linux-kernel - [PATCH RFC] cleanup/scoped_cond_guard: Fix multiple statements in fail

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Date: Mon, 05 Feb 2024 10:07:23 -0800
From: Ira Weiny <ira.weiny@...el.com>
To: Peter Zijlstra <peterz@...radead.org>, 
 Dan Williams <dan.j.williams@...el.com>, 
 "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>, 
 Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: Ingo Molnar <mingo@...nel.org>, Oleg Nesterov <oleg@...hat.com>, 
 linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org, 
 Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH RFC] cleanup/scoped_cond_guard: Fix multiple statements in
 fail

In attempting to create a cond_guard() macro[1] Fabio asked me to do
some testing of the macros he was creating.  The model for this macro
was scoped_cond_guard() and the ability to declare a block for the error
path.

A simple test for scoped_cond_guard() was created to learn how it
worked and to model cond_guard() after it.  Specifically compound
statements were tested as suggested to be used in cond_guard().[2]

static int test_scoped_cond_guard(void)
{
        scoped_cond_guard(rwsem_write_try,
                        { printk(KERN_DEBUG "Failed\n"); return -EINVAL; },
                        &my_sem) {
                printk(KERN_DEBUG "Protected\n");
        }
        return 0;
}

This test fails with the current code:

lib/test-cleanup.c: In function ‘test_scoped_cond_guard’:
/include/linux/cleanup.h:190:17: error: ‘else’ without a previous ‘if’
  190 |                 else
      |                 ^~~~
lib/test-cleanup.c:79:9: note: in expansion of macro ‘scoped_cond_guard’
   79 |         scoped_cond_guard(rwsem_write_try,
      |         ^~~~~~~~~~~~~~~~~

This is due to an extra statement between the if and else blocks created
by the ';' in the macro.

Ensure the if block is delineated properly for the use of compound
statements within the macro.

[1] https://lore.kernel.org/all/20240204173105.935612-1-fabio.maria.de.francesco@linux.intel.com/
[2] https://lore.kernel.org/all/65b938c1ad435_5cc6f294eb@dwillia2-mobl3.amr.corp.intel.com.notmuch/

Signed-off-by: Ira Weiny <ira.weiny@...el.com>
---
NOTE: There is no user of this syntax yet but this is the way that Dan
and I thought the macro worked.  An alternate syntax would be to require
termination of the statement (ie use ';') in the use of the macro; see
below.  But this change seemed better as the compiler should drop the
extra statements created and allows for a bit more flexibility in the
use of the macro.

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 88af56600325..6cc4bfe61bc7 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -186,7 +186,7 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
 #define scoped_cond_guard(_name, _fail, args...) \
        for (CLASS(_name, scope)(args), \
             *done = NULL; !done; done = (void *)1) \
-               if (!__guard_ptr(_name)(&scope)) _fail; \
+               if (!__guard_ptr(_name)(&scope)) _fail \
                else

 /*
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 2fabd497d659..fae110e8b89f 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -441,7 +441,7 @@ static int ptrace_attach(struct task_struct *task, long request,
         * SUID, SGID and LSM creds get determined differently
         * under ptrace.
         */
-       scoped_cond_guard (mutex_intr, return -ERESTARTNOINTR,
+       scoped_cond_guard (mutex_intr, return -ERESTARTNOINTR;,
                           &task->signal->cred_guard_mutex) {

                scoped_guard (task_lock, task) {
---
 include/linux/cleanup.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 88af56600325..d45452ce6222 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -186,7 +186,7 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
 #define scoped_cond_guard(_name, _fail, args...) \
 	for (CLASS(_name, scope)(args), \
 	     *done = NULL; !done; done = (void *)1) \
-		if (!__guard_ptr(_name)(&scope)) _fail; \
+		if (!__guard_ptr(_name)(&scope)) { _fail; } \
 		else
 
 /*

---
base-commit: 03c972291873663f15c78ff4ca07cbf5025735f8
change-id: 20240201-cond_guard-afa0566cecdf

Best regards,
-- 
Ira Weiny <ira.weiny@...el.com>