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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id b15-20020a170903228f00b001784a45511asm8184760plh.79.2022.09.16.07.38.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 16 Sep 2022 07:38:38 -0700 (PDT) Date: Fri, 16 Sep 2022 07:38:37 -0700 From: Kees Cook To: Josh Triplett Cc: Eric Biederman , Alexander Viro , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] fs/exec.c: Add fast path for ENOENT on PATH search before allocating mm Message-ID: <202209160727.5FC78B735@keescook> References: <5c7333ea4bec2fad1b47a8fa2db7c31e4ffc4f14.1663334978.git.josh@joshtriplett.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5c7333ea4bec2fad1b47a8fa2db7c31e4ffc4f14.1663334978.git.josh@joshtriplett.org> ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1663339120; a=rsa-sha256; cv=none; b=r2JJWYH024XtcDPGOA74+BSedOcAsd0EP8L4hDnhYmd4KjEu/gdmJ+IEmdEaDRiv7QKDzW Lqnn8MKOhwqCX1V4utb0M3Tk/kzKgZ+iomBiarK+3YlO+T+4kjXWljgg8lTh55jWo0zCD4 ewt5bI1arzhepRwzVNRLM6LfuFUlknk= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=chromium.org header.s=google header.b=LHdnib7N; dmarc=pass (policy=none) header.from=chromium.org; spf=pass (imf01.hostedemail.com: domain of keescook@chromium.org designates 209.85.215.177 as permitted sender) smtp.mailfrom=keescook@chromium.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1663339120; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=cKDTNYSujMHhHC8oskKxTsGs14KRpTgwA0hXaRQXqJE=; b=xv+JX2cIvG4xjoReWYC5C6HbVlJgEWFFDcPBRAxOoUbkmy73mbfzXa4XPQJIP8T2TH+Z7U pWeauSG7fR5bEePXyRGf5hCO78M2Exv3uAwyO3qKowbNBzxf/2dMDXwM98y8WjJa0TevvC Thy8d6+QA9B6aMKVYtDOQh6iudE82fM= X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: A04A4400AA Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=chromium.org header.s=google header.b=LHdnib7N; dmarc=pass (policy=none) header.from=chromium.org; spf=pass (imf01.hostedemail.com: domain of keescook@chromium.org designates 209.85.215.177 as permitted sender) smtp.mailfrom=keescook@chromium.org X-Stat-Signature: 983wmuqchegy39uq9odfs6d7r8qrk8cj X-HE-Tag: 1663339120-633860 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Sep 16, 2022 at 02:41:30PM +0100, Josh Triplett wrote: > Currently, execve allocates an mm and parses argv and envp before > checking if the path exists. However, the common case of a $PATH search > may have several failed calls to exec before a single success. Do a > filename lookup for the purposes of returning ENOENT before doing more > expensive operations. At first I didn't understand how you were seeing this, since I'm so used to watching shell scripts under tracing, which correctly use stat(): $ strace bash -c foo stat("/home/keescook/bin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory) stat("/usr/local/sbin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory) stat("/usr/local/bin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory) stat("/usr/sbin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory) stat("/usr/bin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory) stat("/sbin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory) stat("/bin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory) But I see, yes, glibc tries to actually call execve(), which, as you say, is extremely heavy: $ strace ./execvpe ... execve("/home/kees/bin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory) execve("/usr/local/sbin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory) execve("/usr/local/bin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory) execve("/usr/sbin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory) execve("/usr/bin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory) execve("/sbin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory) execve("/bin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory) This really seems much more like a glibc bug. The shell does it correctly... -Kees -- Kees Cook