Re: [PATCH v2] lib/string.c: implement stpcpy

Nick Desaulniers
2020-08-15T21:46:26Z

On Sat, Aug 15, 2020 at 2:24 PM Joe Perches <joe@perches.com> wrote:
>
> On Sat, 2020-08-15 at 13:47 -0700, Nick Desaulniers wrote:
> > On Sat, Aug 15, 2020 at 9:34 AM Kees Cook <keescook@chromium.org> wrote:
> > > On Fri, Aug 14, 2020 at 07:09:44PM -0700, Nick Desaulniers wrote:
> > > > LLVM implemented a recent "libcall optimization" that lowers calls to
> > > > `sprintf(dest, "%s", str)` where the return value is used to
> > > > `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> > > > in parsing format strings. Calling `sprintf` with overlapping arguments
> > > > was clarified in ISO C99 and POSIX.1-2001 to be undefined behavior.
> > > >
> > > > `stpcpy` is just like `strcpy` except it returns the pointer to the new
> > > > tail of `dest`. This allows you to chain multiple calls to `stpcpy` in
> > > > one statement.
> > >
> > > O_O What?
> > >
> > > No; this is a _terrible_ API: there is no bounds checking, there are no
> > > buffer sizes. Anything using the example sprintf() pattern is _already_
> > > wrong and must be removed from the kernel. (Yes, I realize that the
> > > kernel is *filled* with this bad assumption that "I'll never write more
> > > than PAGE_SIZE bytes to this buffer", but that's both theoretically
> > > wrong ("640k is enough for anybody") and has been known to be wrong in
> > > practice too (e.g. when suddenly your writing routine is reachable by
> > > splice(2) and you may not have a PAGE_SIZE buffer).
> > >
> > > But we cannot _add_ another dangerous string API. We're already in a
> > > terrible mess trying to remove strcpy[1], strlcpy[2], and strncpy[3]. This
> > > needs to be addressed up by removing the unbounded sprintf() uses. (And
> > > to do so without introducing bugs related to using snprintf() when
> > > scnprintf() is expected[4].)
> >
> > Well, everything (-next, mainline, stable) is broken right now (with
> > ToT Clang) without providing this symbol. I'm not going to go clean
> > the entire kernel's use of sprintf to get our CI back to being green.
>
> Maybe this should get place in compiler-clang.h so it isn't
> generic and public.

https://bugs.llvm.org/show_bug.cgi?id=47162#c7 and
https://bugs.llvm.org/show_bug.cgi?id=47144
Seem to imply that Clang is not the only compiler that can lower a
sequence of libcalls to stpcpy. Do we want to wait until we have a
fire drill w/ GCC to move such an implementation from
include/linux/compiler-clang.h back in to lib/string.c?

>
> Something like:
>
> ---
> include/linux/compiler-clang.h | 27 +++++++++++++++++++++++++++
> 1 file changed, 27 insertions(+)
>
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index cee0c728d39a..6279f1904e39 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -61,3 +61,30 @@
> #if __has_feature(shadow_call_stack)
> # define __noscs __attribute__((__no_sanitize__("shadow-call-stack")))
> #endif
> +
> +#ifndef __HAVE_ARCH_STPCPY
> +/**
> + * stpcpy - copy a string from src to dest returning a pointer to the new end
> + * of dest, including src's NULL terminator. May overrun dest.
> + * @dest: pointer to buffer being copied into.
> + * Must be large enough to receive copy.
> + * @src: pointer to the beginning of string being copied from.
> + * Must not overlap dest.
> + *
> + * This function exists _only_ to support clang's possible conversion of
> + * sprintf calls to stpcpy.
> + *
> + * stpcpy differs from strcpy in two key ways:
> + * 1. inputs must not overlap.
> + * 2. return value is dest's NUL termination character after copy.
> + * (for strcpy, the return value is a pointer to src)
> + */
> +
> +static inline char *stpcpy(char __restrict *dest, const char __restrict *src)
> +{
> + while ((*dest++ = *src++) != '\0') {
> + ; /* nothing */
> + }
> + return --dest;
> +}
> +#endif
>
>

--
Thanks,
~Nick Desaulniers