Compiler Attributes: add support for __copy (gcc >= 9)

[ 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>

2 files changed, 8 insertions, 0 deletions

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 8e9b0cb8db41..d26fe424de6b 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -319,6 +319,10 @@
 #define __no_sanitize_address __attribute__((no_sanitize_address))
 #endif
 
+#if GCC_VERSION >= 90100
+#define __copy(symbol)                 __attribute__((__copy__(symbol)))
+#endif
+
 #endif	/* gcc version >= 40000 specific checks */
 
 #if !defined(__noclone)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 4f3dfabb680f..2c552147a5c3 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -433,6 +433,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
 # define __latent_entropy
 #endif
 
+#ifndef __copy
+# define __copy(symbol)
+#endif
+
 /*
  * Tell gcc if a function is cold. The compiler will assume any path
  * directly leading to the call is unlikely.