Remote debugging using gdbserver

Remote debugging is rather straightforward:

On the target platform, launch the application with GDBserver, while specifying the host and port for listening to an incoming TCP connection:

gdbserver HOST:PORT PROG [ARGS ...]
gdbserver 10.19.103.55:123 test_rsh_sw.bin r
sh/1bit/

On the development workstation, launch the cross-target GDB:

arm-none-linux-gnueabi-gdb PROG

Be sure to specify the non-stripped executable. At the GDB console, type:

target remote HOST:PORT


break main
continue

These commands will connect GDB to the GDBserver running on the target platform, set a breakpoint at the start of the program, and let it run until it reaches that first breakpoint.

You can also attach GDBserver to a process that's already running:

gdbserver HOST:PORT --attach PID

The process is then stopped, and you can then debug it with a remote GDB.

df DOES NOT return/yum hangs "Running transaction tests"

This is bcoz of a nfs mount hanging around
Use
umount -f

What is an address space

When a program is built(compilation, assembler invocation, and linking)
it is stored in a file with a special format. One such format is
ELF(Executable and Linking Format)in Unix like systems. It has the
necessary information ( in the ELF header and program header) to help
the program loader to load and create the process image in memory at
runtime, and also do run time linking (for shared libraries linked
dynamically).
At the time program is built, storage for all uninitialised data (static or
global) are not allocated, but only the total size is noted in program
header table of the ELF file. Later when program is being loaded, loader
read the metadata from the ELF file and allocate one large chunk (page
aligned) of memory to hose all such variables. This section is called
BSS(Block Static Storage or Block started by symbol).
Data( storage for all initialized static and global variables) and STACK
section should be familiar to you. HEAP is created by dynamic invocation
by memory-alloc routines of your program. In addition to this there will
be shared memory mappings as well. If you process a file using mmap()
system call interface, those regions are also mapped to your program.
Now the sum total of all such memory is called the address space of your
program.

Now what’s an address? Is that a virtual address, physical address
or logical address?
The value you see when you apply an & operator to a C variable
is actually the program-relative logical address. It has to be
processed by segmentation unit(coupled with the value in the CS
register) to create a linear address and then processed by the
paging unit to create the physical address in your RAM. So these
details are not at the control of C, but is at the control of the
memory management subsystems of the operating system.

USB Enumeration Process

Typical USB 2.0 Sequence

The steps below are a typical sequence of events that occurs during enumeration of a USB 2.0 device under Windows. Device firmware shouldn’t assume that enumeration requests and events will occur in a particular order. To function successfully, a device must detect and respond to any control request or other bus event at any time.

1. The system has a new device. A user attaches a device to a USB port, or the system powers up with a device attached. The port may be on the root hub at the host or on a hub that connects downstream from the host. The hub provides power to the port, and the device is in the Powered state. The device can draw up to 100 mA from the bus.

2. The hub detects the device. The hub monitors the voltages on the signal lines (D+ and D-) at each of its ports. The hub has a pull-down resistor of 14.25k–24.8kW on each line. A device has a pull-up resistor of 900–1575W on D+ for a full-speed device or D- for a low-speed device. High-speed-capable devices attach at full speed. On attaching to a port, the device’s pull-up brings its line high, enabling the hub to detect that a device is attached. On detecting a device, the hub continues to provide power but doesn’t yet transmit USB traffic to the device. Chapter 15 has more on how hubs detect devices.

3. The host learns of the new device. Each hub uses its interrupt endpoint to report events at the hub. The report indicates only whether the hub or a port (and if so, which port) has experienced an event. On learning of an event, the host sends the hub a Get Port Status request to find out more. Get Port Status and the other hub-class requests described are standard requests that all hubs support. The information returned tells the host when a device is newly attached.

4. The hub detects whether a device is low or full speed. Just before resetting the device, the hub determines whether the device is low or full speed by examining the voltages on the two signal lines. The hub detects the device’s speed by determining which line has a higher voltage when idle. The hub sends the information to the host in response to the next Get Port Status request. A USB 1.x hub may instead detect the device’s speed just after a bus reset. USB 2.0 requires speed detection before the reset so the hub knows whether to check for a high-speed-capable device during reset as described below.

5. The hub resets the device. When a host learns of a new device, the host sends the hub a Set Port Feature request that asks the hub to reset the port. The hub places the device’s USB data lines in the Reset condition for at least 10 ms. Reset is a special condition where both D+ and D- are logic low. (Normally, the lines have opposite logic states.) The hub sends the reset only to the new device. Other hubs and devices on the bus don’t see the reset.

6. The host learns if a full-speed device supports high speed. Detecting whether a device supports high speed uses two special signal states. In the Chirp J state, only the D+ line is driven and in the Chirp K state, only the D- line is driven.

During the reset, a device that supports high speed sends a Chirp K. A high-speed-capable hub detects the Chirp K and responds with a series of alternating Chirp K and Chirp J. On detecting the pattern KJKJKJ, the device removes its full-speed pull-up and performs all further communications at high speed. If the hub doesn’t respond to the device’s Chirp K, the device knows it must continue to communicate at full speed. All high-speed devices must be capable of responding to control requests at full speed.

