I know that we can use several commands to access and read memory: for example, print, p, x...
But how can I change the contents of memory at any specific location (while debugging in GDB)?
The easiest is setting a program variable (see GDB: assignment):
(gdb) l
6 {
7 int i;
8 struct file *f, *ftmp;
9
(gdb) set variable i = 10
(gdb) p i
$1 = 10
Or you can just update arbitrary (writable) location by address:
(gdb) set {int}0x83040 = 4
There's more. Read the manual.
As Nikolai has said you can use the gdb 'set' command to change the value of a variable.
You can also use the 'set' command to change memory locations.
eg. Expanding on Nikolai's example:
(gdb) l
6 {
7 int i;
8 struct file *f, *ftmp;
9
(gdb) set variable i = 10
(gdb) p i
$1 = 10
(gdb) p &i
$2 = (int *) 0xbfbb0000
(gdb) set *((int *) 0xbfbb0000) = 20
(gdb) p i
$3 = 20
This should work for any valid pointer, and can be cast to any appropriate data type.
Expanding on the answers provided here.
You can just do set idx = 1 to set a variable, but that syntax is not recommended because the variable name may clash with a set sub-command. As an example set w=1 would not be valid.
This means that you should prefer the syntax: set variable idx = 1 or set var idx = 1.
Last but not least, you can just use your trusty old print command, since it evaluates an expression. The only difference being that he also prints the result of the expression.
(gdb) p idx = 1
$1 = 1
You can read more about gdb here.
Writing memory:
(gdb) set *0x20001234 = 0xABABABAB
Reading memory:
(gdb) x 0x20001234
0x20001234: 0xabababab
One of the most useful things is to change the value of Registers directly.
0x000000000800088e <+67>: lea rdi,[rip+0x118] # 0x80009ad
To change the value of rdi register:
set $rdi = 0x8201010
Related
Is there any way to extract variables that are being passed to a particular function as parameters in a given c code?
For an example,
main()
{
int a = 10;
float b = 2.0f;
funcA(a,b);
}
Need is to extract the information that variable a & variable b are passed to funcA in a given C code.
Is there a way to extract these information using gdb and it's function breakpoints?
when you are in any function typing frame will give you parameters information of function.
Below are some other commands you can use to get to know more about local variables and parameters.
info locals
frame
info args
more gdb commands
sample :
(gdb) b main
Note: breakpoint 1 also set at pc 0x40053e.
Breakpoint 2 at 0x40053e: file main.c, line 6.
(gdb) r
Starting program: /home/a.out
Breakpoint 1, main () at main.c:6
6 int a = 10;
(gdb) n
7 float b = 2.0f;
(gdb) info locals
a = 10
b = 0
x = 0
(gdb) frame
#0 main () at main.c:7
7 float b = 2.0f;
(gdb) next
9 int x = funcA(a,b);
(gdb) step
funcA (a=10, b=2) at main.c:16
16 int sum = 0;
(gdb) frame
#0 funcA (a=10, b=2) at main.c:16
16 int sum = 0;
(gdb) info args
a = 10
b = 2
(gdb) info locals
sum = 0
(gdb) p a
$1 = 10
(gdb) print b
$2 = 2
(gdb)
I am trying to place a conditional breakpoint on a function that would check the contents of an array. My idea was to use memcmp() in the condition:
typedef struct {
uint8_t arr[4];
} arg_t;
Then in gdb (declaring an array inline):
b func() if memcmp(arg.arr, (uint8_t[]){1, 2, 3, 4}, sizeof(arg.arr)) == 0
However, this does not work:
(gdb) c
Continuing.
Error in testing breakpoint condition:
Too many array elements
I can do it with if arg.arr[0] == 1 && arg.arr[1] == 2 && ... but in my real case access to the array (containing an IPv6 address) is rather convoluted, so it becomes unwieldy rather quickly.
UPDATE: After Mark's comment, I tried the following test program:
#include <stdio.h>
#include <stdint.h>
#include <string.h>
int main(void) {
uint8_t a[] = { 1, 2, 3, 4 };
printf("memcmp returns %d\n", memcmp(a, (uint8_t[4]){1,2,3,4}, sizeof(a)));
return 0;
}
After compiling it, I placed a conditional break on the return using the explicit array size syntax, and it worked but it does not seem to stop as expected:
(gdb) br hello.c:8 if memcmp(a, (uint8_t[4]){1,2,3,4}, sizeof(a)) == 0
Breakpoint 1 at 0x75c: file hello.c, line 8.
