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Vincent MailholandGreg Kroah-Hartman
f9e7a75648
container_of() can be called in a nested manner to retrieve the grand
parent structure as illustrated below:
struct foo {
int a;
};
struct bar {
struct foo foo;
};
#define to_foo(a_ptr) container_of(a_ptr, struct foo, a)
#define to_bar(a_ptr) container_of(to_foo(a_ptr), struct bar, foo)
The issue is that the above construct will cause __mptr, the local
variable of container_of(), to shadow itself because of the nested
call. This then triggers a warning in sparse and W=2 builds.
While this warning is benign, it still causes some overhead as proven by
below list of commits in which people made local workarounds:
- commit 7eab14de73 ("mdio, phy: fix -Wshadow warnings triggered by
nested container_of()")
- commit 8d8c313124 ("clk: define to_clk_regmap() as inline
function")
- commit bfb972c5e1 ("IB/verbs: avoid nested container_of()")
- commit 093adbcedf ("btrfs: switch helper macros to static inlines
in sysfs.h")
- commit c1d35dfa0f ("rt2x00: Fix sparse warning on nested
container_of()")
(the list is probably not exhaustive).
As a matter of fact, the local variable __mptr is only used once in
container_of(). As such, it is not strictly needed. Inline that local
__mptr variable to remove once and for all the risk of variable
shadowing when nesting container_of() and prevent people from writing
further local fixes.
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20260714-containerof_refactor-v1-3-b5c31164d2ad@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
41 lines
1.4 KiB
C
41 lines
1.4 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_CONTAINER_OF_H
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#define _LINUX_CONTAINER_OF_H
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#include <linux/build_bug.h>
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#include <linux/stddef.h>
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#define typeof_member(T, m) typeof(((T*)0)->m)
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/**
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* container_of - cast a member of a structure out to the containing structure
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* @ptr: the pointer to the member.
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* @type: the type of the container struct this is embedded in.
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* @member: the name of the member within the struct.
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*
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* WARNING: any const qualifier of @ptr is lost.
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* Do not use container_of() in new code.
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*/
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#define container_of(ptr, type, member) ({ \
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static_assert(__same_type(*(ptr), typeof_member(type, member)) || \
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__same_type(*(ptr), void), \
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"pointer type mismatch in container_of()"); \
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(type *)((void *)(ptr) - offsetof(type, member)); })
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/**
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* container_of_const - cast a member of a structure out to the containing
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* structure and preserve the const-ness of the pointer
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* @ptr: the pointer to the member
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* @type: the type of the container struct this is embedded in.
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* @member: the name of the member within the struct.
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*
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* Always prefer container_of_const() instead of container_of() in new code.
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*/
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#define container_of_const(ptr, type, member) \
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_Generic(ptr, \
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const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
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default: ((type *)container_of(ptr, type, member)) \
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)
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#endif /* _LINUX_CONTAINER_OF_H */
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