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

["Dávid Bolvanský" <david.bolvansky@gmail.com>]

-fno-builtin-stpcpy can be used to disable stpcpy but Nick at llvm bugzilla wrote that these flags are broken with LTO.


> Dňa 15. 8. 2020 o 23:24 užívateľ Joe Perches <joe@perches.com> napísal:
> 
> 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.
> 
> 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
> 
>