Re: [PATCH 5/9] fs: remove various compat readv/writev helpers

[Christoph Hellwig]

On Wed, Sep 23, 2020 at 03:25:49PM +0100, Al Viro wrote:
> On Wed, Sep 23, 2020 at 08:05:43AM +0200, Christoph Hellwig wrote:
> >  COMPAT_SYSCALL_DEFINE3(readv, compat_ulong_t, fd,
> > -		const struct compat_iovec __user *,vec,
> > +		const struct iovec __user *, vec,
> 
> Um...  Will it even compile?
> 
> >  #ifdef __ARCH_WANT_COMPAT_SYS_PREADV64
> >  COMPAT_SYSCALL_DEFINE4(preadv64, unsigned long, fd,
> > -		const struct compat_iovec __user *,vec,
> > +		const struct iovec __user *, vec,
> 
> Ditto.  Look into include/linux/compat.h and you'll see
> 
> asmlinkage long compat_sys_preadv64(unsigned long fd,
>                 const struct compat_iovec __user *vec,
>                 unsigned long vlen, loff_t pos);
> 
> How does that manage to avoid the compiler screaming bloody
> murder?

That's a very good question.  But it does not just compile but actually
works.  Probably because all the syscall wrappers mean that we don't
actually generate the normal names.  I just tried this:

--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -468,7 +468,7 @@ asmlinkage long sys_lseek(unsigned int fd, off_t offset,
 asmlinkage long sys_read(unsigned int fd, char __user *buf, size_t count);
 asmlinkage long sys_write(unsigned int fd, const char __user *buf,
                            size_t count);
-asmlinkage long sys_readv(unsigned long fd,
+asmlinkage long sys_readv(void *fd,

for fun, and the compiler doesn't care either..