From b2137c3b6d7a45622967aac78b46a4bd2cff6c42 Mon Sep 17 00:00:00 2001 From: Andy Chiu Date: Tue, 8 Apr 2025 02:08:29 +0800 Subject: [PATCH] riscv: ftrace: prepare ftrace for atomic code patching MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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. | : | 0: auipc t0, hi(ftrace_caller) | 4: jalr t0, lo(ftrace_caller) | | -8: &ftrace_nop_ops | : | 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. [ alex: Fix build errors with !CONFIG_DYNAMIC_FTRACE ] Co-developed-by: Björn Töpel Signed-off-by: Björn Töpel Signed-off-by: Andy Chiu Link: https://lore.kernel.org/r/20250407180838.42877-5-andybnac@gmail.com Signed-off-by: Alexandre Ghiti Signed-off-by: Palmer Dabbelt --- arch/riscv/include/asm/ftrace.h | 49 +++++------- arch/riscv/kernel/ftrace.c | 137 +++++++++++++++++--------------- arch/riscv/kernel/mcount-dyn.S | 9 +-- 3 files changed, 98 insertions(+), 97 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 (). The second instruction is patched to jalr to + * start the trace. + * + *: + * 0: nop + * 4: nop + * *: - * 0: auipc t0/ra, 0x? - * 4: jalr t0/ra, ?(t0/ra) + * 0: auipc t0, 0x? + * 4: jalr t0, ?(t0) * *: - * 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..ea04f09f9d4d 100644 --- a/arch/riscv/kernel/ftrace.c +++ b/arch/riscv/kernel/ftrace.c @@ -8,11 +8,22 @@ #include #include #include +#include #include #include #include #ifdef CONFIG_DYNAMIC_FTRACE +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; +} + void ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex) { mutex_lock(&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; 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]; - - make_call_t0(rec->ip, addr, call); + unsigned long distance, orig_addr, pc = rec->ip - MCOUNT_AUIPC_SIZE; - 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; } struct ftrace_modify_param { @@ -166,23 +174,22 @@ void arch_ftrace_update_code(int command) stop_machine(__ftrace_modify_code, ¶m, cpu_online_mask); } -#endif +#else /* CONFIG_DYNAMIC_FTRACE */ +unsigned long ftrace_call_adjust(unsigned long addr) +{ + return addr; +} +#endif /* CONFIG_DYNAMIC_FTRACE */ #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS 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 -- 2.25.1