From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-3.6 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_DNSWL_HI,RP_MATCHES_RCVD shortcircuit=no autolearn=ham autolearn_force=no version=3.4.0 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id D9A7320286 for ; Wed, 13 Sep 2017 18:15:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751500AbdIMSPU (ORCPT ); Wed, 13 Sep 2017 14:15:20 -0400 Received: from cloud.peff.net ([104.130.231.41]:37380 "HELO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1751484AbdIMSPS (ORCPT ); Wed, 13 Sep 2017 14:15:18 -0400 Received: (qmail 26882 invoked by uid 109); 13 Sep 2017 18:15:18 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with SMTP; Wed, 13 Sep 2017 18:15:18 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 18304 invoked by uid 111); 13 Sep 2017 18:15:53 -0000 Received: from Unknown (HELO sigill.intra.peff.net) (10.0.1.3) by peff.net (qpsmtpd/0.94) with SMTP; Wed, 13 Sep 2017 14:15:53 -0400 Authentication-Results: peff.net; auth=none Received: by sigill.intra.peff.net (sSMTP sendmail emulation); Wed, 13 Sep 2017 14:15:16 -0400 Date: Wed, 13 Sep 2017 14:15:16 -0400 From: Jeff King To: demerphq Cc: Git , =?utf-8?B?w4Z2YXIgQXJuZmrDtnLDsA==?= Bjarmason Subject: [PATCH v2] config: avoid "write_in_full(fd, buf, len) < len" pattern Message-ID: <20170913181515.p7u2ouine3ysblqc@sigill.intra.peff.net> References: <20170913170807.cyx7rrpoyhaauvol@sigill.intra.peff.net> <20170913171104.yu7ags4aq2zdwz6r@sigill.intra.peff.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20170913171104.yu7ags4aq2zdwz6r@sigill.intra.peff.net> Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Wed, Sep 13, 2017 at 01:11:04PM -0400, Jeff King wrote: > I scoured the code base for cases of this, but it turns out > that these two in git_config_set_multivar_in_file_gently() > are the only ones. This case is actually quite interesting: > we don't have a size_t, but rather use the subtraction of > two pointers. Which you might think would be a signed > ptrdiff_t, but clearly both gcc and clang treat it as > unsigned (possibly because the conditional just above > guarantees that the result is greater than zero). Sorry, this is totally wrong. As Jonathan pointed out (and as I was puzzled over here), the result of a pointer subtraction _is_ a signed type, and works fine. What I missed is that copy_begin and copy_end here are actually size_t variables, not the pointers. Sorry for the confusion, and here's an updated version of the patch with this paragraph amended (the patch itself is identical): -- >8 -- Subject: [PATCH] config: avoid "write_in_full(fd, buf, len) < len" pattern The return type of write_in_full() is a signed ssize_t, because we may return "-1" on failure (even if we succeeded in writing some bytes). But "len" itself is may be an unsigned type (the function takes a size_t, but of course we may have something else in the calling function). So while it seems like: if (write_in_full(fd, buf, len) < len) die_errno("write error"); would trigger on error, it won't if "len" is unsigned. The compiler sees a signed/unsigned comparison and promotes the signed value, resulting in (size_t)-1, the highest possible size_t (or again, whatever type the caller has). This cannot possibly be smaller than "len", and so the conditional can never trigger. I scoured the code base for cases of this, but it turns out that these two in git_config_set_multivar_in_file_gently() are the only ones. Here our "len" is the difference between two size_t variables, making the result an unsigned size_t. We can fix this by just checking for a negative return value directly, as write_in_full() will never return any value except -1 or the full count. There's no addition to the test suite here, since you need to convince write() to fail in order to see the problem. The simplest reproduction recipe I came up with is to trigger ENOSPC: # make a limited-size filesystem dd if=/dev/zero of=small.disk bs=1M count=1 mke2fs small.disk mkdir mnt sudo mount -o loop small.disk mnt cd mnt sudo chown $USER:$USER . # make a config file with some content git config --file=config one.key value git config --file=config two.key value # now fill up the disk dd if=/dev/zero of=fill # and try to delete a key, which requires copying the rest # of the file to config.lock, and will fail on write() git config --file=config --unset two.key That final command should (and does after this patch) produce an error message due to the failed write, and leave the file intact. Instead, it silently ignores the failure and renames config.lock into place, leaving you with a totally empty config file! Reported-by: demerphq Signed-off-by: Jeff King --- config.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/config.c b/config.c index d0d8ce823a..eee4ac0355 100644 --- a/config.c +++ b/config.c @@ -2608,8 +2608,7 @@ int git_config_set_multivar_in_file_gently(const char *config_filename, /* write the first part of the config */ if (copy_end > copy_begin) { if (write_in_full(fd, contents + copy_begin, - copy_end - copy_begin) < - copy_end - copy_begin) + copy_end - copy_begin) < 0) goto write_err_out; if (new_line && write_str_in_full(fd, "\n") != 1) @@ -2631,8 +2630,7 @@ int git_config_set_multivar_in_file_gently(const char *config_filename, /* write the rest of the config */ if (copy_begin < contents_sz) if (write_in_full(fd, contents + copy_begin, - contents_sz - copy_begin) < - contents_sz - copy_begin) + contents_sz - copy_begin) < 0) goto write_err_out; munmap(contents, contents_sz); -- 2.14.1.874.ge7b2e05270