> - */
> - if (memcmp(expected, replaced, sizeof(replaced))) {
> - pr_err("%p: expected (%08x %08x) but got (%08x %08x)\n",
> - (void *)hook_pos, expected[0], expected[1],
> replaced[0],
> - replaced[1]);
> - return -EINVAL;
> + if (validate) {
> + call[0] = to_auipc_t0(offset);
> + /*
> + * Read the text we want to modify;
> + * return must be -EFAULT on read error
> + */
> + if (copy_from_kernel_nofault(replaced, (void *)source, 2 *
> MCOUNT_INSN_SIZE))
> + return -EFAULT;
> +
> + if (replaced[0] != call[0]) {
> + pr_err("%p: expected (%08x) but got (%08x)\n",
> + (void *)source, call[0], replaced[0]);
> + return -EINVAL;
> + }
> }
>
> - return 0;
> -}
> -
> -static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
> - bool enable, bool ra)
> -{
> - unsigned int call[2];
> - unsigned int nops[2] = {RISCV_INSN_NOP4, RISCV_INSN_NOP4};
> -
> - 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_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 *)(source + MCOUNT_AUIPC_SIZE), call + 1,
> MCOUNT_JALR_SIZE))
> return -EPERM;
>
> return 0;
> @@ -84,22 +76,21 @@ static int __ftrace_modify_call(unsigned long hook_pos,
> unsigned long target,
>
> int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> {
> - unsigned int call[2];
> + unsigned long distance, orig_addr, pc = rec->ip - MCOUNT_AUIPC_SIZE;
>
> - make_call_t0(rec->ip, addr, call);
> -
> - if (patch_insn_write((void *)rec->ip, call, MCOUNT_INSN_SIZE))
> - return -EPERM;
> + orig_addr = (unsigned long)&ftrace_caller;
> + distance = addr > orig_addr ? addr - orig_addr : orig_addr - addr;
> + if (distance > JALR_RANGE)
> + return -EINVAL;
>
> - return 0;
> + return __ftrace_modify_call(pc, addr, false);
> }
>
> -int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
> - unsigned long addr)
> +int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec, unsigned long
> addr)
> {
> - unsigned int nops[2] = {RISCV_INSN_NOP4, RISCV_INSN_NOP4};
> + u32 nop4 = RISCV_INSN_NOP4;
>
> - if (patch_insn_write((void *)rec->ip, nops, MCOUNT_INSN_SIZE))
> + if (patch_insn_write((void *)rec->ip, &nop4, MCOUNT_NOP4_SIZE))
> return -EPERM;
>
> return 0;
> @@ -114,21 +105,38 @@ int ftrace_make_nop(struct module *mod, struct
> dyn_ftrace *rec,
> */
> int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
> {
> - int out;
> + unsigned long pc = rec->ip - MCOUNT_AUIPC_SIZE;
> + unsigned int nops[2], offset;
> + int ret;
> +
> + offset = (unsigned long) &ftrace_caller - pc;
> + nops[0] = to_auipc_t0(offset);
> + nops[1] = RISCV_INSN_NOP4;
>
> mutex_lock(&text_mutex);
> - out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
> + ret = patch_insn_write((void *)pc, nops, 2 * MCOUNT_INSN_SIZE);
> mutex_unlock(&text_mutex);
>
> - return out;
> + return ret;
> }
>
> +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;
> + WRITE_ONCE(ftrace_call_dest, func);
> + /*
> + * The data fence ensure that the update to ftrace_call_dest happens
> + * before the write to function_trace_op later in the generic ftrace.
> + * If the sequence is not enforced, then an old ftrace_call_dest may
> + * race loading a new function_trace_op set in ftrace_modify_all_code
> + *
> + * If we are in stop_machine, then we don't need to call remote fence
> + * as there is no concurrent read-side of ftrace_call_dest.
> + */
> + smp_wmb();
> + if (!irqs_disabled())
> + smp_call_function(ftrace_sync_ipi, NULL, 1);
> + return 0;
Here we allow the read of cpu_online_mask being reordered with the store to
ftrace_call_dest.
Can't this happen then?
Hart X | Hart new
load cpu_online_mask |
| store cpu_online_mask
| tlb shootdown/icache flush
store ftrace_call_dest |
ftrace_sync_ipi | // not hit
| sees old value ?
Thanks, Robbin
> }
>
> struct ftrace_modify_param {
> @@ -172,17 +180,11 @@ void arch_ftrace_update_code(int command)
> int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
> unsigned long addr)
> {
> + unsigned long caller = rec->ip - MCOUNT_AUIPC_SIZE;
> unsigned int call[2];
> - unsigned long caller = rec->ip;
> - int ret;
>
> 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, false);
> + return __ftrace_modify_call(caller, addr, true);
> }
> #endif
>
> diff --git a/arch/riscv/kernel/mcount-dyn.S b/arch/riscv/kernel/mcount-dyn.S
> index 3f06b40bb6c8..8aa554d56096 100644
> --- a/arch/riscv/kernel/mcount-dyn.S
> +++ b/arch/riscv/kernel/mcount-dyn.S
> @@ -13,7 +13,6 @@
>
> .text
>
> -#define FENTRY_RA_OFFSET 8
> #define ABI_SIZE_ON_STACK 80
> #define ABI_A0 0
> #define ABI_A1 8
> @@ -62,8 +61,7 @@
> * After the stack is established,
> *
> * 0(sp) stores the PC of the traced function which can be accessed
> -* by &(fregs)->epc in tracing function. Note that the real
> -* function entry address should be computed with -FENTRY_RA_OFFSET.
> +* by &(fregs)->epc in tracing function.
> *
> * 8(sp) stores the function return address (i.e. parent IP) that
> * can be accessed by &(fregs)->ra in tracing function.
> @@ -140,7 +138,7 @@
> .endm
>
> .macro PREPARE_ARGS
> - addi a0, t0, -FENTRY_RA_OFFSET
> + addi a0, t0, -MCOUNT_JALR_SIZE // ip (callsite's jalr insn)
> la a1, function_trace_op
> REG_L a2, 0(a1)
> mv a1, ra
> @@ -153,7 +151,8 @@ 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 ra, 0(ra)
>
> RESTORE_ABI_REGS
> bnez t1, .Ldirect
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