7. The hub establishes a signal path between the device and the bus. The host verifies that the device has exited the reset state by sending a Get Port Status request. A bit in the returned data indicates whether the device is still in the reset state. If necessary, the host repeats the request until the device has exited the reset state.

When the hub removes the reset, the device is in the Default state. The device’s USB registers are in their reset states, and the device is ready to respond to control transfers at endpoint zero. The device communicates with the host using the default address of 00h.

8. The host sends a Get Descriptor request to learn the maximum packet size of the default pipe. The host sends the request to device address 00h, endpoint zero. Because the host enumerates only one device at a time, only one device will respond to communications addressed to device address 00h even if several devices attach at once.

The eighth byte of the device descriptor contains the maximum packet size supported by endpoint zero. A Windows host requests 64 bytes but after receiving just one packet (whether or not it has 64 bytes), the host begins the Status stage of the transfer. On completing the Status stage, Windows requests the hub to reset the device as in step 5 above. The USB 2.0 specification doesn’t require a reset here. The reset is a precaution that ensures that the device will be in a known state when the reset ends.

9. The host assigns an address. When the reset is complete, the host controller assigns a unique address to the device by sending a Set Address request. The device completes the Status stage of the request using the default address and then implements the new address. The device is now in the Address state. All communications from this point on use the new address. The address is valid until the device is detached, a hub resets the port, or the system reboots. On the next enumeration, the host may assign a different address to the device.

10. The host learns about the device’s abilities. The host sends a Get Descriptor request to the new address to read the device descriptor. This time the host retrieves the entire descriptor. The descriptor contains the maximum packet size for endpoint zero, the number of configurations the device supports, and other basic information about the device.

The host continues to learn about the device by requesting the one or more configuration descriptors specified in the device descriptor. A request for a configuration descriptor is actually a request for the configuration descriptor followed by all of its subordinate descriptors up to the number of bytes requested. A Windows host begins by requesting just the configuration descriptor’s nine bytes. Included in these bytes is the total length of the configuration descriptor and its subordinate descriptors.

Windows then requests the configuration descriptor again, this time requesting the number of bytes in the retrieved total length. The device responds by sending the configuration descriptor followed by all of the configuration’s subordinate descriptors, including interface descriptor(s), with each interface descriptor followed by any endpoint descriptors for the interface. Some configurations also have class- or vendor-specific descriptors. This chapter has more on what the descriptors contain.

11. The host assigns and loads a device driver (except for composite devices). After learning about a device from its descriptors, the host looks for the best match in a driver to manage communications with the device. Windows hosts use INF files to identify the best match. The INF file may be a system file for a USB class or a vendor-provided file that contains the device’s Vendor ID and Product ID. Chapter 9 has more about selecting a driver.

For devices that have been enumerated previously, Windows may use stored information instead of searching the INF files. After the operating system assigns and loads the driver, the driver may request the device to resend descriptors or send other class-specific descriptors.

An exception to this sequence is composite devices, which can have different drivers assigned to multiple interfaces in a configuration. The host can assign these drivers only after enabling the interfaces, so the host must first configure the device as described below.



12. The host’s device driver selects a configuration. After learning about a device from the descriptors, the device driver requests a configuration by sending a Set Configuration request with the desired configuration number. Many devices support only one configuration. If a device supports multiple configurations, the driver can decide which configuration to request based on information the driver has about how the device will be used, or the driver can ask the user what to do or just select the first configuration. (Many drivers only select the first configuration.) On receiving the request, the device implements the requested configuration. The device is now in the Configured state and the device’s interface(s) are enabled.

For composite devices, the host can now assign drivers. As with other devices, the host uses the information retrieved from the device to find a driver for each active interface in the configuration. The device is then ready for use.

Hubs are also USB devices, and the host enumerates a newly attached hub in the same way as other devices. If the hub has devices attached, the host enumerates these after the hub informs the host of their presence.

Attached state. If the hub isn’t providing power to a device’s VBUS line, the device is in the Attached state. The absence of power may occur if the hub has detected an over-current condition or if the host requests the hub to remove power from the port. With no power on VBUS, the host and device can’t communicate, so from their perspective, the situation is the same as when the device isn’t attached.

Suspend State. A device enters the Suspend state after detecting no bus activity, including SOF markers, for at least 3 ms. In the Suspend state, the device should limit its use of bus power. Both configured and unconfigured devices must support this state. Chapter 16 has more about the Suspend state.

