| Debugging with GDB: Compiling and Injecting Code |
|---|
GDB supports on-demand compilation and code injection into programs running under GDB. GCC 5.0 or higher built with libcc1.so must be installed for this functionality to be enabled. This functionality is implemented with the following commands.
compile code source-code
compile code -raw -- source-code
Compile source-code with the compiler language found as the current language in GDB (see Languages). If compilation and injection is not supported with the current language specified in GDB, or the compiler does not support this feature, an error message will be printed. If source-code compiles and links successfully, GDB will load the object-code emitted, and execute it within the context of the currently selected inferior. It is important to note that the compiled code is executed immediately. After execution, the compiled code is removed from GDB and any new types or variables you have defined will be deleted.
The command allows you to specify source-code in two ways. The simplest method is to provide a single line of code to the command. E.g.:
compile code printf ("hello world\n");
If you specify options on the command line as well as source code, they may conflict. The ‘--’ delimiter can be used to separate options from actual source code. E.g.:
compile code -r -- printf ("hello world\n");
Alternatively you can enter source code as multiple lines of text. To enter this mode, invoke the ‘compile code’ command without any text following the command. This will start the multiple-line editor and allow you to type as many lines of source code as required. When you have completed typing, enter ‘end’ on its own line to exit the editor.
compile code
>printf ("hello\n");
>printf ("world\n");
>end
Specifying ‘-raw’, prohibits GDB from wrapping the
provided source-code in a callable scope. In this case, you must
specify the entry point of the code by defining a function named
_gdb_expr_. The ‘-raw’ code cannot access variables of the
inferior. Using ‘-raw’ option may be needed for example when
source-code requires ‘#include’ lines which may conflict with
inferior symbols otherwise.
compile file filename
compile file -raw filename
Like compile code, but take the source code from filename.
compile file /home/user/example.c
compile print expr
compile print /f expr
Compile and execute expr with the compiler language found as the current language in GDB (see Languages). By default the value of expr is printed in a format appropriate to its data type; you can choose a different format by specifying ‘/f ’, where f is a letter specifying the format; see Output Formats.
compile print
compile print /f
Alternatively you can enter the expression (source code producing it) as multiple lines of text. To enter this mode, invoke the ‘compile print’ command without any text following the command. This will start the multiple-line editor.
The process of compiling and injecting the code can be inspected using:
set debug compile
Turns on or off display of GDB process of compiling and injecting the code. The default is off.
show debug compile
Displays the current state of displaying GDB process of compiling and injecting the code.
compile commandGDB needs to specify the right compilation options for the code to be injected, in part to make its ABI compatible with the inferior and in part to make the injected code compatible with GDB’s injecting process.
The options used, in increasing precedence:
gdbarch)These options depend on target processor type and target operating
system, usually they specify at least 32-bit (-m32) or 64-bit
(-m64) compilation option.
GCC (since version 4.7) stores the options used for compilation
into DW_AT_producer part of DWARF debugging information according
to the GCC option -grecord-gcc-switches. One has to
explicitly specify -g during inferior compilation otherwise
GCC produces no DWARF. This feature is only relevant for
platforms where -g produces DWARF by default, otherwise one may
try to enforce DWARF by using -gdwarf-4.
set compile-args
You can override compilation options using the following command:
set compile-args
Set compilation options used for compiling and injecting code with the
compile commands. These options override any conflicting ones
from the target architecture and/or options stored during inferior
compilation.
show compile-args
Displays the current state of compilation options override. This does not show all the options actually used during compilation, use set debug compile for that.
compile commandThere are a few caveats to keep in mind when using the compile
command. As the caveats are different per language, the table below
highlights specific issues on a per language basis.
When the language in GDB is set to ‘C’, the compiler will
attempt to compile the source code with a ‘C’ compiler. The source
code provided to the compile command will have much the same
access to variables and types as it normally would if it were part of
the program currently being debugged in GDB.
Below is a sample program that forms the basis of the examples that follow. This program has been compiled and loaded into GDB, much like any other normal debugging session.
void function1 (void)
{
int i = 42;
printf ("function 1\n");
}
void function2 (void)
{
int j = 12;
function1 ();
}
int main(void)
{
int k = 6;
int *p;
function2 ();
return 0;
}
For the purposes of the examples in this section, the program above has
been compiled, loaded into GDB, stopped at the function
main, and GDB is awaiting input from the user.
