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

Fangrui Song
2020-08-16T00:19:29Z

On 2020-08-15, 'Nick Desaulniers' via Clang Built Linux wrote:
>On Sat, Aug 15, 2020 at 2:31 PM Joe Perches <joe@perches.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@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.
>
>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) )