Re: [PATCH v4 2/3] LoongArch: Add ptrace single step support

From: Youling Tang
Date: Fri Feb 17 2023 - 04:50:35 EST

Hi, Qing

On 02/17/2023 10:37 AM, Qing Zhang wrote:
Use the generic ptrace_resume code for PTRACE_SYSCALL, PTRACE_CONT,
PTRACE_KILL and PTRACE_SINGLESTEP. This implies defining
arch_has_single_step in and implementing the
user_enable_single_step and user_disable_single_step functions.

LongArch has no hardware single-step register. the hardware single-step
function multiplex fetch instruction watchpoint(FWPS) and specifies that
the next instruction must trigger the watch exception by setting the mask bit.
Some scenarios use CSR.FWPS.Skip to ignore the next hit result, not to trigger
the watchpoint exception, and proceed to the next instruction.

Signed-off-by: Qing Zhang <zhangqing@xxxxxxxxxxx>
---
arch/loongarch/include/asm/inst.h | 39 +++++++++++++++
arch/loongarch/include/asm/loongarch.h | 3 ++
arch/loongarch/include/asm/processor.h | 3 ++
arch/loongarch/include/asm/ptrace.h | 2 +
arch/loongarch/kernel/hw_breakpoint.c | 35 +++++++++++--
arch/loongarch/kernel/ptrace.c | 68 ++++++++++++++++++++++++++
arch/loongarch/kernel/traps.c | 34 +++++++++++--
7 files changed, 176 insertions(+), 8 deletions(-)

diff --git a/arch/loongarch/include/asm/inst.h b/arch/loongarch/include/asm/inst.h
index ba18ce8fbdf2..00c6f261d9a2 100644
--- a/arch/loongarch/include/asm/inst.h
+++ b/arch/loongarch/include/asm/inst.h
@@ -368,6 +368,45 @@ static inline bool is_stack_alloc_ins(union loongarch_instruction *ip)
is_imm12_negative(ip->reg2i12_format.immediate);
}

+static inline bool branch_ins_target_pc(union loongarch_instruction *ip)
+{
+ switch (ip->reg0i26_format.opcode) {
+ case b_op:
+ case bl_op:
+ if (ip->reg0i26_format.immediate_l == 0
+ && ip->reg0i26_format.immediate_h == 0)
+ return false;
+ }
+
+ switch (ip->reg1i21_format.opcode) {
+ case beqz_op:
+ case bnez_op:
+ case bceqz_op:
+ if (ip->reg1i21_format.immediate_l == 0
+ && ip->reg1i21_format.immediate_h == 0)
+ return false;
+ }
+
+ switch (ip->reg2i16_format.opcode) {
+ case jirl_op:
+ if (ip->reg2i16_format.rj == 0x1
+ && ip->reg2i16_format.rd == 0x1
LOONGARCH_GPR_RA can be used instead of 0x1.

+ && ip->reg2i16_format.immediate == 0)
+ return false;
+ break;
+ case beq_op:
+ case bne_op:
+ case blt_op:
+ case bge_op:
+ case bltu_op:
+ case bgeu_op:
+ if (ip->reg2i16_format.immediate == 0)
+ return false;
+ }
+
+ return true;
+}
+
void simu_pc(struct pt_regs *regs, union loongarch_instruction insn);
void simu_branch(struct pt_regs *regs, union loongarch_instruction insn);

diff --git a/arch/loongarch/include/asm/loongarch.h b/arch/loongarch/include/asm/loongarch.h
index e9aed583a064..65b7dcdea16d 100644
--- a/arch/loongarch/include/asm/loongarch.h
+++ b/arch/loongarch/include/asm/loongarch.h
@@ -1055,6 +1055,9 @@ static __always_inline void iocsr_write64(u64 val, u32 reg)
#define LOONGARCH_CSR_DERA 0x501 /* debug era */
#define LOONGARCH_CSR_DESAVE 0x502 /* debug save */

+#define CSR_FWPC_SKIP_SHIFT 16
+#define CSR_FWPC_SKIP (_ULCAST_(1) << CSR_FWPC_SKIP_SHIFT)
+
/*
* CSR_ECFG IM
*/
diff --git a/arch/loongarch/include/asm/processor.h b/arch/loongarch/include/asm/processor.h
index db060c5a976f..3ea0f1910c23 100644
--- a/arch/loongarch/include/asm/processor.h
+++ b/arch/loongarch/include/asm/processor.h
@@ -131,6 +131,9 @@ struct thread_struct {
struct perf_event *hbp_break[LOONGARCH_MAX_BRP];
struct perf_event *hbp_watch[LOONGARCH_MAX_WRP];

