From: Emil Velikov <emil.l.velikov@gmail.com>
Date: Wed, 7 May 2025 11:31:45 +0000 (+0100)
Subject: shared: rework fd_lookup_path() helper
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=9ad9a8d6b2f55ce400e77856e0429703df916ca2;p=thirdparty%2Fkmod.git

shared: rework fd_lookup_path() helper

Reduce the stack and heap memory usage of the helper by using a caller
provided buffer. This allows us to avoid copying the path, yet again,
within the caller.

Aside: readlink(3) returns -1 on all errors, it won't return the
prospective size if the output buffer is too small.

v2:
 - swap fd_lookup_path() path/pathlen argument order

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/345
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
---

diff --git a/shared/tmpfile-util.c b/shared/tmpfile-util.c
index 1548d2d0..709e4840 100644
--- a/shared/tmpfile-util.c
+++ b/shared/tmpfile-util.c
@@ -20,21 +20,21 @@
 
 FILE *tmpfile_openat(int dirfd, mode_t mode, struct tmpfile *file)
 {
-	const char *tmpname_tmpl = "tmpfileXXXXXX";
+	const char *tmpname_tmpl = "/tmpfileXXXXXX";
 	const char *tmpname;
 	char tmpfile_path[PATH_MAX];
 	int fd, n;
-	_cleanup_free_ char *targetdir;
 	FILE *fp;
 
-	targetdir = fd_lookup_path(dirfd);
-	if (targetdir == NULL)
+	n = fd_lookup_path(dirfd, tmpfile_path, sizeof(tmpfile_path));
+	if (n < 0)
 		goto create_fail;
 
-	n = snprintf(tmpfile_path, sizeof(tmpfile_path), "%s/%s", targetdir, tmpname_tmpl);
-	if (n < 0 || n >= (int)sizeof(tmpfile_path))
+	if (n + strlen(tmpname_tmpl) + 1 > sizeof(tmpfile_path))
 		goto create_fail;
 
+	memcpy(tmpfile_path + n, tmpname_tmpl, strlen(tmpname_tmpl) + 1);
+
 	fd = mkstemp(tmpfile_path);
 	if (fd < 0)
 		goto create_fail;
diff --git a/shared/util.c b/shared/util.c
index 2c6cfa96..fcb22350 100644
--- a/shared/util.c
+++ b/shared/util.c
@@ -476,10 +476,9 @@ int mkdir_parents(const char *path, mode_t mode)
 	return mkdir_p(path, end - path, mode);
 }
 
-char *fd_lookup_path(int fd)
+int fd_lookup_path(int fd, char *path, size_t pathlen)
 {
 	char proc_path[PATH_MAX];
-	char fd_path[PATH_MAX];
 	ssize_t len;
 
 	snprintf(proc_path, sizeof(proc_path), "/proc/self/fd/%d", fd);
@@ -488,13 +487,14 @@ char *fd_lookup_path(int fd)
 	 * We are using mkstemp to create a temporary file. We need to read the link since
 	 * the mkstemp creates file with an absolute path
 	 */
-	len = readlink(proc_path, fd_path, sizeof(fd_path) - 1);
-	if (len < 0 || len >= (ssize_t)sizeof(fd_path))
-		return NULL;
+	len = readlink(proc_path, path, pathlen - 1);
+	if (len < 0)
+		return (int)len;
 
-	fd_path[len] = '\0';
+	/* readlink(3), may null-terminate but should not be relied upon */
+	path[len] = '\0';
 
-	return strdup(fd_path);
+	return len;
 }
 
 static unsigned long long ts_usec(const struct timespec *ts)
diff --git a/shared/util.h b/shared/util.h
index c36d39e5..7363f426 100644
--- a/shared/util.h
+++ b/shared/util.h
@@ -54,7 +54,7 @@ static inline _must_check_ _nonnull_all_ bool path_is_absolute(const char *p)
 int mkdir_p(const char *path, int len, mode_t mode);
 int mkdir_parents(const char *path, mode_t mode);
 unsigned long long stat_mstamp(const struct stat *st);
-char *fd_lookup_path(int fd);
+_nonnull_all_ int fd_lookup_path(int fd, char *path, size_t pathlen);
 
 /* time-related functions
  * ************************************************************************ */