Bug 1660901 - Support the fstat-like subset of fstatat in the Linux sandbox policies. r=gcp

Differential Revision: https://phabricator.services.mozilla.com/D88499

security/sandbox/linux/SandboxFilter.cpp

@@ -243,6 +243,12 @@ class SandboxPolicyCommon : public SandboxPolicyBase {
     auto path = reinterpret_cast<const char*>(aArgs.args[1]);
     auto buf = reinterpret_cast<statstruct*>(aArgs.args[2]);
     auto flags = static_cast<int>(aArgs.args[3]);
+
+    if (fd != AT_FDCWD && (flags & AT_EMPTY_PATH) != 0 &&
+        strcmp(path, "") == 0) {
+      return ConvertError(fstatsyscall(fd, buf));
+    }
+
     if (fd != AT_FDCWD && path[0] != '/') {
       SANDBOX_LOG_ERROR("unsupported fd-relative fstatat(%d, \"%s\", %p, %d)",
                         fd, path, buf, flags);

security/sandbox/linux/broker/SandboxBrokerUtils.h

@@ -19,10 +19,12 @@
 typedef struct stat64 statstruct;
 #  define statsyscall stat64
 #  define lstatsyscall lstat64
+#  define fstatsyscall fstat64
 #elif defined(__NR_stat)
 typedef struct stat statstruct;
 #  define statsyscall stat
 #  define lstatsyscall lstat
+#  define fstatsyscall fstat
 #else
 #  error Missing stat syscall include.
 #endif