Uwaga na buga! GDB w służbie programisty. Barcamp Semihalf S09:E01

GDB
w służbie programisty
Łukasz Majczak
Copyright © 2018 Semihalf. All rights reserved. Confidential and proprietary information.

O czym będziemy rozmawiać
● Platforma: linux, x86-64
● Podstawowe przydatne komendy
● Wbudowane zmienne i funkcje
● Automatyzacja pracy
● Aplikacji wielowątkowe

Podstawowe komendy
Jak zacząć ?
● gdb + file + run
● gdb <program> + run
● gdb attach <program>

Podstawowe komendy
break ... if cond [thread thread-id]
Set a breakpoint with condition cond; evaluate the expression cond each time the
breakpoint is reached, and stop only if the value is nonzero--that is, if cond evaluates
as true. `...' stands for one of the possible arguments: function, +offset, -offset,
linenum, filename:linenum, filename:function, *address.

Podstawowe komendy
break ... if cond [thread thread-id]
commands
silent
printf “my_var is %dn”,my_var
set my_var = my_var + 1
continue
end

Podstawowe komendy
[r|a|-]watch [-l|-location] expr [thread
thread-id] [mask maskvalue]
Set a watchpoint for an expression. GDB will break when the expression expr is written into by the program
and its value changes. If the command includes a [thread thread-id] argument, GDB breaks only when the
thread identified by thread-id changes the value of expr.Ordinarily a watchpoint respects the scope of
variables in expr (see below). The -location argument tells GDB to instead watch the memory referred to by
expr. In this case, GDB will evaluate expr, take the address of the result, and watch the memory at that
address. The type of the result is used to determine the size of the watched memory. If the expression’s
result does not have an address, then GDB will print an error.
The [mask maskvalue] argument allows creation of masked watchpoints, if the current architecture supports
this feature A masked watchpoint specifies a mask in addition to an address to watch. The mask specifies
that some bits of an address should be ignored when matching the address accessed by the inferior against
the watchpoint address. Thus, a masked watchpoint watches many addresses simultaneously.The mask
argument implies -location. Examples:
(gdb) watch foo mask 0xffff00ff
(gdb) watch *0xdeadbeef mask 0xffffff00

Podstawowe komendy
catch event
Stop when event occurs. The event can be any of the following:
● throw,
● rethrow,
● catch,
● exec,
● syscall,
● fork,
● vfork,
● load,
● unload,
● signal

X
x [Address expression]
x /[Format] [Address expression]
x /[Length][Format] [Address expression]
Format: o,x,d,u,t,f,a,c,s,i followed by:b,h,w,g
print
print expr
print /f expr
Format: o,x,d,u,t,a,c,f
Podstawowe komendy

explore
explore arg
arg is either an expression (in the source
language), or a type visible in the current
context of the program being debugged.
whatis
whatis expr
Print the data type of expression
expr. expr is not actually
evaluated, and any side-effecting
operations (such as assignments or
function calls) inside it do not
take place
Podstawowe komendy

Podstawowe komendy
set var ...
To alter the value of a variable, evaluate an assignment expression.
Example:
set var my_var = 48
set {int}0xdeadbeef = 48

backtrace
This command will print one line per
frame for frames in the stack. By
default, all stack frames are
printed.
frame
When used without any argument, this
command does not change which frame is
selected, but prints a brief description
of the currently selected stack frame. It
can be abbreviated f. With an argument,
this command is used to select a stack
frame.
Podstawowe komendy

step
step [count]
Continue running your program until control
reaches a different source line, then stop
it and return control to GDB. This command
is abbreviated s.
next
next [count]
Continue to the next source line in the
current (innermost) stack frame. This
is similar to step, but function calls
that appear within the line of code are
executed without stopping. Execution
stops when control reaches a different
line of code at the original stack
level that was executing when you gave
the next command. This command is
abbreviated n.
Podstawowe komendy

Podstawowe komendy
● continue, until, finish
● info thread / reg / break
● disassemble
● jump
● call, return

Wbudowane zmienne GDB
Convenience Variables
GDB provides convenience variables that you can use within GDB to hold on to a value and refer to it later. These
variables exist entirely within GDB; they are not part of your program, and setting a convenience variable has no
direct effect on further execution of your program. That is why you can use them freely.
Convenience variables are prefixed with ‘$’. Any name preceded by ‘$’ can be used for a convenience variable,
unless it is one of the predefined machine-specific register names.

Wbudowane funkcje GDB
Convenience Functions
GDB also supplies some convenience functions. These have a syntax similar to
convenience variables. A convenience function can be used in an expression just like an
ordinary function; however, a convenience function is implemented internally to GDB.

