linux-kernel - Re: [PATCH] tools/nolibc: align signature of ioctl() with POSIX

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Message-ID: <c9782760-b371-4b82-8e6e-d169ab94a5ff@t-8ch.de>
Date: Thu, 23 Jan 2025 14:24:13 +0100
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Willy Tarreau <w@....eu>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tools/nolibc: align signature of ioctl() with POSIX

On 2025-01-23 14:16:25+0100, Willy Tarreau wrote:
> Hi Thomas,
> 
> On Thu, Jan 23, 2025 at 02:04:29PM +0100, Thomas Weißschuh wrote:
> > POSIX defines the signature of ioctl() as follows,
> > to allow passing a pointer or integer without casting:
> > 	int ioctl(int fildes, int request, ... /* arg */);
> > 
> > Nolibc ioctl() expects a pointer, forcing the user to manually cast.
> > Using va_arg to make the signature more flexible would work but seems to
> > prevent inlining of the function. Instead use a macro. "fd" and "req"
> > will still be typechecked through sys_ioctl().
> > 
> > Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> > ---
> >  tools/include/nolibc/sys.h | 8 ++------
> >  1 file changed, 2 insertions(+), 6 deletions(-)
> > 
> > diff --git a/tools/include/nolibc/sys.h b/tools/include/nolibc/sys.h
> > index d4a5c2399a66b200ebf7ab249569cce2285481a5..5cb2c66cc8cccc4d4a1126acfd0b30a6efc886c3 100644
> > --- a/tools/include/nolibc/sys.h
> > +++ b/tools/include/nolibc/sys.h
> > @@ -532,7 +532,7 @@ uid_t getuid(void)
> >  
> >  
> >  /*
> > - * int ioctl(int fd, unsigned long req, void *value);
> > + * int ioctl(int fd, unsigned long req, ... value);
> >   */
> >  
> >  static __attribute__((unused))
> > @@ -541,11 +541,7 @@ int sys_ioctl(int fd, unsigned long req, void *value)
> >  	return my_syscall3(__NR_ioctl, fd, req, value);
> >  }
> >  
> > -static __attribute__((unused))
> > -int ioctl(int fd, unsigned long req, void *value)
> > -{
> > -	return __sysret(sys_ioctl(fd, req, value));
> > -}
> > +#define ioctl(fd, req, value) __sysret(sys_ioctl(fd, req, (void *)(value)))
> 
> You risk to get a warning about casting a pointer from an integer of
> a different size if you pass an int there. I think should should perform
> a double cast instead:
> 
>   #define ioctl(fd, req, value) __sysret(sys_ioctl(fd, req, (void *)(uintptr_t)(value)))
> 
> That way any int should cast fine, and pointers should as well.

I don't think this should ever happen. A warning there is actually
useful. The POSIX signature forces users to pass something that is
compatible with (void *), otherwise the vararg handling would be
invalid.


Thomas