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

[Joe Perches <joe@perches.com>]

On Sat, 2020-08-15 at 14:28 -0700, Nick Desaulniers wrote:
> 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?

My guess is yes, wait until gcc, if ever, needs it.