(gdb) run
Starting program: /mnt/c/stuff/src/test/hello
memcmp returns 0
[Inferior 1 (process 153) exited normally]
I was trying to evaluate the return value of the function at the break site manually:
(gdb) br hello.c:8
Note: breakpoint 1 also set at pc 0x800075c.
Breakpoint 2 at 0x800075c: file hello.c, line 8.
(gdb) dele 1
(gdb) run
Starting program: /mnt/c/stuff/src/test/hello
memcmp returns 0
Breakpoint 2, main () at hello.c:8
8 return 0;
(gdb) p memcmp(a, (uint8_t[4]){1,2,3,4}, sizeof(a))
$1 = (int (*)(const void *, const void *, size_t)) 0x7fffff18aba0 <__memcmp_avx2_movbe>
I was surprised to see this, I suspect this may be due to memcmp() being a compiler intrinsic to the avx2 instruction, in which case I might need to cast it somehow?
This may not be what you want, but for complex tests, I get the program to help gdb by adding some check routines.
Method 1:
I create a void checkon (void) { if (expr_to_stop_on) badnews(); }
And then have: void badnews { stopme = 1; }
I then instrument the code with calls to checkon
I then tell gdb to do b badnews.
Method 2:
An alternative may be to create: int checkfail(void) { return expr_to_stop_on; }
Then, tell gdb to watch myarray.
This creates a watchpoint (using H/W assist). The watchpoint is like a breakpoint.
You can then do: cond 1 checkfail()
Method 3:
Similar to method 2, but instead of a watchpoint, use a tracepoint [with the same cond command (Actually, you may need to use actions instead).
This essentially single steps the program and allows a set of commands to be executed at every tracepoint.
This can be slow because every line has to do these things.
I think method 2 is probably the best bet.
(Posting my own answer to be able to close this, but credit goes to #mark-plotnick)
It works with the gdb builtin $_memeq instead of using memcmp - suspecting because of the latter not being a real function in my environment but YMMV:
(gdb) br arrbreak.c:8 if $_memeq(a, (uint8_t[4]){1,2,3,4}, sizeof(a))
Breakpoint 2 at 0x800075c: file arrbreak.c, line 8.
(gdb) run
Starting program: /mnt/c/stuff/src/test/arrbreak
memcmp returns 0
Breakpoint 2, main () at arrbreak.c:8
8 return 0;
(gdb) p $_memeq(a, (uint8_t[4]){1,2,3,4}, sizeof(a))
$1 = 1
(gdb) p $_memeq(a, (uint8_t[4]){1,2,3,5}, sizeof(a))
$2 = 0
(gdb)
I am running an application through gdb and I want to set a breakpoint for any time a specific variable is accessed / changed. Is there a good method for doing this? I would also be interested in other ways to monitor a variable in C/C++ to see if/when it changes.
watch only breaks on write, rwatch let you break on read, and awatch let you break on read/write.
You can set read watchpoints on memory locations:
gdb$ rwatch *0xfeedface
Hardware read watchpoint 2: *0xfeedface
but one limitation applies to the rwatch and awatch commands; you can't use gdb variables
in expressions:
gdb$ rwatch $ebx+0xec1a04f
Expression cannot be implemented with read/access watchpoint.
So you have to expand them yourself:
gdb$ print $ebx
$13 = 0x135700
gdb$ rwatch *0x135700+0xec1a04f
Hardware read watchpoint 3: *0x135700 + 0xec1a04f
gdb$ c
Hardware read watchpoint 3: *0x135700 + 0xec1a04f
Value = 0xec34daf
0x9527d6e7 in objc_msgSend ()
Edit: Oh, and by the way. You need either hardware or software support. Software is obviously much slower. To find out if your OS supports hardware watchpoints you can see the can-use-hw-watchpoints environment setting.
gdb$ show can-use-hw-watchpoints
Debugger's willingness to use watchpoint hardware is 1.
