From: Miguel Ojeda miguel.ojeda.sandonis@gmail.com
[ Upstream commit c0d9782f5b6d7157635ae2fd782a4b27d55a6013 ]
From the GCC manual:
copy copy(function)
The copy attribute applies the set of attributes with which function has been declared to the declaration of the function to which the attribute is applied. The attribute is designed for libraries that define aliases or function resolvers that are expected to specify the same set of attributes as their targets. The copy attribute can be used with functions, variables, or types. However, the kind of symbol to which the attribute is applied (either function or variable) must match the kind of symbol to which the argument refers. The copy attribute copies only syntactic and semantic attributes but not attributes that affect a symbol’s linkage or visibility such as alias, visibility, or weak. The deprecated attribute is also not copied.
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
The upcoming GCC 9 release extends the -Wmissing-attributes warnings (enabled by -Wall) to C and aliases: it warns when particular function attributes are missing in the aliases but not in their target, e.g.:
void __cold f(void) {} void __alias("f") g(void);
diagnoses:
warning: 'g' specifies less restrictive attribute than its target 'f': 'cold' [-Wmissing-attributes]
Using __copy(f) we can copy the __cold attribute from f to g:
void __cold f(void) {} void __copy(f) __alias("f") g(void);
This attribute is most useful to deal with situations where an alias is declared but we don't know the exact attributes the target has.
For instance, in the kernel, the widely used module_init/exit macros define the init/cleanup_module aliases, but those cannot be marked always as __init/__exit since some modules do not have their functions marked as such.
Cc: stable@vger.kernel.org # 4.14+ Suggested-by: Martin Sebor msebor@gcc.gnu.org Reviewed-by: Nick Desaulniers ndesaulniers@google.com Signed-off-by: Miguel Ojeda miguel.ojeda.sandonis@gmail.com Signed-off-by: Stefan Agner stefan@agner.ch --- include/linux/compiler-gcc.h | 4 ++++ include/linux/compiler_types.h | 4 ++++ 2 files changed, 8 insertions(+)
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h index a8ff0ca0c321..3ebee1ce6f98 100644 --- a/include/linux/compiler-gcc.h +++ b/include/linux/compiler-gcc.h @@ -201,6 +201,10 @@ #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1 #endif
+#if GCC_VERSION >= 90100 +#define __copy(symbol) __attribute__((__copy__(symbol))) +#endif + #if !defined(__noclone) #define __noclone /* not needed */ #endif diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index c2ded31a4cec..2b8ed70c4c77 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -180,6 +180,10 @@ struct ftrace_likely_data { #define __diag_GCC(version, severity, string) #endif
+#ifndef __copy +# define __copy(symbol) +#endif + #define __diag_push() __diag(push) #define __diag_pop() __diag(pop)
From: Miguel Ojeda miguel.ojeda.sandonis@gmail.com
[ Upstream commit a6e60d84989fa0e91db7f236eda40453b0e44afa ]
The upcoming GCC 9 release extends the -Wmissing-attributes warnings (enabled by -Wall) to C and aliases: it warns when particular function attributes are missing in the aliases but not in their target.
In particular, it triggers for all the init/cleanup_module aliases in the kernel (defined by the module_init/exit macros), ending up being very noisy.
These aliases point to the __init/__exit functions of a module, which are defined as __cold (among other attributes). However, the aliases themselves do not have the __cold attribute.
Since the compiler behaves differently when compiling a __cold function as well as when compiling paths leading to calls to __cold functions, the warning is trying to point out the possibly-forgotten attribute in the alias.
In order to keep the warning enabled, we decided to silence this case. Ideally, we would mark the aliases directly as __init/__exit. However, there are currently around 132 modules in the kernel which are missing __init/__exit in their init/cleanup functions (either because they are missing, or for other reasons, e.g. the functions being called from somewhere else); and a section mismatch is a hard error.
A conservative alternative was to mark the aliases as __cold only. However, since we would like to eventually enforce __init/__exit to be always marked, we chose to use the new __copy function attribute (introduced by GCC 9 as well to deal with this). With it, we copy the attributes used by the target functions into the aliases. This way, functions that were not marked as __init/__exit won't have their aliases marked either, and therefore there won't be a section mismatch.