+ /* Used by ptrace single_step */
+ unsigned long single_step;
+
/*
* FPU & vector registers, must be at last because
* they are conditionally copied at fork().
diff --git a/arch/loongarch/include/asm/ptrace.h b/arch/loongarch/include/asm/ptrace.h
index 58596c4f8a0f..66a0e6c480a3 100644
--- a/arch/loongarch/include/asm/ptrace.h
+++ b/arch/loongarch/include/asm/ptrace.h
@@ -150,4 +150,6 @@ static inline void user_stack_pointer_set(struct pt_regs *regs,
regs->regs[3] = val;
}

+#define arch_has_single_step() (1)
+
#endif /* _ASM_PTRACE_H */
diff --git a/arch/loongarch/kernel/hw_breakpoint.c b/arch/loongarch/kernel/hw_breakpoint.c
index 6431cd319c32..75d3652fbe00 100644
--- a/arch/loongarch/kernel/hw_breakpoint.c
+++ b/arch/loongarch/kernel/hw_breakpoint.c
@@ -153,6 +153,22 @@ static int hw_breakpoint_slot_setup(struct perf_event **slots, int max_slots,
*/
void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
{
+ int i;
+ struct thread_struct *t = &tsk->thread;
+
+ for (i = 0; i < LOONGARCH_MAX_BRP; i++) {
+ if (t->hbp_break[i]) {
+ unregister_hw_breakpoint(t->hbp_break[i]);
+ t->hbp_break[i] = NULL;
+ }
+ }
+
+ for (i = 0; i < LOONGARCH_MAX_WRP; i++) {
+ if (t->hbp_watch[i]) {
+ unregister_hw_breakpoint(t->hbp_watch[i]);
+ t->hbp_watch[i] = NULL;
+ }
+ }
}

void ptrace_hw_copy_thread(struct task_struct *tsk)
@@ -498,11 +514,20 @@ arch_initcall(arch_hw_breakpoint_init);
void hw_breakpoint_thread_switch(struct task_struct *next)
{
struct pt_regs *regs = task_pt_regs(next);
-
- /* Update breakpoints */
- update_bp_registers(regs, 1, 0);
- /* Update watchpoints */
- update_bp_registers(regs, 1, 1);
+ u64 addr, mask;
+
+ if (test_bit(TIF_SINGLESTEP, &task_thread_info(next)->flags)) {
+ addr = read_wb_reg(CSR_CFG_ADDR, 0, 0);
+ mask = read_wb_reg(CSR_CFG_MASK, 0, 0);
+ if ((task_pt_regs(next)->csr_era & ~mask) == (addr & ~mask))
+ csr_write32(CSR_FWPC_SKIP, LOONGARCH_CSR_FWPS);
+ regs->csr_prmd |= CSR_PRMD_PWE;
+ } else {
+ /* Update breakpoints */
+ update_bp_registers(regs, 1, 0);
+ /* Update watchpoints */
+ update_bp_registers(regs, 1, 1);
+ }
}

void hw_breakpoint_pmu_read(struct perf_event *bp)
diff --git a/arch/loongarch/kernel/ptrace.c b/arch/loongarch/kernel/ptrace.c
index bee4194177fd..52a3ee4366f4 100644
--- a/arch/loongarch/kernel/ptrace.c
+++ b/arch/loongarch/kernel/ptrace.c
@@ -20,6 +20,7 @@
#include <linux/context_tracking.h>
#include <linux/elf.h>
#include <linux/errno.h>
+#include <linux/hw_breakpoint.h>
#include <linux/mm.h>
#include <linux/ptrace.h>
#include <linux/regset.h>
@@ -30,6 +31,7 @@
#include <linux/stddef.h>
#include <linux/seccomp.h>
#include <linux/uaccess.h>
+#include <linux/thread_info.h>

#include <asm/byteorder.h>
#include <asm/cpu.h>
@@ -39,6 +41,7 @@
#include <asm/page.h>
#include <asm/pgtable.h>
#include <asm/processor.h>
+#include <asm/ptrace.h>
#include <asm/reg.h>
#include <asm/syscall.h>