What you're looking for is called a watchpoint.
Usage
(gdb) watch foo: watch the value of variable foo
(gdb) watch *(int*)0x12345678: watch the value pointed by an address, casted to whatever type you want
(gdb) watch a*b + c/d: watch an arbitrarily complex expression, valid in the program's native language
Watchpoints are of three kinds:
watch: gdb will break when a write occurs
rwatch: gdb will break wnen a read occurs
awatch: gdb will break in both cases
You may choose the more appropriate for your needs.
For more information, check this out.
Assuming the first answer is referring to the C-like syntax (char *)(0x135700 +0xec1a04f) then the answer to do rwatch *0x135700+0xec1a04f is incorrect. The correct syntax is rwatch *(0x135700+0xec1a04f).
The lack of ()s there caused me a great deal of pain trying to use watchpoints myself.
I just tried the following:
$ cat gdbtest.c
int abc = 43;
int main()
{
abc = 10;
}
$ gcc -g -o gdbtest gdbtest.c
$ gdb gdbtest
...
(gdb) watch abc
Hardware watchpoint 1: abc
(gdb) r
Starting program: /home/mweerden/gdbtest
...
Old value = 43
New value = 10
main () at gdbtest.c:6
6 }
(gdb) quit
So it seems possible, but you do appear to need some hardware support.
Use watch to see when a variable is written to, rwatch when it is read and awatch when it is read/written from/to, as noted above. However, please note that to use this command, you must break the program, and the variable must be in scope when you've broken the program:
Use the watch command. The argument to the watch command is an
expression that is evaluated. This implies that the variabel you want
to set a watchpoint on must be in the current scope. So, to set a
watchpoint on a non-global variable, you must have set a breakpoint
that will stop your program when the variable is in scope. You set the
watchpoint after the program breaks.
In addition to what has already been answered/commented by asksol and Paolo M
I didn't at first read understand, why do we need to cast the results. Though I read this: https://sourceware.org/gdb/onlinedocs/gdb/Set-Watchpoints.html, yet it wasn't intuitive to me..
So I did an experiment to make the result clearer:
Code: (Let's say that int main() is at Line 3; int i=0 is at Line 5 and other code.. is from Line 10)
int main()
{
int i = 0;
int j;
i = 3840 // binary 1100 0000 0000 to take into account endianness
other code..
}
then i started gdb with the executable file
in my first attempt, i set the breakpoint on the location of variable without casting, following were the results displayed
Thread 1 "testing2" h
Breakpoint 2 at 0x10040109b: file testing2.c, line 10.
(gdb) s
7 i = 3840;
(gdb) p i
$1 = 0
(gdb) p &i
$2 = (int *) 0xffffcbfc
(gdb) watch *0xffffcbfc
Hardware watchpoint 3: *0xffffcbfc
(gdb) s
[New Thread 13168.0xa74]
Thread 1 "testing2" hit Breakpoint 2, main () at testing2.c:10
10 b = a;
(gdb) p i
$3 = 3840
(gdb) p *0xffffcbfc
$4 = 3840
(gdb) p/t *0xffffcbfc
$5 = 111100000000
as we could see breakpoint was hit for line 10 which was set by me. gdb didn't break because although variable i underwent change yet the location being watched didn't change (due to endianness, since it continued to remain all 0's)
in my second attempt, i did the casting on the address of the variable to watch for all the sizeof(int) bytes. this time:
(gdb) p &i
$6 = (int *) 0xffffcbfc
(gdb) p i
$7 = 0
(gdb) watch *(int *) 0xffffcbfc
Hardware watchpoint 6: *(int *) 0xffffcbfc
(gdb) b 10
Breakpoint 7 at 0x10040109b: file testing2.c, line 10.
(gdb) i b
Num Type Disp Enb Address What
6 hw watchpoint keep y *(int *) 0xffffcbfc
7 breakpoint keep y 0x000000010040109b in main at testing2.c:10
(gdb) n
[New Thread 21508.0x3c30]
Thread 1 "testing2" hit Hardware watchpoint 6: *(int *) 0xffffcbfc
Old value = 0
New value = 3840
Thread 1 "testing2" hit Breakpoint 7, main () at testing2.c:10
10 b = a;
gdb break since it detected the value has changed.
