Save wp-config.php atomically via temporary file to prevent corruption on write failure - #68

Actionable comments posted: 3

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Inline comments:
Review comments at @src/WPConfigTransformer.php:
- Around line 397-404: Update the write-result check in the code around
file_put_contents() so it rejects both a false result and a byte count that
differs from strlen($contents). Preserve the existing temp-file cleanup and
exception behavior for either failure.
- Around line 406-410: Update the temporary-file replacement flow using
$real_path and $temp_file to verify that the original permissions can be read
and @chmod() succeeds, and preserve the original owner and group with
chown/chgrp when permitted. If permissions are unreadable or preservation fails,
use the direct-write path instead of renaming a mode-0600 temporary file.

Review comments at @tests/SaveTest.php:
- Around line 55-65: Update testSaveFallbackWhenDirectoryNotWritable to skip
when posix_geteuid is available and reports UID 0, so the fallback test only
runs when directory permissions are enforced.

---

Nitpick comments:
Review comments at @tests/SaveTest.php:
- Around line 18-30: Update SaveTest::tear_down() to remove any remaining
symlink and leftover wp_config_* temporary files before removing the test
directory, so cleanup still completes after an assertion or save failure.

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Patch Testing Report

Patch tested: #68 (head 28db235), compared with main (909f76b)

Environment

• WordPress: n/a (library test). The end-to-end check used wp config set from wp-cli/wp-cli dev-main (a1735b0)
• PHP: 8.5.9 on Linux (Alpine 3.24 container), 8.2.12 on Windows (XAMPP, ZTS x64)
• Server: n/a (command line test)
• Database: n/a
• Browser: n/a (command line test)
• OS: Linux containers on Docker 29.7.2 (WSL2 kernel 6.18; overlayfs and tmpfs), Windows 11 build 26300 (NTFS)
• Theme: n/a
• MU Plugins: None
• Plugins: None
• Tests: PHPUnit 12.5.40, wp-cli-tests 5.3.5

Steps taken

1. Ran the unit tests on main and on the PR, as root and as an unprivileged user, from a copy of the repository inside the container (so chmod behaves as on Linux).
2. Called WPConfigTransformer::update( 'constant', 'DB_NAME', ... ), which is what wp config set does, on a 444 byte wp-config.php, with the main and the PR version of the class, in the cases in the table. I recorded owner, group, mode, inode and content before and after, and checked if user www-data can still read the file.
3. Repeated the owner case with a real wp config set DB_NAME e2e_db --allow-root.
4. Simulated failures: ulimit -f 0 (the repro from wp-cli/config-command#233), a full tmpfs, a tmpfs with one free 4 KiB page, a tmpfs with no free inodes, and a kill at the rename syscall (strace -e inject=rename:signal=KILL).
5. On Windows (NTFS, PHP 8.2.12): a normal save, and a save while another process has wp-config.php open.
6. ✅ Patch is solving the problem: when the write fails (ulimit -f 0, full disk, short write), wp-config.php keeps its old content. On main it is left at 0 bytes.
7. ❌ Patch is failing: the new file gets the owner and group of the user who runs WP-CLI, so the web server can lose read access to wp-config.php. And saves that work on main now fail: a single-file bind mount, and a file that is open in another process on Windows.

Expected result

• A failed write never leaves an empty or truncated wp-config.php.
• A successful save keeps owner, group, mode and symlinks as they were, and works in every setup where the in-place write works today.

Additional Notes

Results:

Findings:

1. Owner and group change. rename() puts a new file in place, so the file gets the owner and primary group of the process. The mode is copied, but owner and group are not. With a 0640 wp-config.php (a common hardening setting), the web server then cannot read it. End to end with the real command:

   before: -rw-r----- www-data:www-data /tmp/site/wp-config.php
   $ wp config set DB_NAME e2e_db --path=/tmp/site --allow-root
   Success: Updated the constant 'DB_NAME' in the 'wp-config.php' file with the value 'e2e_db'.
   exit=0
   after:  -rw-r----- root:root /tmp/site/wp-config.php
   $ su -s /bin/sh www-data -c 'php -r "include \"/tmp/site/wp-config.php\"; echo DB_NAME, PHP_EOL;"'
   Warning: include(/tmp/site/wp-config.php): Failed to open stream: Permission denied in Command line code on line 1
   Fatal error: Uncaught Error: Undefined constant "DB_NAME" in Command line code:1
   

   On main the file stays www-data:www-data and www-data reads e2e_db. So after a successful wp config set, PHP running as www-data can no longer load wp-config.php and the site goes down. The same happens without root when the file has a shared group, for example deploy:www-data 0640: the group becomes deploy. The review comment on line 410 raised ownership in general terms; this is the concrete case.

