Date: Mon, 10 Apr 2017 09:42:36 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: *scanf, wrong types in va_arg, and strict aliasing On Mon, Apr 10, 2017 at 09:31:48AM +0000, Pascal Cuoq wrote: > Hello again, > > > the scanf implementation does the same thing as the printf implementation, in vfscanf.c, line 110: > > https://git.musl-libc.org/cgit/musl/tree/src/stdio/vfscanf.c?id=54807d47acecab778498ced88ce8f62bfa16e379#n110 > > This line is eventually reached, for instance, for the snippet: > > int n; sscanf("0", "%d", &n); > > And the argument being consumed has type int*. This is not a case that 7.16..1.1:2 allows. Indeed, that's wrong, but POSIX makes it hard or even impossible to do right because of the %n$ localization forms. Whereas printf's requirement for these is: "When numbered argument specifications are used, specifying the Nth argument requires that all the leading arguments, from the first to the (N-1)th, are specified in the format string." scanf's requirement is a bit different: "When numbered argument specifications are used, specifying the Nth argument requires that all the leading arguments, from the first to the (N-1)th, are pointers." This means that use of %2$ without a %1$ is permitted as long as a pointer (what kind?!) is passed in the first variadic position after the format string. I think the best we could do is to _try_ to do it right, by tracking the types like we do for printf, and only fallback to generic void* when a conversion specifier is missing. But that's a lot of extra code needed for something with no user-visible improvements. Alternatively, we could write a compiler-barrier in trivial asm to wrap an external va_arg, which is ugly but would also be useful in places like fcntl/ioctl that might read variadic args with unknown type to pass to the kernel. Any opinions on what we should do? Rich
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