I see some value in some place, but unsure where it has originated in my program. How do I figure out where this value initially comes from?
I expect the following event types to be logged:
A value originated from constant, arithmetical expression or syscall - the initial event;
The value was assigned to (or retrieved from) some variable;
The value was passed as an argument or returned from some function;
The value was stored to (or retrieved from) some struct;
By annotating source code with something specific, I triggered the history dump for this value.
For example, for this sample code:
#include <stdlib.h>
struct SomeStruct {
int a;
int b;
};
struct SomeStruct *globalvar;
int f1(struct SomeStruct* par) {
return par->a;
}
int f2(struct SomeStruct* par, int q) {
par->a = q;
return par->b;
}
void trace_value(int g) {} /* dummy */
int main(void) {
int f = 31337;
globalvar = malloc(sizeof(*globalvar));
f2(globalvar, f);
struct SomeStruct q = *globalvar;
int g = f1(&q);
trace_value(g);
return 0;
}
it should return something like
value 31337 originated from constant at fate.c:18
assigned to variable at fate.c:18
retrieved from variable at fate.c:21
passed as argument to function at fate.c:21
received as arument to a function at fate.c:12
assigned to struct field at fate.c:13
copied as a part of struct at fate.c:22
retrieved from struct field at fate.c:9
returned from function at fate.c:10
assigned to variable at fate.c:23
retrieved from variable at fate.c:25
traced at fate.c:25
How do I do this or something similar? I expect Valgrind or GDB or some combination should be able to do this.
Combined idea1 of using reverse gdb and idea2 from MarkPlotnick's comment of using gdb watchpoints. Here is the demo session, more complete than in original answer:
$ gcc -ggdb -Dtrace_value=exit fate.c -o fate
$ gdb -quiet -args ./fate
Reading symbols from /home/vi/code/_/fate...done.
(gdb) break main
Breakpoint 1 at 0x8048482: file fate.c, line 18.
(gdb) r
Starting program: /home/vi/code/_/fate
warning: Could not load shared library symbols for linux-gate.so.1.
Do you need "set solib-search-path" or "set sysroot"?
Breakpoint 1, main () at fate.c:18
18 int f = 31337;
(gdb) record
(gdb) break 25
(gdb) # traced at fate.c:25
Breakpoint 2 at 0x80484d2: file fate.c, line 25.
(gdb) c
Continuing.
Breakpoint 2, main () at fate.c:25
25 trace_value(g);
(gdb) # retrieved from variable at fate.c:25
(gdb) watch g
Hardware watchpoint 3: g
(gdb) reverse-continue
Continuing.
Hardware watchpoint 3: g
Old value = 31337
New value = 134513899
0x080484ce in main () at fate.c:23
23 int g = f1(&q);
(gdb) # assigned to variable at fate.c:23
(gdb) # returned from function at fate.c:10
(gdb) reverse-step
f1 (par=0xffffd670) at fate.c:10
10 }
(gdb) list
5
6 struct SomeStruct *globalvar;
7
8 int f1(struct SomeStruct* par) {
9 return par->a;
10 }
11
12 int f2(struct SomeStruct* par, int q) {
13 par->a = q;
14 return par->b;
(gdb) # retrieved from struct field at fate.c:9
(gdb) print par
$3 = (struct SomeStruct *) 0xffffd670
(gdb) print ((struct SomeStruct *) 0xffffd670)->a
$4 = 31337
(gdb) watch ((struct SomeStruct *) 0xffffd670)->a
Hardware watchpoint 4: ((struct SomeStruct *) 0xffffd670)->a
(gdb) reverse-continue
Continuing.
Hardware watchpoint 4: ((struct SomeStruct *) 0xffffd670)->a
Old value = 31337
New value = -134716508
0x080484ba in main () at fate.c:22
22 struct SomeStruct q = *globalvar;
(gdb) # copied as a part of struct at fate.c:22
(gdb) print globalvar->a
$5 = 31337
(gdb) watch globalvar->a
Hardware watchpoint 5: globalvar->a
(gdb) reverse-continue
Continuing.
