[PATCH 2/2] [RFC] disable -Wattribute-alias warning for SYSCALL_DEFINEx()

[Arnd Bergmann <arnd@arndb.de>]

gcc-8 warns for every single definition of a system call entry
point, e.g.:

include/linux/compat.h:56:18: error: 'compat_sys_rt_sigprocmask' alias between functions of incompatible types 'long int(int,  compat_sigset_t *, compat_sigset_t *, compat_size_t)' {aka 'long int(int,  struct <anonymous> *, struct <anonymous> *, unsigned int)'} and 'long int(long int,  long int,  long int,  long int)' [-Werror=attribute-alias]
  asmlinkage long compat_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))\
                  ^~~~~~~~~~
include/linux/compat.h:45:2: note: in expansion of macro 'COMPAT_SYSCALL_DEFINEx'
  COMPAT_SYSCALL_DEFINEx(4, _##name, __VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~
kernel/signal.c:2601:1: note: in expansion of macro 'COMPAT_SYSCALL_DEFINE4'
 COMPAT_SYSCALL_DEFINE4(rt_sigprocmask, int, how, compat_sigset_t __user *, nset,
 ^~~~~~~~~~~~~~~~~~~~~~
include/linux/compat.h:60:18: note: aliased declaration here
  asmlinkage long compat_SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__))\
                  ^~~~~~~~~~

The new warning seems reasonable in principle, but it doesn't
help us here, since we rely on the type mismatch to sanitize the
system call arguments. After I reported this as GCC PR82435, a new
-Wno-attribute-alias option was added that could be used to turn the
warning off globally on the command line, but I'd prefer to do it a
little more fine-grained.

Interestingly, turning a warning off and on again inside of
a single macro doesn't always work, in this case I had to add
an extra statement inbetween and decided to copy the __SC_TEST
one from the native syscall to the compat syscall macro.  See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83256 for more details
about this.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82435
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 include/linux/compat.h   | 7 ++++++-
 include/linux/syscalls.h | 3 +++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/linux/compat.h b/include/linux/compat.h
index 1165036d091f..27b4a429df77 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -49,14 +49,19 @@
 	COMPAT_SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)
 
 #define COMPAT_SYSCALL_DEFINEx(x, name, ...)				\
+	__diag_push();							\
+	__diag_ignore(GCC_8, "-Wattribute-alias");			\
 	asmlinkage long compat_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))\
 		__attribute__((alias(__stringify(compat_SyS##name))));  \
 	static inline long C_SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__));\
 	asmlinkage long compat_SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__));\
 	asmlinkage long compat_SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__))\
 	{								\
-		return C_SYSC##name(__MAP(x,__SC_DELOUSE,__VA_ARGS__));	\
+		long ret = C_SYSC##name(__MAP(x,__SC_DELOUSE,__VA_ARGS__));\
+		__MAP(x,__SC_TEST,__VA_ARGS__);				\
+		return ret;						\
 	}								\
+	__diag_pop();							\
 	static inline long C_SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__))
 
 #ifdef CONFIG_COMPAT
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 4df16a70b0d7..dcf6ceabda44 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -208,6 +208,8 @@ static inline int is_syscall_trace_event(struct trace_event_call *tp_event)
 
 #define __PROTECT(...) asmlinkage_protect(__VA_ARGS__)
 #define __SYSCALL_DEFINEx(x, name, ...)					\
+	__diag_push();							\
+	__diag_ignore(GCC_8, "-Wattribute-alias");			\
 	asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))	\
 		__attribute__((alias(__stringify(SyS##name))));		\
 	static inline long SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__));	\
@@ -219,6 +221,7 @@ static inline int is_syscall_trace_event(struct trace_event_call *tp_event)
 		__PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__));	\
 		return ret;						\
 	}								\
+	__diag_pop();							\
 	static inline long SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__))
 
 /*
-- 
2.9.0