[Bug breakpoints/30335] New: Watchpoints aren't triggered when calling program functions

[Bug breakpoints/30335] New: Watchpoints aren't triggered when calling program functions

[joe at lothan dot net]

https://sourceware.org/bugzilla/show_bug.cgi?id=30335

            Bug ID: 30335
           Summary: Watchpoints aren't triggered when calling program
                    functions
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: breakpoints
          Assignee: unassigned at sourceware dot org
          Reporter: joe at lothan dot net
  Target Milestone: ---
             Build: Current

When calling program functions using `call` or `print`, existing watchpoints
are not triggered even if the value changes. For example:

$ cat test.c
#include <stdio.h>

int towatch = 0;

void change_var() {
    towatch ++;
    printf("new towatch: %d\n", towatch);
}

int main() {
    for (int i = 0; i < 5; i++)
        change_var();
}
$ gcc -g -o test test.c
$ ./test
new towatch: 1
new towatch: 2
new towatch: 3
new towatch: 4
new towatch: 5
$ gdb ./test
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Reading symbols from ./test...
(gdb) break main
Breakpoint 1 at 0x118b: file test.c, line 11.
(gdb) run
Starting program: /home/joe/test
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, main () at test.c:11
11          for (int i = 0; i < 5; i++)
(gdb) watch towatch
Hardware watchpoint 2: towatch
(gdb) p towatch
$1 = 0
(gdb) # I'd expect the watchpoint to trigger with the following call, but it
doesn't
(gdb) call change_var()
new towatch: 1
(gdb) p towatch
$2 = 1
(gdb) continue
Continuing.

Hardware watchpoint 2: towatch

Old value = 0
New value = 2
change_var () at test.c:7
7           printf("new towatch: %d\n", towatch);


This also happens if I pass the `-location` parameter with `watch`: 

$ gdb -q ./test
Reading symbols from ./test...
(gdb) break main
Breakpoint 1 at 0x118b: file test.c, line 11.
(gdb) run
Starting program: /home/joe/test
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, main () at test.c:11
11          for (int i = 0; i < 5; i++)
(gdb) watch -location towatch
Hardware watchpoint 2: -location towatch
(gdb) call change_var()
new towatch: 1
(gdb)


And when using a software watchpoints: 

$ gdb -q ./test
Reading symbols from ./test...
(gdb) break main
Breakpoint 1 at 0x118b: file test.c, line 11.
(gdb) run
Starting program: /home/joe/test
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, main () at test.c:11
11          for (int i = 0; i < 5; i++)
(gdb) show can-use-hw-watchpoints
Debugger's willingness to use watchpoint hardware is 1.
(gdb) set can-use-hw-watchpoints 0
(gdb) watch towatch
Watchpoint 2: towatch
(gdb) i breakpoints 2
Num     Type           Disp Enb Address            What
2       watchpoint     keep y                      towatch
(gdb) call change_var()
new towatch: 1
(gdb)

Thanks to tromey and BigKnife for the help in #gdb. First time submitting a bug
in gdb, can provide more info if need be.

Cheers,
Joe

Environment:
gdb version 14.0.50.20230411-git
gdb configuration x86_64-pc-linux-gnu
Ubuntu 22.04.1 LTS
Linux 5.15.0-58-generic
gcc version 11.3.0

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[Bug breakpoints/30335] Watchpoints aren't triggered when calling program functions

[joe at lothan dot net]

https://sourceware.org/bugzilla/show_bug.cgi?id=30335

--- Comment #1 from Joe Lothan <joe at lothan dot net> ---
Forgot to mention that breakpoints do trigger when calling program functions,
it's just watchpoints that do not. And interestingly enough, when a breakpoint
is hit, it allows the watchpoint to be hit later in the program function
execution.

Using the same example: 
$ gdb -q ./test
Reading symbols from ./test...
(gdb) break main
Breakpoint 1 at 0x118b: file test.c, line 11.
(gdb) run
Starting program: /home/joe/test
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, main () at test.c:11
11          for (int i = 0; i < 5; i++)
(gdb) watch towatch
Hardware watchpoint 2: towatch
(gdb) break change_var
Breakpoint 3 at 0x555555555151: file test.c, line 6.
(gdb) call change_var()

Breakpoint 3, change_var () at test.c:6
6           towatch ++;
The program being debugged stopped while in a function called from GDB.
Evaluation of the expression containing the function
(change_var) will be abandoned.
When the function is done executing, GDB will silently stop.
(gdb) finish
Run till exit from #0  change_var () at test.c:6

Hardware watchpoint 2: towatch

Old value = 0
New value = 1
change_var () at test.c:7
7           printf("new towatch: %d\n", towatch);
(gdb)

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