1======== 2Debug FS 3======== 4 5.. contents:: 6 7Overview 8-------- 9 10The *DebugFS* feature is primarily aimed at exposing firmware debug data to 11higher SW layers such as a non-secure component. Such component can be the 12TFTF test payload or a Linux kernel module. 13 14Virtual filesystem 15------------------ 16 17The core functionality lies in a virtual file system based on a 9p file server 18interface (`Notes on the Plan 9 Kernel Source`_ and 19`Linux 9p remote filesystem protocol`_). 20The implementation permits exposing virtual files, firmware drivers, and file blobs. 21 22Namespace 23~~~~~~~~~ 24 25Two namespaces are exposed: 26 27 - # is used as root for drivers (e.g. #t0 is the first uart) 28 - / is used as root for virtual "files" (e.g. /fip, or /dev/uart) 29 309p interface 31~~~~~~~~~~~~ 32 33The associated primitives are: 34 35- Unix-like: 36 37 - open(): create a file descriptor that acts as a handle to the file passed as 38 an argument. 39 - close(): close the file descriptor created by open(). 40 - read(): read from a file to a buffer. 41 - write(): write from a buffer to a file. 42 - seek(): set the file position indicator of a file descriptor either to a 43 relative or an absolute offset. 44 - stat(): get information about a file (type, mode, size, ...). 45 46.. code:: c 47 48 int open(const char *name, int flags); 49 int close(int fd); 50 int read(int fd, void *buf, int n); 51 int write(int fd, void *buf, int n); 52 int seek(int fd, long off, int whence); 53 int stat(char *path, dir_t *dir); 54 55- Specific primitives : 56 57 - mount(): create a link between a driver and spec. 58 - create(): create a file in a specific location. 59 - bind(): expose the content of a directory to another directory. 60 61.. code:: c 62 63 int mount(char *srv, char *mnt, char *spec); 64 int create(const char *name, int flags); 65 int bind(char *path, char *where); 66 67This interface is embedded into the BL31 run-time payload when selected by build 68options. The interface multiplexes drivers or emulated "files": 69 70- Debug data can be partitioned into different virtual files e.g. expose PMF 71 measurements through a file, and internal firmware state counters through 72 another file. 73- This permits direct access to a firmware driver, mainly for test purposes 74 (e.g. a hardware device that may not be accessible to non-privileged/ 75 non-secure layers, or for which no support exists in the NS side). 76 77SMC interface 78------------- 79 80The communication with the 9p layer in BL31 is made through an SMC conduit 81(`SMC Calling Convention`_), using a specific SiP Function Id. An NS 82shared buffer is used to pass path string parameters, or e.g. to exchange 83data on a read operation. Refer to :ref:`ARM SiP Services <arm sip services>` 84for a description of the SMC interface. 85 86Security considerations 87----------------------- 88 89- Due to the nature of the exposed data, the feature is considered experimental 90 and importantly **shall only be used in debug builds**. 91- Several primitive imply string manipulations and usage of string formats. 92- Special care is taken with the shared buffer to avoid TOCTOU attacks. 93 94Limitations 95----------- 96 97- In order to setup the shared buffer, the component consuming the interface 98 needs to allocate a physical page frame and transmit its address. 99- In order to map the shared buffer, BL31 requires enabling the dynamic xlat 100 table option. 101- Data exchange is limited by the shared buffer length. A large read operation 102 might be split into multiple read operations of smaller chunks. 103- On concurrent access, a spinlock is implemented in the BL31 service to protect 104 the internal work buffer, and re-entrancy into the filesystem layers. 105- Notice, a physical device driver if exposed by the firmware may conflict with 106 the higher level OS if the latter implements its own driver for the same 107 physical device. 108 109Applications 110------------ 111 112The SMC interface is accessible from an NS environment, that is: 113 114- a test payload, bootloader or hypervisor running at NS-EL2 115- a Linux kernel driver running at NS-EL1 116- a Linux userspace application through the kernel driver 117 118-------------- 119 120*Copyright (c) 2019-2020, Arm Limited and Contributors. All rights reserved.* 121 122.. _SMC Calling Convention: https://developer.arm.com/docs/den0028/latest 123.. _Notes on the Plan 9 Kernel Source: http://lsub.org/who/nemo/9.pdf 124.. _Linux 9p remote filesystem protocol: https://www.kernel.org/doc/Documentation/filesystems/9p.txt 125.. _ARM SiP Services: arm-sip-service.rst 126