Wbudowane funkcje GDB
Convenience Functions
● $_isvoid (expr)
● $_memeq(buf1, buf2, length)
● $_regex(str, regex)
● $_streq(str1, str2)
● $_strlen(str)
● $_caller_is(name[, number_of_frames])
● $_caller_matches(regexp[, number_of_frames])
● $_any_caller_is(name[, number_of_frames])
● $_any_caller_matches(regexp[, number_of_frames])
● $_as_string(value)

Automatyzacja
A user-defined command is a sequence of GDB commands to which you assign a new
name as a command. This is done with the define command. User commands may accept
up to 10 arguments separated by whitespace. Arguments are accessed within the user
command via $arg0...$arg9. A trivial example:
define adder
print $arg0 + $arg1 + $arg2
To execute the command use:
adder 1 2 3

Automatyzacja
define commandname
...
end
Define a command named commandname. If there is already a command by that name,
you are asked to confirm that you want to redefine it. The definition of the command is
made up of other GDB command lines, which are given following the define command. The
end of these commands is marked by a line containing end.

Automatyzacja
if arg
Takes a single argument, which is an expression to evaluate. It is followed by a series of
commands that are executed only if the expression is true (nonzero). There can then
optionally be a line else, followed by a series of commands that are only executed if the
expression was false. The end of the list is marked by a line containing end.

Automatyzacja
while arg
The syntax is similar to if: the command takes a single argument, which is an expression to
evaluate, and must be followed by the commands to execute, one per line, terminated by
an end. The commands are executed repeatedly as long as the expression evaluates to
true.

Automatyzacja
document commandname
Document the user-defined command commandname, so that it can be accessed by help.
The command commandname must already be defined. This command reads lines of
documentation just as define reads the lines of the command definition, ending with end.
After the document command is finished, help on command commandname displays the
documentation you have written. You may use the document command again to change
the documentation of a command. Redefining the command with define does not change
the documentation.

Automatyzacja
show user commandname
Display the GDB commands used to define commandname (but not its documentation). If
no commandname is given, display the definitions for all user-defined commands.

Automatyzacja
Example
(gdb) show user
User command "example":
set $i=0
while $i<16
print tab[$i++]
end
(gdb)

All-stop vs non-stop
All-stop
In all-stop mode, whenever your program stops under GDB for
any reason, all threads of execution stop, not just the current
thread. This allows you to examine the overall state of the
program, including switching between threads, without worrying
that things may change underfoot.
Conversely, whenever you restart the program, all threads start
executing. This is true even when single-stepping with
commands like step or next.
Non-stop
For some multi-threaded targets, GDB supports an
optional mode of operation in which you can examine
stopped program threads in the debugger while other
threads continue to execute freely. This minimizes
intrusion when debugging live systems, such as programs
where some threads have real-time constraints or must
continue to respond to external events. This is referred to
as non-stop mode.
Aplikacje wielowątkowe

Background Execution
GDB’s execution commands have two variants: the normal foreground (synchronous)
behavior, and a background (asynchronous) behavior. In foreground execution, GDB waits
for the program to report that some thread has stopped before prompting for another
command. In background execution, GDB immediately gives a command prompt so that
you can issue other commands while your program runs.
If the target doesn’t support async mode, GDB issues an error message if you attempt to
use the background execution commands.

To specify background execution, add a & to the command. For example, the background
form of the continue command is continue&, or just c&. The execution commands that
accept background execution are:
run, attach, step, stepi, next, nexti, continue, finish, until

You can interrupt your program while it is running in the background by using the interrupt
command.
interrupt
interrupt -a

Reverse execute
When you are debugging a program, it is not unusual to realize that you have gone too far,
and some event of interest has already happened. If the target environment supports it,
GDB can allow you to “rewind” the program by running it backward.
A target environment that supports reverse execution should be able to “undo” the changes
in machine state that have taken place as the program was executing normally. Variables,
registers etc. should revert to their previous values. Obviously this requires a great deal of
sophistication on the part of the target environment; not all target environments can support
reverse execution.

● reverse-continue ('rc') -- Continue program being debugged but run it in reverse
● reverse-finish -- Execute backward until just before the selected stack frame is
called
● reverse-next ('rn') -- Step program backward, proceeding through subroutine calls.
● reverse-nexti ('rni') -- Step backward one instruction, but proceed through called
subroutines.
● reverse-step ('rs') -- Step program backward until it reaches the beginning of a
previous source line
● reverse-stepi -- Step backward exactly one instruction
● set exec-direction (forward/reverse) -- Set direction of execution.
● All subsequent execution commands (continue, step, until etc.) will run the program
being debugged in the selected direction.
Reverse execute

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Uwaga na buga! GDB w służbie programisty. Barcamp Semihalf S09:E01

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