From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-22.4 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 77A4AC43461 for ; Mon, 14 Sep 2020 16:18:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2DA8121973 for ; Mon, 14 Sep 2020 16:18:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="nDk1vkg2" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726457AbgINQSi (ORCPT ); Mon, 14 Sep 2020 12:18:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54264 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726137AbgINQOR (ORCPT ); Mon, 14 Sep 2020 12:14:17 -0400 Received: from mail-pj1-x1044.google.com (mail-pj1-x1044.google.com [IPv6:2607:f8b0:4864:20::1044]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4D246C06174A for ; Mon, 14 Sep 2020 09:14:15 -0700 (PDT) Received: by mail-pj1-x1044.google.com with SMTP id u3so63215pjr.3 for ; Mon, 14 Sep 2020 09:14:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=JtLqKIqxK7wc9FLa1pAfxW1qizIDuJ12ED71Joxln3w=; b=nDk1vkg2ksSv79eO346Zawe7Qui+QvmmSjYNyW1T/U9xLLgjgvLqkeGnSjNPc8zV0h Ypjxt9hZf1dSy1XAR8u8tgFTmg/NasgxxMhGwoVOBjfIuAIPVO9Tb8x9KIdgG2sVC1wH w85WYgNRbx0GMMh16uTAMpzrb95cMXRFkdRkmgN4jrrs+x9MeS8hTiMYta/yvoEEUkN7 nO4tXsUT/gyvqQ+Aj+K7kRFT++aF1qTY95WfFWWJVXMKjjLXrryHziLOxFM5p/6FBr53 JjROWiDmpPPNA6s310ap1GQ1Qd9E3E08got/meIkfC/5h6TunXEe4+pZo8vNWd1GHRnu muUw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=JtLqKIqxK7wc9FLa1pAfxW1qizIDuJ12ED71Joxln3w=; b=bD9aCsy0xpHIt29OVmWZw8WoyO9SKFaIJEwwC6yDYQyhn9KKDUG6uk9DrOSmd1pnSP mvjn+hJWAYdnBZA6PdApHZElp0eiQOgcIVf5BcHkyL2njvDzid6UUAg1YBjLufnAxYGe bPWY9XQ//T7QasktM6jnwpCQPcxHoRzOzExJHZlAFUqa0h93tzyi2/s1Hax6odcw4p1u //wfnQJN3DcFyt82AuuFAPhIHtLBfAS0EQux4dsQXw1C6Y+wdR/0j/8dXBpQ4hAeJz8V MpoWgW5dThx3z1nJcncmVc0Xu0K1Ly4cppun5hlZIF9TIs+diLN3W8ntOgAqjF2MBWhl mQyA== X-Gm-Message-State: AOAM533WJyYA1hGHGCPdzcVcYdQOKyFp084E5iRW/haQIxeML3r32mm0 6oJzyrGe3MOpqRtBkGRdJwWdoW1GrxjRezMtp0ivqQ== X-Google-Smtp-Source: ABdhPJyX96XmrrBKOtPkCawoH6WCN1fmRQ+2sSY3qHhmnO5EFJlpdvBHzl3xnDKV42eDwJUmuWCB/4KmAxv3gmp9qtE= X-Received: by 2002:a17:90b:889:: with SMTP id bj9mr101969pjb.101.1600100054525; Mon, 14 Sep 2020 09:14:14 -0700 (PDT) MIME-Version: 1.0 References: <20200914160958.889694-1-ndesaulniers@google.com> In-Reply-To: <20200914160958.889694-1-ndesaulniers@google.com> From: Nick Desaulniers Date: Mon, 14 Sep 2020 09:14:03 -0700 Message-ID: Subject: Re: [PATCH v4] lib/string.c: implement stpcpy To: Andrew Morton Cc: Kees Cook , Arvind Sankar , Masahiro Yamada , LKML , clang-built-linux , Sami Tolvanen , Andy Lavr , Joe Perches , Rasmus Villemoes , Nathan Chancellor , "# 3.4.x" Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 14, 2020 at 9:10 AM 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. > > Reported-by: Sami Tolvanen > Suggested-by: Andy Lavr > Suggested-by: Arvind Sankar > Suggested-by: Joe Perches > Suggested-by: Kees Cook > Suggested-by: Masahiro Yamada > Suggested-by: Rasmus Villemoes > Signed-off-by: Nick Desaulniers > Tested-by: Nathan Chancellor > Reviewed-by: Kees Cook > 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 > --- > Changes V4: > * Roll up Kees' comment fixup from > https://lore.kernel.org/lkml/202009060302.4574D8D0E0@keescook/#t. > * Keep Nathan's tested by tag. > * Add Kees' reviewed by tag from > https://lore.kernel.org/lkml/202009031446.3865FE82B@keescook/. > > 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..b6b8847218b5 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 a pointer > + * to the new %NUL-terminating character in @dest. (For strcpy, the return > + * value is a pointer to the start of @dest. This interface is considered And, I forgot the stupid close parens...I will resend a v5... > + * 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 > -- > 2.28.0.618.gf4bc123cb7-goog > -- Thanks, ~Nick Desaulniers