2. Saves that work today now fail. rename() is refused in two cases I hit, and the old file is left unchanged in both:

   • A Docker single-file bind mount (-v ./wp-config.php:/var/www/html/wp-config.php): rename(/site/t,/site/wp-config.php): Resource busy.
   • Windows, when another process has wp-config.php open, even with FILE_SHARE_DELETE: rename(...): Access is denied (code: 5).

   The @ hides these messages, so the user only sees Failed to update the config file. Because the original is untouched at that point, falling back to the in-place write is safe there and keeps today's behaviour.

3. The Windows comment on line 417. A normal rename over an existing file works on Windows with PHP 8.2.12: the content was updated and the NTFS file ID changed, so the rename really ran. The failure is only the open-file case above.

4. The partial-write comment on line 404. On a real short write (tmpfs with one free page), file_put_contents() returned false with Warning: file_put_contents(): Only 4096 of 5833 bytes written, possibly out of free disk space, and the PR threw and removed the temp file. So the false === check already covers this for string contents.

5. Temp file left behind. If PHP dies between tempnam() and rename() (kill, OOM, SIGXFSZ), wp_config_<random> stays next to wp-config.php. After the chmod it has the original mode, for example 0644, and it holds the full config including DB_PASSWORD. The name is random and has no .php extension. The risk is low, but it is worth knowing. I have no change for it.

6. tempnam() fallback. When tempnam() cannot create a file in the directory, it does not return false. It creates the file in the system temp directory and raises a Notice. rename() then crosses file systems and PHP copies the file, so that save is not atomic, and the false === $temp_file branch is never reached. My diff checks that the temp file is in the same directory and otherwise writes in place.

7. Tests. As root, testSaveFallbackWhenDirectoryNotWritable passes, but it takes the atomic path, because root can write to a 0555 directory (see the "run as root" row). The root skip suggested on line 65 fixes that. Unit test results as an unprivileged user: main 95 tests OK, PR 98 tests OK. As root, the existing SetupTest::testFileNotWritable fails on both main and the PR (root can write a read-only file); that is not related to this PR.

Screenshots/Screencast with results

• None (command line test). The relevant output is quoted above.

Support Content

diff --git a/src/WPConfigTransformer.php b/src/WPConfigTransformer.php
--- a/src/WPConfigTransformer.php
+++ b/src/WPConfigTransformer.php
@@ -385,42 +385,77 @@ class WPConfigTransformer {
 		}
 
 		$real_path = realpath( $this->wp_config_path ) ?: $this->wp_config_path;
-		$dir       = dirname( $real_path );
 
-		if ( is_writable( $dir ) ) {
-			$temp_file = tempnam( $dir, 'wp_config_' );
+		if ( $this->replace_file( $real_path, $contents ) ) {
+			return true;
+		}
 
-			if ( false === $temp_file ) {
-				$temp_file = $dir . '/.' . basename( $real_path ) . '.' . uniqid( 'tmp_', true );
-			}
+		// The file could not be replaced without changing it in other ways. Write in place, as before.
+		$result = file_put_contents( $this->wp_config_path, $contents, LOCK_EX );
 
-			$result = file_put_contents( $temp_file, $contents, LOCK_EX );
+		if ( false === $result ) {
+			throw new Exception( 'Failed to update the config file.' );
+		}
 
-			if ( false === $result ) {
-				if ( file_exists( $temp_file ) ) {
-					@unlink( $temp_file );
-				}
-				throw new Exception( 'Failed to update the config file.' );
-			}
+		return true;
+	}
 
