From patchwork Fri Sep 18 20:14:07 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Sami Tolvanen X-Patchwork-Id: 11786013 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id B312F618 for ; Fri, 18 Sep 2020 20:14:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8CCAE235FD for ; Fri, 18 Sep 2020 20:14:58 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="VAY1d+BK" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726306AbgIRUO6 (ORCPT ); Fri, 18 Sep 2020 16:14:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52700 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726309AbgIRUOm (ORCPT ); Fri, 18 Sep 2020 16:14:42 -0400 Received: from mail-yb1-xb49.google.com (mail-yb1-xb49.google.com [IPv6:2607:f8b0:4864:20::b49]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E57F0C0613CF for ; Fri, 18 Sep 2020 13:14:41 -0700 (PDT) Received: by mail-yb1-xb49.google.com with SMTP id l67so6661678ybb.7 for ; Fri, 18 Sep 2020 13:14:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=sender:date:in-reply-to:message-id:mime-version:references:subject :from:to:cc; bh=skSGpFbYMPCYyMt3xD2UXrEbXnhCXJcxNWr+Xyy+zNQ=; b=VAY1d+BKJBvFH1+plE6Ne6NojDosoRo9j26NV8dlVGXM//ycIlCDZru6h0GwYf6PGK oVDTueoqNoahOkpRrBlNbroVu/FB1veyRVPAgf/45GgzWqf6Xsq0Y+GXu2hi+twUmF4M UWlrMiOG7qzpx/rugvq9ImdavvJF+euLmvDRVYx6xTfAs6YbiMljlvnEF0AxzVWwqwE0 Wlnm4GTIQPvLwoJ7Q6dUKY/MHQfiWIXB6ndcTD3C4Ky4vDcOB9wdWqOfH1nmSHu8+xDC GmhmT101k2ut9EE13MQG/mtNTs23zRSYi6edJX+ofgh4sM6lA1iJVwk3pkcZdBV+lO+m xpbA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=skSGpFbYMPCYyMt3xD2UXrEbXnhCXJcxNWr+Xyy+zNQ=; b=S+CH67CUjfC5MIN9s89QnzD7yTk058GypK/4LtgAi8dFRUidQ2FgJnP3YYC7CYtTa+ WFnh+0JUguMThbjTxSPBI6qwH+YVIDwMQ2LQ8JLnIZXQsb9EUrMQIj/9QeA1WeSVD3gP iiArevbpWgUzesmsqKrETotz0hj/lqdm/CAbZiMZWQCwfG3O2sm0xdNrzMcLD42KKwOl GPDc4IC/Z0tGO8VBbICoM578nx6RDSZZNTbR4VNUkK/56dncrRVAfHuPB4k7fR7XAkTc tBU3nB23BOv0ksNtp1/n7h5+rs0L6477PUAuYM2pB4yZ9HitKtpwtew0ZSJqvahI37SE UaMQ== X-Gm-Message-State: AOAM533Mpt2rCIuw3PrtPSAcKNfHBafjVYMNLXFDvU+d1/TkXV35wdAC WOc2gE6elw3ITcMVIKt5ABGFiMh1ltFnYRa7KGA= X-Google-Smtp-Source: ABdhPJzcj7wCxo8E12yukgL7sAuybdWWQ1AJ64l97WPGZG7w7s7z827ZYArDZS6m8vlRS9reaOndS152UiN0Two7+9I= Sender: "samitolvanen via sendgmr" X-Received: from samitolvanen1.mtv.corp.google.com ([2620:15c:201:2:f693:9fff:fef4:1b6d]) (user=samitolvanen job=sendgmr) by 2002:a25:6f89:: with SMTP id k131mr40244617ybc.214.1600460081050; Fri, 18 Sep 2020 13:14:41 -0700 (PDT) Date: Fri, 18 Sep 2020 13:14:07 -0700 In-Reply-To: <20200918201436.2932360-1-samitolvanen@google.com> Message-Id: <20200918201436.2932360-2-samitolvanen@google.com> Mime-Version: 1.0 References: <20200918201436.2932360-1-samitolvanen@google.com> X-Mailer: git-send-email 2.28.0.681.g6f77f65b4e-goog Subject: [PATCH v3 01/30] lib/string.c: implement stpcpy From: Sami Tolvanen To: Masahiro Yamada , Will Deacon Cc: Peter Zijlstra , Steven Rostedt , Greg Kroah-Hartman , "Paul E. McKenney" , Kees Cook , Nick Desaulniers , clang-built-linux@googlegroups.com, kernel-hardening@lists.openwall.com, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, x86@kernel.org, Sami Tolvanen Precedence: bulk List-ID: X-Mailing-List: linux-kbuild@vger.kernel.org From: Nick Desaulniers 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 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 --- lib/string.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/lib/string.c b/lib/string.c index 6012c385fb31..4288e0158d47 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 + * 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