Re: [PATCH v3] lib/string.c: implement stpcpy

[Nathan Chancellor <natechancellor@gmail.com>]

On Tue, Aug 25, 2020 at 06:58:36AM -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.  `stpcpy` is just like `strcpy` except it
> returns the pointer to the new tail of `dest`.  This optimization was
> introduced into clang-12.
> 
> Implement this so that we don't observe linkage failures due to missing
> symbol definitions for `stpcpy`.
> 
> Similar to last year's fire drill with:
> commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
> 
> The kernel is somewhere between a "freestanding" environment (no full libc)
> and "hosted" environment (many symbols from libc exist with the same
> type, function signature, and semantics).
> 
> As H. Peter Anvin notes, there's not really a great way to inform the
> compiler that you're targeting a freestanding environment but would like
> to opt-in to some libcall optimizations (see pr/47280 below), rather than
> opt-out.
> 
> Arvind notes, -fno-builtin-* behaves slightly differently between GCC
> and Clang, and Clang is missing many __builtin_* definitions, which I
> consider a bug in Clang and am working on fixing.
> 
> Masahiro summarizes the subtle distinction between compilers justly:
>   To prevent transformation from foo() into bar(), there are two ways in
>   Clang to do that; -fno-builtin-foo, and -fno-builtin-bar.  There is
>   only one in GCC; -fno-buitin-foo.
> 
> (Any difference in that behavior in Clang is likely a bug from a missing
> __builtin_* definition.)
> 
> Masahiro also notes:
>   We want to disable optimization from foo() to bar(),
>   but we may still benefit from the optimization from
>   foo() into something else. If GCC implements the same transform, we
>   would run into a problem because it is not -fno-builtin-bar, but
>   -fno-builtin-foo that disables that optimization.
> 
>   In this regard, -fno-builtin-foo would be more future-proof than
>   -fno-built-bar, but -fno-builtin-foo is still potentially overkill. We
>   may want to prevent calls from foo() being optimized into calls to
>   bar(), but we still may want other optimization on calls to foo().
> 
> It seems that compilers today don't quite provide the fine grain control
> over which libcall optimizations pseudo-freestanding environments would
> prefer.
> 
> Finally, Kees notes that this interface is unsafe, so we should not
> encourage its use.  As such, I've removed the declaration from any
> header, but it still needs to be exported to avoid linkage errors in
> modules.
> 
> Cc: stable@vger.kernel.org
> Link: https://bugs.llvm.org/show_bug.cgi?id=47162
> Link: https://bugs.llvm.org/show_bug.cgi?id=47280
> Link: https://github.com/ClangBuiltLinux/linux/issues/1126
> Link: https://man7.org/linux/man-pages/man3/stpcpy.3.html
> Link: https://pubs.opengroup.org/onlinepubs/9699919799/functions/stpcpy.html
> Link: https://reviews.llvm.org/D85963
> Suggested-by: Andy Lavr <andy.lavr@gmail.com>
> Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
> Suggested-by: Joe Perches <joe@perches.com>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Reported-by: Sami Tolvanen <samitolvanen@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Tested-by: Nathan Chancellor <natechancellor@gmail.com>

> ---
> Changes V3:
> * Drop Sami's Tested by tag; newer patch.
> * Add EXPORT_SYMBOL as per Andy.
> * Rewrite commit message, rewrote part of what Masahiro said to be
>   generic in terms of foo() and bar().
> * Prefer %NUL-terminated to NULL terminated. NUL is the ASCII character
>   '\0', as per Arvind and Rasmus.
> 
> Changes V2:
> * Added Sami's Tested by; though the patch changed implementation, the
>   missing symbol at link time was the problem Sami was observing.
> * Fix __restrict -> __restrict__ typo as per Joe.
> * Drop note about restrict from commit message as per Arvind.
> * Fix NULL -> NUL as per Arvind; NUL is ASCII '\0'. TIL
> * Fix off by one error as per Arvind; I had another off by one error in
>   my test program that was masking this.
> 
>  lib/string.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/lib/string.c b/lib/string.c
> index 6012c385fb31..6bd0cf0fb009 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -272,6 +272,30 @@ ssize_t strscpy_pad(char *dest, const char *src, size_t count)
>  }
>  EXPORT_SYMBOL(strscpy_pad);
>  
> +/**
> + * stpcpy - copy a string from src to dest returning a pointer to the new end
> + *          of dest, including src's %NUL-terminator. May overrun dest.
> + * @dest: pointer to end of string being copied into. Must be large enough
> + *        to receive copy.
> + * @src: pointer to the beginning of string being copied from. Must not overlap
> + *       dest.
> + *
> + * stpcpy differs from strcpy in a key way: the return value is the new
> + * %NUL-terminated character. (for strcpy, the return value is a pointer to
> + * src. This interface is considered unsafe as it doesn't perform bounds
> + * checking of the inputs. As such it's not recommended for usage. Instead,
> + * its definition is provided in case the compiler lowers other libcalls to
> + * stpcpy.
> + */
> +char *stpcpy(char *__restrict__ dest, const char *__restrict__ src);
> +char *stpcpy(char *__restrict__ dest, const char *__restrict__ src)
> +{
> +	while ((*dest++ = *src++) != '\0')
> +		/* nothing */;
> +	return --dest;
> +}
> +EXPORT_SYMBOL(stpcpy);
> +
>  #ifndef __HAVE_ARCH_STRCAT
>  /**
>   * strcat - Append one %NUL-terminated string to another
> -- 
> 2.28.0.297.g1956fa8f8d-goog
>