-			$perms = @fileperms( $real_path );
-			if ( false !== $perms ) {
-				// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged
-				@chmod( $temp_file, $perms & 0777 );
-			}
+	/**
+	 * Writes the contents to a temporary file next to the config file and renames it over the config file.
+	 *
+	 * Returns false without touching the config file when the new file would not get the same owner, group
+	 * and permissions, or when the rename is refused (for example a single-file bind mount, or a file that
+	 * another process has open on Windows). The caller then writes in place.
+	 *
+	 * @throws Exception If the temporary file cannot be written, for example when the disk is full.
+	 *
+	 * @param string $path     Resolved path to the config file.
+	 * @param string $contents New config contents.
+	 *
+	 * @return bool True if the config file was replaced.
+	 */
+	protected function replace_file( $path, $contents ) {
+		$dir  = dirname( $path );
+		$stat = stat( $path );
 
-			if ( ! @rename( $temp_file, $real_path ) ) {
-				if ( file_exists( $temp_file ) ) {
-					@unlink( $temp_file );
-				}
-				throw new Exception( 'Failed to update the config file.' );
-			}
-		} else {
-			$result = file_put_contents( $this->wp_config_path, $contents, LOCK_EX );
+		if ( false === $stat || ! is_writable( $dir ) ) {
+			return false;
+		}
+
+		// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged -- tempnam() notices when it falls back to the system temp directory.
+		$temp_file = @tempnam( $dir, 'wp_config_' );
 
-			if ( false === $result ) {
-				throw new Exception( 'Failed to update the config file.' );
+		// A temporary file outside the directory cannot be renamed atomically.
+		if ( false === $temp_file || realpath( dirname( $temp_file ) ) !== realpath( $dir ) ) {
+			if ( false !== $temp_file ) {
+				unlink( $temp_file );
 			}
+			return false;
+		}
+
+		if ( false === file_put_contents( $temp_file, $contents ) ) {
+			unlink( $temp_file );
+			throw new Exception( 'Failed to update the config file.' );
+		}
+
+		// phpcs:disable WordPress.PHP.NoSilencedErrors.Discouraged -- failures are checked below.
+		@chmod( $temp_file, $stat['mode'] & 0777 );
+		@chown( $temp_file, $stat['uid'] );
+		@chgrp( $temp_file, $stat['gid'] );
+		// phpcs:enable
+
+		clearstatcache( true, $temp_file );
+		$temp_stat = stat( $temp_file );
+
+		$same = false !== $temp_stat
+			&& $temp_stat['uid'] === $stat['uid']
+			&& $temp_stat['gid'] === $stat['gid']
+			&& ( $temp_stat['mode'] & 0777 ) === ( $stat['mode'] & 0777 );
+
+		// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged -- a refused rename falls back to an in-place write.
+		if ( ! $same || ! @rename( $temp_file, $path ) ) {
+			unlink( $temp_file );
+			return false;
 		}
 
 		return true;
diff --git a/tests/SaveTest.php b/tests/SaveTest.php
--- a/tests/SaveTest.php
+++ b/tests/SaveTest.php
@@ -53,6 +53,10 @@ class SaveTest extends TestCase {
 	}
 
 	public function testSaveFallbackWhenDirectoryNotWritable() {
+		if ( function_exists( 'posix_geteuid' ) && 0 === posix_geteuid() ) {
+			$this->markTestSkipped( 'root can write to a 0555 directory, so the fallback is not reached.' );
+		}
+
 		// Make directory non-writable
 		chmod( $this->test_dir, 0555 );
 

# write fails, as in config-command#233
su deploy -c "ulimit -f 0; php set.php ..."

# killed just before the rename
strace -f -e trace=rename,renameat,renameat2 -e inject=rename,renameat,renameat2:signal=KILL php set.php ...

# killed just after the truncate (main)
strace -f -P /path/wp-config.php -e trace=write -e inject=write:signal=KILL php set.php ...

# full disk, short write, no inodes
docker run --tmpfs /mnt/full:size=64k --tmpfs /mnt/ino:size=1m,nr_inodes=8 ...

# single-file bind mount
docker run -v ./wp-config.php:/site/wp-config.php ...