Hardware watchpoint 5: globalvar->a
Old value = 31337
New value = 0
0x0804846f in f2 (par=0x804a008, q=31337) at fate.c:13
13 par->a = q;
(gdb) # assigned to struct field at fate.c:13
(gdb) # received as arument to a function at fate.c:12
(gdb) list
8 int f1(struct SomeStruct* par) {
9 return par->a;
10 }
11
12 int f2(struct SomeStruct* par, int q) {
13 par->a = q;
14 return par->b;
15 }
16
17 int main() {
(gdb) bt
#0 0x0804846f in f2 (par=0x804a008, q=31337) at fate.c:13
#1 0x080484b0 in main () at fate.c:21
(gdb) reverse-finish
Run back to call of #0 0x0804846f in f2 (par=0x804a008, q=31337) at fate.c:13
0x080484ab in main () at fate.c:21
21 f2(globalvar, f);
(gdb) # passed as argument to function at fate.c:21
(gdb) # retrieved from variable at fate.c:21
(gdb) watch f
Hardware watchpoint 6: f
(gdb) reverse-finish
"finish" not meaningful in the outermost frame.
(gdb) reverse-continue
Continuing.
Warning:
Could not insert hardware watchpoint 6.
Could not insert hardware breakpoints:
You may have requested too many hardware breakpoints/watchpoints.
(gdb) delete
Delete all breakpoints? (y or n) y
(gdb) watch f
Hardware watchpoint 7: f
(gdb) reverse-continue
Continuing.
No more reverse-execution history.
main () at fate.c:18
18 int f = 31337;
(gdb) # assigned to variable at fate.c:18
(gdb) # value 31337 originated from constant at fate.c:18
All expected messages in the question statement correspond to some info you have seen in gdb output (as shown in comments).
I believe it could be accomplished manually (i.e. running on gdb session) in runtime by technique called "reverse debugging". I haven't tried it yet, but GDB version 7.0 documentation claims, that it is supported on some platforms.
The method would be something like:
localize single step where variable is used in the last place (that is, your "starting point")
analyze source code (so you need debugging symbol and code section available) of stack frame (e.g. by list), so you get how this value is obtained (or possibly modified) witihin (e.g. from parameter passed to function)
step back to previous stack frame and repeat from previous step unless you find its origin
Here is some proof-of-concept session for your sample code. I edited it a bit, as trace_value function was undefined. Note that record command may heavily slow down program's execution.
$ gdb -q a.out
Reading symbols from /home/grzegorz/workspace/a.out...done.
(gdb) b main
Breakpoint 1 at 0x400502: file fate.c, line 22.
(gdb) run
Starting program: /home/grzegorz/workspace/a.out
Breakpoint 1, main () at fate.c:22
22 int f = 31337;
Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.149.el6_6.5.x86_64
(gdb) record
(gdb) b trace_value
Breakpoint 2 at 0x4004f8: file fate.c, line 19.
(gdb) c
Continuing.
Breakpoint 2, trace_value (g=31337) at fate.c:19
19 void trace_value(int g){}
(gdb) info args
g = 31337
(gdb) reverse-finish
Run back to call of #0 trace_value (g=31337) at fate.c:19
0x0000000000400550 in main () at fate.c:29
29 trace_value(g);
(gdb) bt
#0 0x0000000000400550 in main () at fate.c:29
(gdb) list 29
24 globalvar = malloc(sizeof(*globalvar));
25 f2(globalvar, f);
26 struct SomeStruct q = *globalvar;
27 int g = f1(&q);
28
29 trace_value(g);
30
31 return 0;
32 }
Few things maybe require some explanation. You need to set breakpoint for main at first, as this is when the program execution begins, then enable session recording by record command. Then set second breakpoint at trace_value function and use continue command (c in short). This allows you to record whole execution up to moment when trace_value is entered. You may think of it as this "starting point", described above.
This of course not the full story. As I described earlier you need to analyze source code of current stack frame an then decide what to do next. You may use reverse-step or reverse-finish command accordingly to current situation.
I am trying to find out how to print an integer value (I saw that it is x/d) but I am missing something.