Note that the warning would go away marking either the extern declaration, the definition, or both. However, we only mark the definition of the alias, since we do not want callers (which only see the declaration) to be compiled as if the function was __cold (and therefore the paths leading to those calls would be assumed to be unlikely).
Cc: stable@vger.kernel.org # 4.14+ Link: https://lore.kernel.org/lkml/20190123173707.GA16603@gmail.com/ Link: https://lore.kernel.org/lkml/20190206175627.GA20399@gmail.com/ Suggested-by: Martin Sebor msebor@gcc.gnu.org Acked-by: Jessica Yu jeyu@kernel.org Signed-off-by: Miguel Ojeda miguel.ojeda.sandonis@gmail.com Signed-off-by: Stefan Agner stefan@agner.ch --- include/linux/module.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/module.h b/include/linux/module.h index c71044644979..9915397715fc 100644 --- a/include/linux/module.h +++ b/include/linux/module.h @@ -130,13 +130,13 @@ extern void cleanup_module(void); #define module_init(initfn) \ static inline initcall_t __maybe_unused __inittest(void) \ { return initfn; } \ - int init_module(void) __attribute__((alias(#initfn))); + int init_module(void) __copy(initfn) __attribute__((alias(#initfn)));
/* This is only required if you want to be unloadable. */ #define module_exit(exitfn) \ static inline exitcall_t __maybe_unused __exittest(void) \ { return exitfn; } \ - void cleanup_module(void) __attribute__((alias(#exitfn))); + void cleanup_module(void) __copy(exitfn) __attribute__((alias(#exitfn)));
#endif
On Tue, Jun 04, 2019 at 03:24:40PM +0200, Stefan Agner wrote:
From: Miguel Ojeda miguel.ojeda.sandonis@gmail.com
[ Upstream commit c0d9782f5b6d7157635ae2fd782a4b27d55a6013 ]
From the GCC manual:
copy copy(function)
The copy attribute applies the set of attributes with which function has been declared to the declaration of the function to which the attribute is applied. The attribute is designed for libraries that define aliases or function resolvers that are expected to specify the same set of attributes as their targets. The copy attribute can be used with functions, variables, or types. However, the kind of symbol to which the attribute is applied (either function or variable) must match the kind of symbol to which the argument refers. The copy attribute copies only syntactic and semantic attributes but not attributes that affect a symbol’s linkage or visibility such as alias, visibility, or weak. The deprecated attribute is also not copied.https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
The upcoming GCC 9 release extends the -Wmissing-attributes warnings (enabled by -Wall) to C and aliases: it warns when particular function attributes are missing in the aliases but not in their target, e.g.:
void __cold f(void) {} void __alias("f") g(void);diagnoses:
warning: 'g' specifies less restrictive attribute than its target 'f': 'cold' [-Wmissing-attributes]Using __copy(f) we can copy the __cold attribute from f to g:
void __cold f(void) {} void __copy(f) __alias("f") g(void);This attribute is most useful to deal with situations where an alias is declared but we don't know the exact attributes the target has.
For instance, in the kernel, the widely used module_init/exit macros define the init/cleanup_module aliases, but those cannot be marked always as __init/__exit since some modules do not have their functions marked as such.
Cc: stable@vger.kernel.org # 4.14+ Suggested-by: Martin Sebor msebor@gcc.gnu.org Reviewed-by: Nick Desaulniers ndesaulniers@google.com Signed-off-by: Miguel Ojeda miguel.ojeda.sandonis@gmail.com Signed-off-by: Stefan Agner stefan@agner.ch
include/linux/compiler-gcc.h | 4 ++++ include/linux/compiler_types.h | 4 ++++ 2 files changed, 8 insertions(+)
Thanks for this, and the 4.14 updated series, I've applied them all now.
greg k-h
linux-stable-mirror@lists.linaro.org
-
Greg KH -
Stefan Agner