[PATCH v1 5/6] LoongArch: Fix perf hardware breakpoint failure via installation

In hw_breakpoint_control(), the logic to enable CSR_PRMD_PWE (global
watchpoint enable) incorrectly relies on TIF_LOAD_WATCH. This thread
flag is only set during ptrace operations, meaning that the standard
hardware breakpoints created via perf_event_open() will never have
this flag set.

As a result, the CSR_PRMD_PWE switch is skipped, leaving the hardware
breakpoint completely inactive in perf usage.

Fix this by decoupling the CSR_PRMD_PWE from the ptrace specific flag
TIF_LOAD_WATCH, which ensures that CSR_PRMD_PWE is enabled whenever a
hardware breakpoint is installed.

Here is a user-space reproducer to demonstrate the issue:

(1) Test program (test_perf_install.c):

  #include <stdio.h>
  #include <unistd.h>
  #include <sys/syscall.h>
  #include <sys/ioctl.h>
  #include <linux/perf_event.h>
  #include <linux/hw_breakpoint.h>

  static int var = 0;

  int main(void)
  {
  	size_t count = 0;
  	struct perf_event_attr attr = {
  		.type = PERF_TYPE_BREAKPOINT,
  		.size = sizeof(attr),
  		.bp_type = HW_BREAKPOINT_W,
  		.bp_addr = (unsigned long)&var,
  		.bp_len = HW_BREAKPOINT_LEN_1,
  		.exclude_kernel = 1,
  	};

  	int fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
  	ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

  	asm volatile("st.b %1, %0" : "=m"(var) : "r"(11) : "memory");

  	ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
  	read(fd, &count, sizeof(size_t));

  	printf("Watchpoint counts: expected = 1, actual = %zu\n", count);

  	close(fd);
  	return 0;
  }

(2) Test steps:

  $ gcc test_perf_install.c -o test_perf_install
  $ ./test_perf_install

(3) Test results:

Without this patch:

  Watchpoint counts: expected = 1, actual = 0

With this patch:

  Watchpoint counts: expected = 1, actual = 1

Fixes: 3892b11eac5a ("LoongArch: Check TIF_LOAD_WATCH to enable user space watchpoint")
Cc: [email protected]
Signed-off-by: Tiezhu Yang <[email protected]>
---
 arch/loongarch/kernel/hw_breakpoint.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/loongarch/kernel/hw_breakpoint.c b/arch/loongarch/kernel/hw_breakpoint.c
index 3683a52b2368..9dcb122218c2 100644
--- a/arch/loongarch/kernel/hw_breakpoint.c
+++ b/arch/loongarch/kernel/hw_breakpoint.c
@@ -233,7 +233,7 @@ static int hw_breakpoint_control(struct perf_event *bp,
 		}
 		enable = csr_read64(LOONGARCH_CSR_CRMD);
 		csr_write64(CSR_CRMD_WE | enable, LOONGARCH_CSR_CRMD);
-		if (bp->hw.target && test_tsk_thread_flag(bp->hw.target, TIF_LOAD_WATCH))
+		if (bp->hw.target)
 			regs->csr_prmd |= CSR_PRMD_PWE;
 		break;
 	case HW_BREAKPOINT_UNINSTALL:
-- 
2.42.0