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=-17.6 required=3.0 tests=BAYES_00, DKIM_ADSP_CUSTOM_MED,DKIM_INVALID,DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham 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 A3814C433DF for ; Tue, 25 Aug 2020 13:59:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 715A120738 for ; Tue, 25 Aug 2020 13:59:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=google.com header.i=@google.com header.b="kChPl54d" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726347AbgHYN7o (ORCPT ); Tue, 25 Aug 2020 09:59:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57086 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726222AbgHYN64 (ORCPT ); Tue, 25 Aug 2020 09:58:56 -0400 Received: from mail-pf1-x449.google.com (mail-pf1-x449.google.com [IPv6:2607:f8b0:4864:20::449]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5098FC061574 for ; Tue, 25 Aug 2020 06:58:56 -0700 (PDT) Received: by mail-pf1-x449.google.com with SMTP id g6so8705230pfi.1 for ; Tue, 25 Aug 2020 06:58:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=sender:date:message-id:mime-version:subject:from:to:cc; bh=Us+zKhWOc0nF2Stw/sjqwIYhi1P3cVkrUO+fKBNUwt4=; b=kChPl54dn9ymEmCalokFzgP0MuYBhYIUai4sfhGRDTh7cmSDYrE9/f2w2wlzy79P1U odUElDEA6bydiFE9ZvHxHgUU1DeCZfPfixc+QnXh6sqh3mCROa7cWR80GWycaCUfb4IF UBKXlwIVApSR31H2zG7ZxWF6WxQTdY9nQlI9tLzJ7tq7xXyPlCbj3ruaO4VmIFc2t8FP u5EoeZ4mdLG9kegCOTiKILj3FbecfJPwMC/TsHIbN2mbiBeuIHnoLEgj58tO1/iziyT3 ZHyFWI96H8MuXgxec7wRTEFCpZDQz1eBjJcUj1sj21HyfKjhnsCK5NAQONRKGgGjyfaK ImQw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:message-id:mime-version:subject:from :to:cc; bh=Us+zKhWOc0nF2Stw/sjqwIYhi1P3cVkrUO+fKBNUwt4=; b=sJa1SZXttWznqzE51khzqJIamQZ6LLvCKfCrYk4ytrIMWuBbYJB0cprSBNoHOZXgG+ P1k2QSG39plouO4N+mB6JE87snf8Ao3ZPD/UF3kPEBYRzovlzL/T6j59Oz4rSWZ2InRe zvfkbBoBIUzHaGr//GWT6EDQmBNxVl1FOc4IpKvKe6SeBdCQUNz+6kk+lx9mxS2Tj+GS uofHrr5TX4bcVfQYlvnBD6IOz2y0LEi7/WHH3qHzJ9O0rzzThdroUHGYQFdeYOl9pf80 q74/ohJMoDnZZWjMcXPc+KB5ozReybkQzdQRbVmfKmcmgtFr/jxq5QOSOzam4+jgjCNq J/jQ== X-Gm-Message-State: AOAM532ZS6a164pAE1WrBKqcUErAvgG1QYvKdmlzeh0WY5bl0Zm9LXQH N+GL0i9q1ygc/02aMoCz72HHpFeRtwAegpkvDlE= X-Google-Smtp-Source: ABdhPJyZgdjJcZNgecDr3UiTzEn5aQy+0LYKn3Ej4qIsX1xwC3PrG7l2IDzKFUt1pE1eWuX67nXFBj1sKlB/v8+4iqU= X-Received: from ndesaulniers1.mtv.corp.google.com ([2620:15c:211:202:f693:9fff:fef4:4d25]) (user=ndesaulniers job=sendgmr) by 2002:a62:7c97:: with SMTP id x145mr8231879pfc.155.1598363935586; Tue, 25 Aug 2020 06:58:55 -0700 (PDT) Date: Tue, 25 Aug 2020 06:58:36 -0700 Message-Id: <20200825135838.2938771-1-ndesaulniers@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.28.0.297.g1956fa8f8d-goog Subject: [PATCH v3] lib/string.c: implement stpcpy From: Nick Desaulniers To: Masahiro Yamada Cc: clang-built-linux@googlegroups.com, Nick Desaulniers , 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 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 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 --- 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