fd-util: Use /proc/pid/fd instead of /proc/self/fd

Currently, we mount via file descriptors using /proc/self/fd. This
works, but it means that in /proc/mounts and various other files,
the source of the mount will be listed as /proc/self/fd/xxx. For other
software that parses these files, /proc/self/fd/xxx doesn't mean anything,
or worse, it means the completely wrong thing, as it will refer to one of
their own file descriptors instead.

Let's improve the situation by using /proc/pid/fd instead. This allows
processes parsing /proc/mounts to do the right thing more often than not.
One scenario where even this doesn't work if when containers are involved,
as with the pid namespace unshared, even /proc/pid/fd will mean the wrong
thing, but it's no worse than /proc/self/fd which will always means the wrong
thing.

This also doesn't work if we mount via file descriptor and then exit, as the pid will
be gone, but it does work as long as the process that did the mount is alive, which
makes it useful for systemd-dissect --with for example if the program we run in the
image wants to parse /proc/mounts.

(cherry picked from commit 4419735)

src/basic/fd-util.h

@@ -8,6 +8,8 @@
 #include <sys/socket.h>
 
 #include "macro.h"
+#include "format-util.h"
+#include "process-util.h"
 #include "stdio-util.h"
 
 /* maximum length of fdname */
@@ -112,14 +114,17 @@ static inline int dir_fd_is_root_or_cwd(int dir_fd) {
         return dir_fd == AT_FDCWD ? true : path_is_root_at(dir_fd, NULL);
 }
 
-/* The maximum length a buffer for a /proc/self/fd/<fd> path needs */
+/* The maximum length a buffer for a /proc/<pid>/fd/<fd> path needs. We intentionally don't use /proc/self/fd
+ * as these paths might be read by other programs (for example when mounting file descriptors the source path
+ * ends up in /proc/mounts and related files) for which /proc/self/fd will be interpreted differently than
+ * /proc/<pid>/fd. */
 #define PROC_FD_PATH_MAX \
-        (STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int))
+        (STRLEN("/proc//fd/") + DECIMAL_STR_MAX(pid_t) + DECIMAL_STR_MAX(int))
 
 static inline char *format_proc_fd_path(char buf[static PROC_FD_PATH_MAX], int fd) {
         assert(buf);
         assert(fd >= 0);
-        assert_se(snprintf_ok(buf, PROC_FD_PATH_MAX, "/proc/self/fd/%i", fd));
+        assert_se(snprintf_ok(buf, PROC_FD_PATH_MAX, "/proc/" PID_FMT "/fd/%i", getpid_cached(), fd));
         return buf;
 }
 

src/test/test-fd-util.c

@@ -382,11 +382,16 @@ TEST(close_all_fds) {
 }
 
 TEST(format_proc_fd_path) {
-        assert_se(streq_ptr(FORMAT_PROC_FD_PATH(0), "/proc/self/fd/0"));
-        assert_se(streq_ptr(FORMAT_PROC_FD_PATH(1), "/proc/self/fd/1"));
-        assert_se(streq_ptr(FORMAT_PROC_FD_PATH(2), "/proc/self/fd/2"));
-        assert_se(streq_ptr(FORMAT_PROC_FD_PATH(3), "/proc/self/fd/3"));
-        assert_se(streq_ptr(FORMAT_PROC_FD_PATH(2147483647), "/proc/self/fd/2147483647"));
+        _cleanup_free_ char *expected = NULL;
+
+        for (int i = 0; i < 4; i++) {
+                assert_se(asprintf(&expected, "/proc/" PID_FMT "/fd/%i", getpid_cached(), i) >= 0);
+                assert_se(streq_ptr(FORMAT_PROC_FD_PATH(i), expected));
+                expected = mfree(expected);
+        }
+
+        assert_se(asprintf(&expected, "/proc/" PID_FMT "/fd/2147483647", getpid_cached()) >= 0);
+        assert_se(streq_ptr(FORMAT_PROC_FD_PATH(2147483647), expected));
 }
 
 TEST(fd_reopen) {