28. Stack Bounds Checker¶

28.2.1. Task Stack¶

Each task in a system has a fixed size stack associated with it. This stack is allocated when the task is created. As the task executes, the stack is used to contain parameters, return addresses, saved registers, and local variables. The amount of stack space required by a task is dependent on the exact set of routines used. The peak stack usage reflects the worst case of subroutine pushing information on the stack. For example, if a subroutine allocates a local buffer of 1024 bytes, then this data must be accounted for in the stack of every task that invokes that routine.

Recursive routines make calculating peak stack usage difficult, if not impossible. Each call to the recursive routine consumes n bytes of stack space. If the routine recursives 1000 times, then 1000 * n bytes of stack space are required.

28.2.2. Execution¶

The stack bounds checker operates as a set of task extensions. At task creation time, the task’s stack is filled with a pattern to indicate the stack is unused. As the task executes, it will overwrite this pattern in memory. At each task switch, the stack bounds checker’s task switch extension is executed. This extension checks that:

• the last n bytes of the task’s stack have not been overwritten. If this pattern has been damaged, it indicates that at some point since this task was context switch to the CPU, it has used too much stack space.
• the current stack pointer of the task is not within the address range allocated for use as the task’s stack.

If either of these conditions is detected, then a blown stack error is reported using the printk routine.

The number of bytes checked for an overwrite is processor family dependent. The minimum stack frame per subroutine call varies widely between processor families. On CISC families like the Motorola MC68xxx and Intel ix86, all that is needed is a return address. On more complex RISC processors, the minimum stack frame per subroutine call may include space to save a significant number of registers.

Another processor dependent feature that must be taken into account by the stack bounds checker is the direction that the stack grows. On some processor families, the stack grows up or to higher addresses as the task executes. On other families, it grows down to lower addresses. The stack bounds checker implementation uses the stack description definitions provided by every RTEMS port to get for this information.

28.3.1. Initializing the Stack Bounds Checker¶

The stack checker is initialized automatically when its task create extension runs for the first time.

The application must include the stack bounds checker extension set in its set of Initial Extensions. This set of extensions is defined as STACK_CHECKER_EXTENSION. If using <rtems/confdefs.h> for Configuration Table generation, then all that is necessary is to define the macro CONFIGURE_STACK_CHECKER_ENABLED before including <rtems/confdefs.h> as shown below:

#define CONFIGURE_STACK_CHECKER_ENABLED
...
#include <rtems/confdefs.h>

28.3.2. Checking for Blown Task Stack¶

The application may check whether the stack pointer of currently executing task is within proper bounds at any time by calling the rtems_stack_checker_is_blown method. This method return FALSE if the task is operating within its stack bounds and has not damaged its pattern area.

28.3.3. Reporting Task Stack Usage¶

The application may dynamically report the stack usage for every task in the system by calling the rtems_stack_checker_report_usage routine. This routine prints a table with the peak usage and stack size of every task in the system. The following is an example of the report generated:

ID      NAME       LOW        HIGH     AVAILABLE      USED
0x04010001  IDLE  0x003e8a60  0x003e9667       2952        200
0x08010002  TA1   0x003e5750  0x003e7b57       9096       1168
0x08010003  TA2   0x003e31c8  0x003e55cf       9096       1168
0x08010004  TA3   0x003e0c40  0x003e3047       9096       1104
0xffffffff  INTR  0x003ecfc0  0x003effbf      12160        128

Notice the last line. The task id is 0xffffffff and its name is INTR. This is not actually a task, it is the interrupt stack.

28.3.4. When a Task Overflows the Stack¶

When the stack bounds checker determines that a stack overflow has occurred, it will attempt to print a message using printk identifying the task and then shut the system down. If the stack overflow has caused corruption, then it is possible that the message cannot be printed.

The following is an example of the output generated:

BLOWN STACK!!! Offending task(0x3eb360): id=0x08010002; name=0x54413120
stack covers range 0x003e5750 - 0x003e7b57 (9224 bytes)
Damaged pattern begins at 0x003e5758 and is 128 bytes long

The above includes the task id and a pointer to the task control block as well as enough information so one can look at the task’s stack and see what was happening.

28.4.1. STACK_CHECKER_IS_BLOWN - Has Current Task Blown Its Stack¶

CALLING SEQUENCE:

bool rtems_stack_checker_is_blown( void );

STATUS CODES:

TRUE	Stack is operating within its stack limits
FALSE	Current stack pointer is outside allocated area

DESCRIPTION:

This method is used to determine if the current stack pointer of the currently executing task is within bounds.

NOTES:

This method checks the current stack pointer against the high and low addresses of the stack memory allocated when the task was created and it looks for damage to the high water mark pattern for the worst case usage of the task being called.

28.4.2. STACK_CHECKER_REPORT_USAGE - Report Task Stack Usage¶

CALLING SEQUENCE:

void rtems_stack_checker_report_usage( void );

STATUS CODES:

NONE

DESCRIPTION:

This routine prints a table with the peak stack usage and stack space allocation of every task in the system.

NOTES:

NONE