@@ -541,3 +544,68 @@ long arch_ptrace(struct task_struct *child, long request,

return ret;
}
+
+void ptrace_triggered(struct perf_event *bp,
+ struct perf_sample_data *data, struct pt_regs *regs)
+{
+ struct perf_event_attr attr;
+
+ attr = bp->attr;
+ attr.disabled = true;
+ modify_user_hw_breakpoint(bp, &attr);
+}
+
+static int set_single_step(struct task_struct *tsk, unsigned long addr)
+{
+ struct thread_struct *thread = &tsk->thread;
+ struct perf_event *bp;
+ struct perf_event_attr attr;
+ struct arch_hw_breakpoint *info;
+
+ bp = thread->hbp_break[0];
+ if (!bp) {
+ ptrace_breakpoint_init(&attr);
+
+ attr.bp_addr = addr;
+ attr.bp_len = HW_BREAKPOINT_LEN_8;
+ attr.bp_type = HW_BREAKPOINT_X;
+
+ bp = register_user_hw_breakpoint(&attr, ptrace_triggered,
+ NULL, tsk);
+ if (IS_ERR(bp))
+ return PTR_ERR(bp);
+
+ thread->hbp_break[0] = bp;
+ } else {
+ int err;
+
+ attr = bp->attr;
+ attr.bp_addr = addr;
+ /* reenable breakpoint */
+ attr.disabled = false;
+ err = modify_user_hw_breakpoint(bp, &attr);
+ if (unlikely(err))
+ return err;
+
+ csr_write64(attr.bp_addr, LOONGARCH_CSR_IB0ADDR);
+ }
+ info = counter_arch_bp(bp);
+ info->mask = 0xffffffffffff;
If `mask` is only used for user space address mask, it should not be
fixed to 0xffffffffffff, the user space address size may have many
situations, which can be defined according to TASK_SIZE.

Use (TASK_SIZE - 1) instead.

+
+ return 0;
+}
+
+/* ptrace API */
+void user_enable_single_step(struct task_struct *task)
+{
+ struct thread_info *ti = task_thread_info(task);
+
+ set_single_step(task, task_pt_regs(task)->csr_era);
+ task->thread.single_step = task_pt_regs(task)->csr_era;
+ set_ti_thread_flag(ti, TIF_SINGLESTEP);
+}
+
+void user_disable_single_step(struct task_struct *task)
+{
+ clear_tsk_thread_flag(task, TIF_SINGLESTEP);
+}
diff --git a/arch/loongarch/kernel/traps.c b/arch/loongarch/kernel/traps.c
index 2b133079e0f3..a59275a43bd1 100644
--- a/arch/loongarch/kernel/traps.c
+++ b/arch/loongarch/kernel/traps.c
@@ -511,9 +511,37 @@ asmlinkage void noinstr do_watch(struct pt_regs *regs)
#ifdef CONFIG_HAVE_HW_BREAKPOINT
irqentry_state_t state = irqentry_enter(regs);

- breakpoint_handler(regs);
- watchpoint_handler(regs);
- force_sig(SIGTRAP);
+ if (test_tsk_thread_flag(current, TIF_SINGLESTEP)) {
+ int llbit = (csr_read32(LOONGARCH_CSR_LLBCTL) & 0x1);
+ unsigned long pc = regs->csr_era;
+ union loongarch_instruction *ip = (union loongarch_instruction *)pc;
+
+ if (llbit) {
+ /*
+ * When the ll-sc combo is encountered, it is regarded as an single
+ * instruction. So don't clear llbit and reset CSR.FWPS.Skip until
+ * the llsc execution is completed.
+ */
Comments should be aligned with code, similar modifications in other
places.

+ csr_write32(CSR_FWPC_SKIP, LOONGARCH_CSR_FWPS);
+ csr_write32(CSR_LLBCTL_KLO, LOONGARCH_CSR_LLBCTL);
+ } else if (pc == current->thread.single_step) {
+ /*
+ * Maybe some hardware Bug caused that certain insns are occasionally
+ * not skipped when CSR.FWPS.Skip is set, such as fld.d/fst.d. So Singlestep
+ * needs to compare whether the csr_era is equal to the value of singlestep
+ * which last time set.
+ */
+ if (branch_ins_target_pc(ip))
+ /* Prevent rd == pc from causing single step to fail to stop */
IMO, that comment description is not accurate enough.

Youling.

+ csr_write32(CSR_FWPC_SKIP, LOONGARCH_CSR_FWPS);
+ } else {
+ force_sig(SIGTRAP);
+ }
+ } else {
+ breakpoint_handler(regs);
+ watchpoint_handler(regs);
+ force_sig(SIGTRAP);
+ }

irqentry_exit(regs, state);
#endif