Things to know of spinlocks

Spin locks can be used in interrupt handlers, whereas semaphores cannot be used because they sleep
If a lock is used in an interrupt handler, you must also disable local interrupts (interrupt requests on the current processor) before obtaining the lock

So the pseudo code of spin lock

{
Disable preempt()
acquire_lock()
}

If used in ISR
{
Disable interrupts()
spin lock()
//critical region
spin_unlock()
}

OR

{
spinlock_t mr_lock = SPIN_LOCK_UNLOCKED;
unsigned long flags;

spin_lock_irqsave(&mr_lock, flags);
/* critical region ... */
spin_unlock_irqrestore(&mr_lock, flags);


}

The fact that a contended spin lock causes threads to spin (essentially wasting processor time) while waiting for the lock to become available is important. This behavior is the point of the spin lock. It is not wise to hold a spin lock for a long time. This is the nature of the spin lock: a lightweight single-holder lock that should be held for short durations. An alternative behavior when the lock is contended is to put the current thread to sleep and wake it up when it becomes available. Then the processor can go off and execute other code. This incurs a bit of overhead most notably the two context switches required to switch out of and back into the blocking thread, which is certainly a lot more code than the handful of lines used to implement a spin lock. Therefore, it is wise to hold spin locks for less than the duration of two context switches. Because most of us have better things to do than measure context switches, just try to hold the lock as little time as possible[1]. The semaphores provide a lock that makes the waiting thread sleep, rather than spin, when contended.

Method
Description

spin_lock()
Acquires given lock

spin_lock_irq()
Disables local interrupts and acquires given lock

spin_lock_irqsave()
Saves current state of local interrupts, disables local interrupts, and acquires given lock

spin_unlock()
Releases given lock

spin_unlock_irq()
Releases given lock and enables local interrupts

spin_unlock_irqrestore()
Releases given lock and restores local interrupts to given previous state

spin_lock_init()
Dynamically initializes given spinlock_t

spin_trylock()
Tries to acquire given lock; if unavailable, returns nonzero

spin_is_locked()
Returns nonzero if the given lock is currently acquired, otherwise it returns zero


certain locking precautions must be taken when working with bottom halves. The function spin_lock_bh() obtains the given lock and disables all bottom halves. The function spin_unlock_bh() performs the inverse.

Because a bottom half may preempt process context code, if data is shared between a bottom half process context, you must protect the data in process context with both a lock and the disabling of bottom halves. Likewise, because an interrupt handler may preempt a bottom half, if data is shared between an interrupt handler and a bottom half, you must both obtain the appropriate lock and disable interrupts.

Kernel Directory Structure

Download: kernel.org
Browse: lxr.linux.no (with cross reference)
Directory structure
include: public headers
kernel: core kernel components (e.g., scheduler)
arch: hardware-dependent code
fs: file systems
mm: memory management
ipc: inter-process communication
drivers: device drivers
usr: user-space code
lib: common libraries

Adding a new system call

Assume we’d like add a system call calles “mycall” that takes two integers as input and return their sum.
1) Add “.long sys_mycall” at the end of the list in the file syscall_table.S.
---Full path for the file syscall_table.S is
usr/src/linux/arch/i386/kernel/syscall_table.S
NOTE: You can use the command sudo gedit syscall_table.S to edit the file
syscall_table.S. Beware that if you do not have the root privileges, system will not allow you to edit
any of the kernel files.
2) In file unistd.h
- Add #define __NR_mycall at the end of the list
(e.g. If the number of the last system call Last_System_Call_Num is 319 then the
line you shall add should be #define __NR_mycall 320.).
- Increment NR_syscalls by one (e.g. If before adding your system call total number of
system calls is 320, then this will be #define NR_syscalls 321.).
---Full path for the file unistd.h is /usr/src/linux/include/asm-i386/unistd.h
3) Add the following line at the end of the file syscalls.h:
asmlinkage long sys_mycall(int i, int j);
---Full path for the file syscalls.h is /usr/src/linux/include/linux/syscalls.h
4) Add mycall/ to core-y += in Makefile.
The line in the end shall look like:
core-y += kernel/ mm/ fs/ ipc/ security/ crypto/ block/ mycall/
---Full path for Makefile is /usr/src/linux/Makefile.
5) Create a new directory in /usr/src/linux and name it mycall.
6) Create a new file called mycall.c in /usr/src/linux/mycall. Contents of the file shall be
as follows:
/*----------Start of mycall.c----------*/
#include <linux/linkage.h>
asmlinkage long sys_mycall(int i, int j) {
return(i+j);
}
/*-----------End of mycall.c-----------*/
*asmlinkage is used to look for the arguments on the kernel stack.
7) Create Makefile in /usr/src/linux/mycall. Makefile shall be like:
########## Start of Makefile ##########
obj-y := mycall.o
########## End of Makefile ##########
8) Create the following userspace program to test your system call and name it testmycall.c. The
contents of this file shall be:
/*---------- Start of testmycall.c File ----------*/
#include <unistd.h>
#include <stdio.h>
#define __NR_mycall
long mycall(int i, int j) {
return syscall(__NR_mycall, i, j);
};
int main() {
printf(“%d\n”, mycall(10, 20));
return 0;
}
/*---------- End of testmycall.c File ----------*/
9) Compile testmycall.c to test whether your system call works.
gcc testmycall.c –o testmycall