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

[Sedat Dilek]

On Sun, Aug 16, 2020 at 2:19 AM 'Fangrui Song' via Clang Built Linux
<clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> On 2020-08-15, 'Nick Desaulniers' via Clang Built Linux wrote:
> >On Sat, Aug 15, 2020 at 2:31 PM Joe Perches <joe@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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@xxxxxxxxxxxx> 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!
>
> Adding a definition without a declaration for stpcpy looks good.
> Clang LTO will work.
>
> (If the kernel does not want to provide these routines,
> is http://git.kernel.org/linus/6edfba1b33c701108717f4e036320fc39abe1912
> probably wrong? (why remove -ffreestanding from the main Makefile) )
>

We had some many issues in arch/x86 where *FLAGS were removed or used
differently and had to re-add them :-(.

So if -ffreestanding is used in arch/x86 and was! used in top-level
Makefile - it makes sense to re-add it back?
( I cannot speak for archs other than x86. )

- Sedat -