linux-kernel - Re: [PATCH] powerpc/Makefile: fix build failure by disabling attribute-alias warning

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Date:   Mon, 28 May 2018 16:56:22 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/Makefile: fix build failure by disabling
 attribute-alias warning



Le 28/05/2018 à 16:37, Segher Boessenkool a écrit :
> On Mon, May 28, 2018 at 02:17:49PM +0000, Christophe Leroy wrote:
>> Latest GCC version emit many warnings similar to the following one.
> 
> You didn't actually show an example?

Yes I forgot:

In file included from arch/powerpc/kernel/syscalls.c:24:
./include/linux/syscalls.h:233:18: warning: 'sys_mmap2' alias between 
functions of incompatible types 'long int(long unsigned int,  size_t, 
long unsigned int,  long unsigned int,  long unsigned int,  long 
unsigned int)' {aka 'long int(long unsigned int,  unsigned int,  long 
unsigned int,  long unsigned int,  long unsigned int,  long unsigned 
int)'} and 'long int(long int,  long int,  long int,  long int,  long 
int,  long int)' [-Wattribute-alias]
   asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__)) \
                   ^~~
./include/linux/syscalls.h:222:2: note: in expansion of macro 
'__SYSCALL_DEFINEx'
   __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
   ^~~~~~~~~~~~~~~~~
./include/linux/syscalls.h:216:36: note: in expansion of macro 
'SYSCALL_DEFINEx'
  #define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, 
__VA_ARGS__)
                                     ^~~~~~~~~~~~~~~
arch/powerpc/kernel/syscalls.c:65:1: note: in expansion of macro 
'SYSCALL_DEFINE6'
  SYSCALL_DEFINE6(mmap2, unsigned long, addr, size_t, len,
  ^~~~~~~~~~~~~~~
./include/linux/syscalls.h:238:18: note: aliased declaration here
   asmlinkage long __se_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__)) \
                   ^~~~~~~~
./include/linux/syscalls.h:222:2: note: in expansion of macro 
'__SYSCALL_DEFINEx'
   __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
   ^~~~~~~~~~~~~~~~~
./include/linux/syscalls.h:216:36: note: in expansion of macro 
'SYSCALL_DEFINEx'
  #define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, 
__VA_ARGS__)
                                     ^~~~~~~~~~~~~~~
arch/powerpc/kernel/syscalls.c:65:1: note: in expansion of macro 
'SYSCALL_DEFINE6'
  SYSCALL_DEFINE6(mmap2, unsigned long, addr, size_t, len,
  ^~~~~~~~~~~~~~~

> 
> This warning should detect serious problems, so don't disable it without
> first investigating please.

It's been a discussion on this topic, ref 
https://lkml.org/lkml/2017/12/5/581

It says "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"


> 
> What do you call "latest version", btw?  Trunk, aka 9.0?  Or the most
> advanced release, 8.1?  Or the latest release (which also is 8.1 :-) )

I encounter it with 8.1
According the refered discusion, it linked to GCC 8

Christophe

> 
> 
> Segher
>