So, my code is the following
1 #include <stdio.h>
2 main(){
3 int a;
4 int b;
5 int c;
6 int d;
7 int multiplied;
8 a = 5;
9 b = 6;
10 c = 7;
11 d = adding(a,b,c);
12 multiplied = multiply(a,b,c);
13 printf("The value of d is %d \n",d);
14 printf("The multiplied values are %d \n", multiplied);
15 }
16 int adding(a,b,c){
17 int e;
18 e = a+b+c;
19 return e;
20 }
21 int multiply(a,b,c){
22 int f = a*b*c;
23 return f;
24 }
// I compiled with -q and I want to print the values of the variables (from their addresses) So...
(gdb) disassemble main
0x080483ed <+9>: mov DWORD PTR [esp+0x2c],0x5
0x080483f5 <+17>: mov DWORD PTR [esp+0x28],0x6
0x080483fd <+25>: mov DWORD PTR [esp+0x24],0x7
0x08048405 <+33>: mov eax,DWORD PTR [esp+0x24] <code>
I put some breakpoints in main / multiply / adding and then I was trying to do the following thing.
I used
print $esp+0x24
and
(gdb) x/4d 0xbffff47c but im not getting the right answers back.
I used the 4d because I thought that an integer is 4 bytes (or maybe again im missing something) but the results arent showing the value 5.
Can you please help me? Thanks and sorry for the bad output / format of gdb.. seriously i cant understand whats wrong
(gdb) print $esp+0x2c
$2 = (void *) 0xbffff494
(gdb) print $esp+0x28
$3 = (void *) 0xbffff490
(gdb) print $esp+0x24
$4 = (void *) 0xbffff48c
(gdb) x/d 0xbffff494
0xbffff494: -1208180748
(gdb) x/d 0xbffff490
0xbffff490: -1208179932
(gdb) x/d 0xbffff48c
0xbffff48c: 134513881
Also this happens ofcourse after the first breakpoint of main and actually the same values are coming all the time in all breakpoints (except the one before main...)
Another interesting thing that I found is the following... Im sure that the first values are garbages. But why it considers 0x5 as an address when it should print the actual value?
Breakpoint 1, main () at functioncalling.c:10
10 a = 5;
(gdb) x/s a
0xb7fc9ff4: "|M\025"
(gdb) cont
Continuing.
Breakpoint 3, adding (a=5, b=6, c=7) at functioncalling.c:21
21 e = a+b+c;
(gdb) x/s a
0x5: <Address 0x5 out of bounds>
I compiled your program with -g and no optimization, and set a breakpoint before line 11. My stack addresses are a bit different from yours, which isn't surprising given the variety of systems out there.
(gdb) print $esp+0x2c
$2 = (void *) 0xbffff44c
This is printing the address of a. To confirm:
(gdb) print &a
$4 = (int *) 0xbffff44c
Use x/wd to show a 4-byte integer in decimal.
(gdb) x/wd $esp+0x2c
0xbffff44c: 5
x/4d will show 4 values (4 is the repeat count) starting at the address. If you omit the size letter w here, the x command will default to the size previously used.
(gdb) x/4d $esp+0x2c
0xbffff44c: 5 134513856 0 -1073744680
There's your 5. As for the 3 other numbers, they are things further up the stack.
(gdb) x/4a $esp+0x2c
0xbffff44c: 0x5 0x80484c0 <__libc_csu_init> 0x0 0xbffff4d8
Your next question:
Another interesting thing that I found is the following... Im sure that the first values are garbages. But why it considers 0x5 as an address when it should print the actual value?
Breakpoint 3, adding (a=5, b=6, c=7) at functioncalling.c:21
21 e = a+b+c;
(gdb) x/s a
0x5: <Address 0x5 out of bounds>
The x command, when given a program's variable as its argument, retrieves its value and uses that as the address. x/s a means to retrieve the value in a and use it as the starting address of a NUL-terminated string. If a were of type char * and contained a suitable value, x/s would print sensible output. To print a's actual value, give the x command the argument &a.
(gdb) x/wd &a
0xbffff44c: 5
This may seem counterintuitive. Just consider the x command to operate just like the C statement printf(fmt, *(argument)) would. The argument to the x command is almost always a literal memory address or an address expression involving the stack pointer, base pointer, or pc registers.