linux-kernel - [PATCH v3] x86/bug: __WARN_FLAGS: prevent shadowing by renaming local variable f to _

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Date:   Thu, 24 Mar 2022 11:37:42 +0900
From:   Vincent Mailhol <mailhol.vincent@...adoo.fr>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     linux-kernel@...r.kernel.org, adobriyan@...il.com,
        jpoimboe@...hat.com, linux-sparse@...r.kernel.org,
        llvm@...ts.linux.dev, luc.vanoostenryck@...il.com,
        nathan@...nel.org, peterz@...radead.org,
        Sasha Levin <sashal@...nel.org>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        Charlemagne Lasse <charlemagnelasse@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH v3] x86/bug: __WARN_FLAGS: prevent shadowing by renaming local variable f to __flags

The macro __WARN_FLAGS() uses a local variable named "f". This being a
common name, there is a risk of shadowing other variables.

For example, GCC would yield:

| In file included from ./include/linux/bug.h:5,
|                  from ./include/linux/cpumask.h:14,
|                  from ./arch/x86/include/asm/cpumask.h:5,
|                  from ./arch/x86/include/asm/msr.h:11,
|                  from ./arch/x86/include/asm/processor.h:22,
|                  from ./arch/x86/include/asm/timex.h:5,
|                  from ./include/linux/timex.h:65,
|                  from ./include/linux/time32.h:13,
|                  from ./include/linux/time.h:60,
|                  from ./include/linux/stat.h:19,
|                  from ./include/linux/module.h:13,
|                  from virt/lib/irqbypass.mod.c:1:
| ./include/linux/rcupdate.h: In function 'rcu_head_after_call_rcu':
| ./arch/x86/include/asm/bug.h:80:21: warning: declaration of 'f' shadows a parameter [-Wshadow]
|    80 |         __auto_type f = BUGFLAG_WARNING|(flags);                \
|       |                     ^
| ./include/asm-generic/bug.h:106:17: note: in expansion of macro '__WARN_FLAGS'
|   106 |                 __WARN_FLAGS(BUGFLAG_ONCE |                     \
|       |                 ^~~~~~~~~~~~
| ./include/linux/rcupdate.h:1007:9: note: in expansion of macro 'WARN_ON_ONCE'
|  1007 |         WARN_ON_ONCE(func != (rcu_callback_t)~0L);
|       |         ^~~~~~~~~~~~
| In file included from ./include/linux/rbtree.h:24,
|                  from ./include/linux/mm_types.h:11,
|                  from ./include/linux/buildid.h:5,
|                  from ./include/linux/module.h:14,
|                  from virt/lib/irqbypass.mod.c:1:
| ./include/linux/rcupdate.h:1001:62: note: shadowed declaration is here
|  1001 | rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
|       |                                               ~~~~~~~~~~~~~~~^

For reference, sparse also warns about it, c.f. [1].

This patch renames the variable from f to __flags (with two underscore
prefixes as suggested in the Linux kernel coding style [2]) in order
to prevent collisions.

[1] https://lore.kernel.org/all/CAFGhKbyifH1a+nAMCvWM88TK6fpNPdzFtUXPmRGnnQeePV+1sw@mail.gmail.com/

[2] Linux kernel coding style, section 12) Macros, Enums and RTL,
paragraph 5) namespace collisions when defining local variables in
macros resembling functions
https://www.kernel.org/doc/html/latest/process/coding-style.html#macros-enums-and-rtl

CC: Charlemagne Lasse <charlemagnelasse@...il.com>
Fixes: bfb1a7c91fb7 ("x86/bug: Merge annotate_reachable() into_BUG_FLAGS() asm")
Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
---
Changelog:

v2 -> v3:

  * add a mention that sparse is warning about this too.
  c.f. https://lore.kernel.org/all/CAKwvOdmSV3Nse+tGMBXvN=QvnOs6-ODZRJB0OF5Pd6BVb-scFw@mail.gmail.com/

v1 -> v2:

   * Update the reference link to the kernel documentation to latest version.
   * Do not break the Fixes: tag to a new line.
   * Added the Acked-by and Reviewed-by tags.
---
 arch/x86/include/asm/bug.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index bab883c0b6fe..66570e95af39 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -77,9 +77,9 @@ do {								\
  */
 #define __WARN_FLAGS(flags)					\
 do {								\
-	__auto_type f = BUGFLAG_WARNING|(flags);		\
+	__auto_type __flags = BUGFLAG_WARNING|(flags);		\
 	instrumentation_begin();				\
-	_BUG_FLAGS(ASM_UD2, f, ASM_REACHABLE);			\
+	_BUG_FLAGS(ASM_UD2, __flags, ASM_REACHABLE);		\
 	instrumentation_end();					\
 } while (0)
 
-- 
2.34.1