[PATCH v3] setup: don't die if realpath(3) fails on getcwd(3)

From: Kevin Locke <kevin@kevinlocke.name>
Date: May 24, 2022, 14:51 UTC
Message-ID: <68c66aef7ca4dba53faec9e6d2f3b70fe58ac33e.1653403877.git.kevin@kevinlocke.name>
In-Reply-To: <7c064f43ed426c9e3b54e1ae5313d6b9332a47cb.1653141169.git.kevin@kevinlocke.name>

Prior to Git 2.35.0, git could be run from an inaccessible working directory so long as the git repository specified by options and/or environment variables was accessible. For example:

    git init repo
    mkdir -p a/b
    cd a/b
    chmod u-x ..
    git -C "${PWD%/a/b}/repo" status

If this example seems a bit contrived, consider running with the repository owner as a substitute UID (e.g. with runuser(1) or sudo(8)) without ensuring the working directory is accessible by that user.

The code added by e6f8861bd4 ("setup: introduce startup_info->original_cwd") to preserve the working directory attempts to normalize the path using strbuf_realpath(). If that fails, as in the case above, it is treated as a fatal error.

This commit treats strbuf_realpath() errors as non-fatal. If an error occurs, setup_original_cwd() will continue without applying removal prevention for cwd, resulting in the pre-2.35.0 behavior. The risk should be minimal, since git will not operate on a repository with inaccessible ancestors, this behavior is only known to occur when cwd is a descendant of the repository, an ancestor of cwd is inaccessible, and no ancestors of the repository are inaccessible.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
---

Notes:
Changes since v2:
 * Use trace2_data_string(), rather than trace_printf(), to report
   realpath failure.

Changes since v1:
 * Set startup_info->original_cwd = NULL when strbuf_realpath() fails,
   rather than setting it to the un-normalized path.
 * Add a trace message when realpath fails to aid debugging.
 * Remove potential realpath failure cause from comment before it.
 * Improve format for reference to e6f8861bd4 in commit message.
 * Clarify when the pre-2.35.0 behavior may occur as a result of this
   commit in the commit message.
 * Remove 'Fixes:' tag from commit message.

 setup.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/setup.c b/setup.c
index a7b36f3ffbf..38bd55cbac1 100644
--- a/setup.c
+++ b/setup.c
@@ -458,11 +458,19 @@ static void setup_original_cwd(void)
 	 *     not startup_info->original_cwd.
 	 */
 
-	/* Normalize the directory */
-	strbuf_realpath(&tmp, tmp_original_cwd, 1);
-	free((char*)tmp_original_cwd);
-	tmp_original_cwd = NULL;
-	startup_info->original_cwd = strbuf_detach(&tmp, NULL);
+	/* Try to normalize the directory. */
+	if (strbuf_realpath(&tmp, tmp_original_cwd, 0)) {
+		free((char*)tmp_original_cwd);
+		tmp_original_cwd = NULL;
+		startup_info->original_cwd = strbuf_detach(&tmp, NULL);
+	} else {
+		trace2_data_string("setup", the_repository,
+				   "realpath-path", tmp_original_cwd);
+		trace2_data_string("setup", the_repository,
+				   "realpath-failure", strerror(errno));
+		tmp_original_cwd = NULL;
+		goto no_prevention_needed;
+	}
 
 	/*
 	 * Get our worktree; we only protect the current working directory

-- 
2.35.1