Re: [PATCH] [stable 4.14] turn off -Wattribute-alias

[Greg KH]

On Thu, Jul 26, 2018 at 12:02:42PM +0200, Arnd Bergmann wrote:
> On Thu, Jul 26, 2018 at 11:22 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, Jul 26, 2018 at 10:13:22AM +0200, Arnd Bergmann wrote:
> >> Starting with gcc-8.1, we get a warning about all system call definitions,
> >> which use an alias between functions with incompatible prototypes, e.g.:
> >>
> >> In file included from ../mm/process_vm_access.c:19:
> >> ../include/linux/syscalls.h:211:18: warning: 'sys_process_vm_readv' alias between functions of incompatible types 'long int(pid_t,  const struct iovec *, long unsigned int,  const struct iovec *, long unsigned int,  long unsigned int)' {aka 'long int(int,  const struct iovec *, long unsigned int,  const struct iovec *, 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:207:2: note: in expansion of macro '__SYSCALL_DEFINEx'
> >>   __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
> >>   ^~~~~~~~~~~~~~~~~
> >> ../include/linux/syscalls.h:201:36: note: in expansion of macro 'SYSCALL_DEFINEx'
> >>  #define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)
> >>                                     ^~~~~~~~~~~~~~~
> >> ../mm/process_vm_access.c:300:1: note: in expansion of macro 'SYSCALL_DEFINE6'
> >>  SYSCALL_DEFINE6(process_vm_readv, pid_t, pid, const struct iovec __user *, lvec,
> >>  ^~~~~~~~~~~~~~~
> >> ../include/linux/syscalls.h:215:18: note: aliased declaration here
> >>   asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)) \
> >>                   ^~~
> >> ../include/linux/syscalls.h:207:2: note: in expansion of macro '__SYSCALL_DEFINEx'
> >>   __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
> >>   ^~~~~~~~~~~~~~~~~
> >> ../include/linux/syscalls.h:201:36: note: in expansion of macro 'SYSCALL_DEFINEx'
> >>  #define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)
> >>                                     ^~~~~~~~~~~~~~~
> >> ../mm/process_vm_access.c:300:1: note: in expansion of macro 'SYSCALL_DEFINE6'
> >>  SYSCALL_DEFINE6(process_vm_readv, pid_t, pid, const struct iovec __user *, lvec,
> >>
> >> This is really noisy and does not indicate a real problem. In the latest
> >> mainline kernel, this was addressed by commit bee20031772a ("disable
> >> -Wattribute-alias warning for SYSCALL_DEFINEx()"), which seems too invasive
> >> to backport.
> >
> > Too invasive?
> >
> > Here's the diffstat:
> >          include/linux/compat.h   |    8 +++++++-
> >          include/linux/syscalls.h |    4 ++++
> >          2 files changed, 11 insertions(+), 1 deletion(-)
> >
> > That seems almost trivial to backport.  Did you try it and it caused
> > other problems?
> >
> > Where ever possible, I always want to use the upstream commits.
> 
> I forward ported that patch a few times across kernels when that macro
> changed in subtle ways. This was particularly tricky as some using the
> new macro inside of another macro sometimes has different effects
> based on seemingly unrelated changes: in particular the addition of
> __MAP() in COMPAT_SYSCALL_DEFINEx that I had to do to work
> around gcc pr83256 can easily break.
> 
> It also depends on commit 8793bb7f4a9d ("kbuild: add macro for
> controlling warnings to linux/compiler.h") before it, which is not long
> but does add new infrastructure.

Ah, yeah, that's a mess.  Ok, I'll go queue these up, thanks.  Maybe I
can finally move to gcc-8 now :)

thanks,

greg k-h