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

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Date:   Sat, 15 Aug 2020 15:17:39 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Daniel Axtens <dja@...ens.net>, Ingo Molnar <mingo@...nel.org>,
        Yury Norov <yury.norov@...il.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v2] lib/string.c: implement stpcpy

On Sat, Aug 15, 2020 at 2:31 PM Joe Perches <joe@...ches.com> wrote:
>
> On Sat, 2020-08-15 at 14:28 -0700, Nick Desaulniers wrote:
> > On Sat, Aug 15, 2020 at 2:24 PM Joe Perches <joe@...ches.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@...omium.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.

The suggestion to use static inline doesn't even make sense. The
compiler is lowering calls to other library routines; `stpcpy` isn't
being explicitly called.  Even if it was, not sure we want it being
inlined.  No symbol definition will be emitted; problem not solved.
And I refuse to add any more code using `extern inline`.  Putting the
definition in lib/string.c is the most straightforward and avoids
revisiting this issue in the future for other toolchains.  I'll limit
access by removing the declaration, and adding a comment to avoid its
use.  But if you're going to use a gnu target triple without using
-ffreestanding because you *want* libcall optimizations, then you have
to provide symbols for all possible library routines!
-- 
Thanks,
~Nick Desaulniers