To access variables and types for any program in GDB, the
program must be compiled and packaged with debug information. The
compile command is not an exception to this rule. Without debug
information, you can still use the compile command, but you will
be very limited in what variables and types you can access.
So with that in mind, the example above has been compiled with debug
information enabled. The compile command will have access to
all variables and types (except those that may have been optimized
out). Currently, as GDB has stopped the program in the
main function, the compile command would have access to
the variable k. You could invoke the compile command
and type some source code to set the value of k. You can also
read it, or do anything with that variable you would normally do in
C. Be aware that changes to inferior variables in the
compile command are persistent. In the following example:
compile code k = 3;
the variable k is now 3. It will retain that value until
something else in the example program changes it, or another
compile command changes it.
Normal scope and access rules apply to source code compiled and
injected by the compile command. In the example, the variables
j and k are not accessible yet, because the program is
currently stopped in the main function, where these variables
are not in scope. Therefore, the following command
compile code j = 3;
will result in a compilation error message.
Once the program is continued, execution will bring these variables in
scope, and they will become accessible; then the code you specify via
the compile command will be able to access them.
You can create variables and types with the compile command as
part of your source code. Variables and types that are created as part
of the compile command are not visible to the rest of the program for
the duration of its run. This example is valid:
compile code int ff = 5; printf ("ff is %d\n", ff);
However, if you were to type the following into GDB after that command has completed:
compile code printf ("ff is %d\n'', ff);
a compiler error would be raised as the variable ff no longer
exists. Object code generated and injected by the compile
command is removed when its execution ends. Caution is advised
when assigning to program variables values of variables created by the
code submitted to the compile command. This example is valid:
compile code int ff = 5; k = ff;
The value of the variable ff is assigned to k. The variable
k does not require the existence of ff to maintain the value
it has been assigned. However, pointers require particular care in
assignment. If the source code compiled with the compile command
changed the address of a pointer in the example program, perhaps to a
variable created in the compile command, that pointer would point
to an invalid location when the command exits. The following example
would likely cause issues with your debugged program:
compile code int ff = 5; p = &ff;
In this example, p would point to ff when the
compile command is executing the source code provided to it.
However, as variables in the (example) program persist with their
assigned values, the variable p would point to an invalid
location when the command exists. A general rule should be followed
in that you should either assign NULL to any assigned pointers,
or restore a valid location to the pointer before the command exits.
Similar caution must be exercised with any structs, unions, and typedefs
defined in compile command. Types defined in the compile
command will no longer be available in the next compile command.
Therefore, if you cast a variable to a type defined in the
compile command, care must be taken to ensure that any future
need to resolve the type can be achieved.
(gdb) compile code static struct a { int a; } v = { 42 }; argv = &v;
(gdb) compile code printf ("%d\n", ((struct a *) argv)->a);
gdb command line:1:36: error: dereferencing pointer to incomplete type âstruct aâ
Compilation failed.
(gdb) compile code struct a { int a; }; printf ("%d\n", ((struct a *) argv)->a);
42
Variables that have been optimized away by the compiler are not
accessible to the code submitted to the compile command.
Access to those variables will generate a compiler error which GDB
will print to the console.
compile command
GDB needs to find GCC for the inferior being debugged
which may not be obvious for remote targets of different architecture
than where GDB is running. Environment variable PATH on
GDB host is searched for GCC binary matching the
target architecture and operating system. This search can be overriden
by set compile-gcc GDB command below. PATH is
taken from shell that executed GDB, it is not the value set by
GDB command set environment). See Environment.
Specifically PATH is searched for binaries matching regular expression
arch(-[^-]*)?-os-gcc according to the inferior target being
debugged. arch is processor name — multiarch is supported, so for
example both i386 and x86_64 targets look for pattern
(x86_64|i.86) and both s390 and s390x targets look
for pattern s390x?. os is currently supported only for
pattern linux(-gnu)?.
On Posix hosts the compiler driver GDB needs to find also
shared library libcc1.so from the compiler. It is searched in
default shared library search path (overridable with usual environment
variable LD_LIBRARY_PATH), unrelated to PATH or set
compile-gcc settings. Contrary to it libcc1plugin.so is found
according to the installation of the found compiler — as possibly
specified by the set compile-gcc command.
set compile-gcc
Set compilation command used for compiling and injecting code with the
compile commands. If this option is not set (it is set to
an empty string), the search described above will occur — that is the
default.
show compile-gcc
Displays the current compile command GCC driver filename.
If set, it is the main command gcc, found usually for example
under name x86_64-linux-gnu-gcc.