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