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`_). The implementation permits 19exposing virtual files, firmware drivers, and file blobs. 20 21Namespace 22~~~~~~~~~ 23 24Two namespaces are exposed: 25 26 - # is used as root for drivers (e.g. #t0 is the first uart) 27 - / is used as root for virtual "files" (e.g. /fip, or /dev/uart) 28 299p interface 30~~~~~~~~~~~~ 31 32The associated primitives are: 33 34- Unix-like: 35 36 - open(): create a file descriptor that acts as a handle to the file passed as 37 an argument. 38 - close(): close the file descriptor created by open(). 39 - read(): read from a file to a buffer. 40 - write(): write from a buffer to a file. 41 - seek(): set the file position indicator of a file descriptor either to a 42 relative or an absolute offset. 43 - stat(): get information about a file (type, mode, size, ...). 44 45.. code:: c 46 47 int open(const char *name, int flags); 48 int close(int fd); 49 int read(int fd, void *buf, int n); 50 int write(int fd, void *buf, int n); 51 int seek(int fd, long off, int whence); 52 int stat(char *path, dir_t *dir); 53 54- Specific primitives : 55 56 - mount(): create a link between a driver and spec. 57 - create(): create a file in a specific location. 58 - bind(): expose the content of a directory to another directory. 59 60.. code:: c 61 62 int mount(char *srv, char *mnt, char *spec); 63 int create(const char *name, int flags); 64 int bind(char *path, char *where); 65 66This interface is embedded into the BL31 run-time payload when selected by build 67options. The interface multiplexes drivers or emulated "files": 68 69- Debug data can be partitioned into different virtual files e.g. expose PMF 70 measurements through a file, and internal firmware state counters through 71 another file. 72- This permits direct access to a firmware driver, mainly for test purposes 73 (e.g. a hardware device that may not be accessible to non-privileged/ 74 non-secure layers, or for which no support exists in the NS side). 75 76SMC interface 77------------- 78 79The communication with the 9p layer in BL31 is made through an SMC conduit 80(`SMC Calling Convention PDD`_), using a specific SiP Function Id. An NS shared 81buffer is used to pass path string parameters, or e.g. to exchange data on a 82read operation. Refer to `ARM SiP Services`_ for a description of the SMC 83interface. 84 85Security considerations 86----------------------- 87 88- Due to the nature of the exposed data, the feature is considered experimental 89 and importantly **shall only be used in debug builds**. 90- Several primitive imply string manipulations and usage of string formats. 91- Special care is taken with the shared buffer to avoid TOCTOU attacks. 92 93Limitations 94----------- 95 96- In order to setup the shared buffer, the component consuming the interface 97 needs to allocate a physical page frame and transmit its address. 98- In order to map the shared buffer, BL31 requires enabling the dynamic xlat 99 table option. 100- Data exchange is limited by the shared buffer length. A large read operation 101 might be split into multiple read operations of smaller chunks. 102- On concurrent access, a spinlock is implemented in the BL31 service to protect 103 the internal work buffer, and re-entrancy into the filesystem layers. 104- Notice, a physical device driver if exposed by the firmware may conflict with 105 the higher level OS if the latter implements its own driver for the same 106 physical device. 107 108Applications 109------------ 110 111The SMC interface is accessible from an NS environment, that is: 112 113- a test payload, bootloader or hypervisor running at NS-EL2 114- a Linux kernel driver running at NS-EL1 115- a Linux userspace application through the kernel driver 116 117References 118---------- 119 120.. [#] `SMC Calling Convention PDD`_ 121.. [#] `Notes on the Plan 9 Kernel Source`_ 122.. [#] `Linux 9p remote filesystem protocol`_ 123.. [#] `ARM SiP Services`_ 124 125-------------- 126 127*Copyright (c) 2019, Arm Limited and Contributors. All rights reserved.* 128 129.. _SMC Calling Convention PDD: http://infocenter.arm.com/help/topic/com.arm.doc.den0028b/ 130.. _Notes on the Plan 9 Kernel Source: http://lsub.org/who/nemo/9.pdf 131.. _Linux 9p remote filesystem protocol: https://www.kernel.org/doc/Documentation/filesystems/9p.txt 132.. _ARM SiP Services: arm-sip-service.rst 133