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=-14.6 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 F0247C433E1 for ; Tue, 25 Aug 2020 18:51:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C127B20738 for ; Tue, 25 Aug 2020 18:51:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="ghGTgj4N" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726820AbgHYSv2 (ORCPT ); Tue, 25 Aug 2020 14:51:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46170 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726066AbgHYSvZ (ORCPT ); Tue, 25 Aug 2020 14:51:25 -0400 Received: from mail-qk1-x741.google.com (mail-qk1-x741.google.com [IPv6:2607:f8b0:4864:20::741]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1CE01C061574; Tue, 25 Aug 2020 11:51:25 -0700 (PDT) Received: by mail-qk1-x741.google.com with SMTP id i20so12030244qkk.8; Tue, 25 Aug 2020 11:51:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=6x0iz9JUsWXmCzILUsCg/89oPKkQvuUuZ5mSr/G7CCc=; b=ghGTgj4Nu47q9B5RTknR8QOLlVRGwQKsRF4h60+v59AdwRLeWtRni/4llQCoxt4n0m v0xZ+78rKDEx4vhho4cTxEFc2uzldInEdxoLfx+wwfs3mN5OIVCub+TKHjdDWSLdxZIE cNKvtgQvA7as/hKzdTDPwgEJAUeJBPYVN/7lWLTlAoz+RxorL+Ws3YQh4dh5ALeMBoRx yr5hzqdCPYjQ61P4MVr3e4fYr8sdtxPv7WxV/ATS+9joExTLkkMj0AL8xw5dYs1nv86d vAlEqZUBbqDnZLD8uXPx9kaFB6Lk9yYKW53H2GbGT2vcy7OWXSY7JOUF5K7MuvXE/f+m MqNQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=6x0iz9JUsWXmCzILUsCg/89oPKkQvuUuZ5mSr/G7CCc=; b=qSj5WfCaOO0M0Wr5oYbcB011lt2jZHq4m1OZ5n+MifytGadhmbRqWD/zHCblSckCgg wht7onq8B/csrpnXOA+8M5TpZX/hIZ87e39qVwYb97V/ubN5BUpkDtXTMOw2jY5duBJ8 xGt/gw0q8xGwccWcLBqr9asZVmJ2yxg5hQ4A37VxUph/ksnDwCEEwqHVUDmDeaFsadQ6 oqlR7qEQgSW8PlzsAhqXqmOJnTniIAfnDv2eL8ospFuXFVFvGs/729VA51MhlXVNooE1 +2ZQh6A4vrAafrz7HS2BXov9LvXUhSFL/arvc6lhoXTH+6euq6AxYwiQSbh7n78uWeDR Jp0w== X-Gm-Message-State: AOAM5329Cn7bTT4OGt0uWWUuqpvC6n7EHikvnpjJ8Jza4yr+OUA7A+Vk IFw9SZtG41nmc160IvicuQU= X-Google-Smtp-Source: ABdhPJyzaWhn7iXWu/e6eMxG4wr1Xi1OEWmXBPDTIUpJN/oWNMEdX1hHR+VF3B1LC8+FgGykLM7v8w== X-Received: by 2002:ae9:e507:: with SMTP id w7mr10448078qkf.264.1598381484105; Tue, 25 Aug 2020 11:51:24 -0700 (PDT) Received: from ubuntu-n2-xlarge-x86 ([2604:1380:45d1:2600::1]) by smtp.gmail.com with ESMTPSA id o72sm12491217qka.113.2020.08.25.11.51.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Aug 2020 11:51:23 -0700 (PDT) Date: Tue, 25 Aug 2020 11:51:21 -0700 From: Nathan Chancellor To: Nick Desaulniers , h@ubuntu-n2-xlarge-x86 Cc: Masahiro Yamada , clang-built-linux@googlegroups.com, stable@vger.kernel.org, Andy Lavr , Arvind Sankar , Joe Perches , Rasmus Villemoes , Sami Tolvanen , Andrew Morton , Kees Cook , Andy Shevchenko , Alexandru Ardelean , Yury Norov , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] lib/string.c: implement stpcpy Message-ID: <20200825185121.GA602430@ubuntu-n2-xlarge-x86> References: <20200825135838.2938771-1-ndesaulniers@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200825135838.2938771-1-ndesaulniers@google.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > Suggested-by: Arvind Sankar > Suggested-by: Joe Perches > Suggested-by: Masahiro Yamada > Suggested-by: Rasmus Villemoes > Reported-by: Sami Tolvanen > Signed-off-by: Nick Desaulniers Tested-by: Nathan Chancellor > --- > 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 >