move to fd based trace file mount

1 files changed, 25 insertions, 8 deletions

diff --git a/src/firejail/fs_trace.c b/src/firejail/fs_trace.c
index 2a7c83049..9ade0bdc3 100644
--- a/src/firejail/fs_trace.c
+++ b/src/firejail/fs_trace.c
@@ -46,21 +46,38 @@ void fs_trace_preload(void) {
                         printf("Creating an empty trace log file: %s\n", arg_tracefile);
                 // create a bind mounted trace logfile that the sandbox can see
                 EUID_USER();
-                FILE *fp = fopen(arg_tracefile, "w");
-                if (!fp)
-                        errExit("fopen");
-                SET_PERMS_STREAM(fp, firejail_uid, firejail_gid, S_IRUSR | S_IWRITE | S_IRGRP | S_IROTH);
-                fclose(fp);
+                int fd = open(arg_tracefile, O_CREAT|O_RDWR, S_IRUSR | S_IWRITE | S_IRGRP | S_IROTH);
+                if (fd == -1) {
+                        perror("open");
+                        fprintf(stderr, "Error: cannot open trace log file %s\n", arg_tracefile);
+                        exit(1);
+                }
+                if (fstat(fd, &s) == -1)
+                        errExit("fstat");
+                if (!S_ISREG(s.st_mode)) {
+                        fprintf(stderr, "Error: cannot write trace log: %s is no regular file\n", arg_tracefile);
+                        exit(1);
+                }
+                if (ftruncate(fd, 0) == -1)
+                        errExit("ftruncate");
                 EUID_ROOT();
-                fp = fopen(RUN_TRACE_FILE, "w");
+                FILE *fp = fopen(RUN_TRACE_FILE, "w");
                 if (!fp)
                         errExit("fopen " RUN_TRACE_FILE);
                 fclose(fp);
                 fs_logger2("touch ", arg_tracefile);
-                if (mount(arg_tracefile, RUN_TRACE_FILE, NULL, MS_BIND|MS_REC, NULL) < 0)
-                        errExit("mount bind " RUN_TRACE_FILE);
+                // mount using the symbolic link in /proc/self/fd
                 if (arg_debug)
                         printf("Bind mount %s to %s\n", arg_tracefile, RUN_TRACE_FILE);
+                char *proc;
+                if (asprintf(&proc, "/proc/self/fd/%d", fd) == -1)
+                        errExit("asprintf");
+                if (mount(proc, RUN_TRACE_FILE, NULL, MS_BIND|MS_REC, NULL) < 0)
+                        errExit("mount bind " RUN_TRACE_FILE);
+                free(proc);
+                close(fd);
+                // now that RUN_TRACE_FILE is user-writable, mount it noexec
+                fs_remount(RUN_TRACE_FILE, MOUNT_NOEXEC, 0);
         }
 }