Is it possible to set a gdb watchpoint programmatically?

Question

I want to set a watchpoint (break on hardware write) temporarily in my C++ program to find memory corruption.

I\'ve seen all the ways to do it manually through gdb,

Answer 1

Based on user512106's great answer, I coded up a little "library" that someone might find useful:

It's on github at https://github.com/whh8b/hwbp_lib. I wish I could have commented directly on his answer, but I don't have enough rep yet.

Based on feedback from the community, I am going to copy/paste the relevant code here:

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

extern int errno;

enum {
    BREAK_EXEC = 0x0,
    BREAK_WRITE = 0x1,
    BREAK_READWRITE = 0x3,
};

enum {
    BREAK_ONE = 0x0,
    BREAK_TWO = 0x1,
    BREAK_FOUR = 0x3,
    BREAK_EIGHT = 0x2,
};

#define ENABLE_BREAKPOINT(x) (0x1<<(x*2))
#define ENABLE_BREAK_EXEC(x) (BREAK_EXEC<<(16+(x*4)))
#define ENABLE_BREAK_WRITE(x) (BREAK_WRITE<<(16+(x*4)))
#define ENABLE_BREAK_READWRITE(x) (BREAK_READWRITE<<(16+(x*4)))

/*
 * This function fork()s a child that will use
 * ptrace to set a hardware breakpoint for 
 * memory r/w at _addr_. When the breakpoint is
 * hit, then _handler_ is invoked in a signal-
 * handling context.
 */
bool install_breakpoint(void *addr, int bpno, void (*handler)(int)) {
    pid_t child = 0;
    uint32_t enable_breakpoint = ENABLE_BREAKPOINT(bpno);
    uint32_t enable_breakwrite = ENABLE_BREAK_WRITE(bpno);
    pid_t parent = getpid();
    int child_status = 0;

    if (!(child = fork()))
    {
        int parent_status = 0;
        if (ptrace(PTRACE_ATTACH, parent, NULL, NULL))
            _exit(1);

        while (!WIFSTOPPED(parent_status))
            waitpid(parent, &parent_status, 0);

        /*
         * set the breakpoint address.
         */
        if (ptrace(PTRACE_POKEUSER,
                   parent,
                   offsetof(struct user, u_debugreg[bpno]),
                   addr))
            _exit(1);

        /*
         * set parameters for when the breakpoint should be triggered.
         */
        if (ptrace(PTRACE_POKEUSER,
                   parent,
                   offsetof(struct user, u_debugreg[7]),
                   enable_breakwrite | enable_breakpoint))
            _exit(1);

        if (ptrace(PTRACE_DETACH, parent, NULL, NULL))
            _exit(1);

        _exit(0);
    }

    waitpid(child, &child_status, 0);

    signal(SIGTRAP, handler);

    if (WIFEXITED(child_status) && !WEXITSTATUS(child_status))
        return true;
    return false;
}

/*
 * This function will disable a breakpoint by
 * invoking install_breakpoint is a 0x0 _addr_
 * and no handler function. See comments above
 * for implementation details.
 */
bool disable_breakpoint(int bpno) 
{
    return install_breakpoint(0x0, bpno, NULL);
}

/*
 * Example of how to use this /library/.
 */
int handled = 0;

void handle(int s) {
    handled = 1;
    return;
}

int main(int argc, char **argv) {
    int a = 0;

    if (!install_breakpoint(&a, 0, handle))
        printf("failed to set the breakpoint!\n");

    a = 1;
    printf("handled: %d\n", handled);

    if (!disable_breakpoint(0))
        printf("failed to disable the breakpoint!\n");

    return 1;
}

I hope that this helps someone!

Will