linux-kernel - [PATCH AUTOSEL 5.18 36/47] gcc-12: disable '-Wdangling-pointer' warning for now

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Date:   Mon, 13 Jun 2022 22:04:29 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>, mmarek@...e.com,
        linux-kbuild@...r.kernel.org
Subject: [PATCH AUTOSEL 5.18 36/47] gcc-12: disable '-Wdangling-pointer' warning for now

From: Linus Torvalds <torvalds@...ux-foundation.org>

[ Upstream commit 49beadbd47c270a00754c107a837b4f29df4c822 ]

While the concept of checking for dangling pointers to local variables
at function exit is really interesting, the gcc-12 implementation is not
compatible with reality, and results in false positives.

For example, gcc sees us putting things on a local list head allocated
on the stack, which involves exactly those kinds of pointers to the
local stack entry:

  In function ‘__list_add’,
      inlined from ‘list_add_tail’ at include/linux/list.h:102:2,
      inlined from ‘rebuild_snap_realms’ at fs/ceph/snap.c:434:2:
  include/linux/list.h:74:19: warning: storing the address of local variable ‘realm_queue’ in ‘*&realm_27(D)->rebuild_item.prev’ [-Wdangling-pointer=]
     74 |         new->prev = prev;
        |         ~~~~~~~~~~^~~~~~

But then gcc - understandably - doesn't really understand the big
picture how the doubly linked list works, so doesn't see how we then end
up emptying said list head in a loop and the pointer we added has been
removed.

Gcc also complains about us (intentionally) using this as a way to store
a kind of fake stack trace, eg

  drivers/acpi/acpica/utdebug.c:40:38: warning: storing the address of local variable ‘current_sp’ in ‘acpi_gbl_entry_stack_pointer’ [-Wdangling-pointer=]
     40 |         acpi_gbl_entry_stack_pointer = &current_sp;
        |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~

which is entirely reasonable from a compiler standpoint, and we may want
to change those kinds of patterns, but not not.

So this is one of those "it would be lovely if the compiler were to
complain about us leaving dangling pointers to the stack", but not this
way.

Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Makefile b/Makefile
index eb3adfec0b22..a8de148aeb04 100644
--- a/Makefile
+++ b/Makefile
@@ -804,6 +804,9 @@ endif
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)
 
+# These result in bogus false positives
+KBUILD_CFLAGS += $(call cc-disable-warning, dangling-pointer)
+
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
 else
-- 
2.35.1