Re: Problem of GDB interaction with interrupted system calls

[teawater]

This signal ctrl-c will not really send to inferior.

(gdb) help info handle

On Oct 31, 12:10 am, Alexandre Rusev <address@hidden> wrote:
> Hi.
>
> When the program at ht end of message debugged under GDB is stopped with
> Ctrl+C
> it's usually found in interrupted system call. (The same result is
> observed for x86 and PPC with kernels 2.6.18 and 2.6.28)
>
> (gdb) where
> #0  0xb7fe2424 in __kernel_vsyscall ()
> #1  0xb7f36ad0 in nanosleep () from /lib/libc.so.6
> #2  0xb7f3690e in sleep () from /lib/libc.so.6
> #3  0x08048600 in qqq () at testBT2.c:45
> #4  0x080487a5 in eee () at testBT2.c:73
> #5  0x0804846a in main () at testBT2.c:17
>
> The PC is pointing at the next instruction, accordingly to GDB.
> But the kernel tries to restart syscall by means of changing PC to PC-4
> (in case of PowerPC and to some other value for x86)
> and it does it's change to PC after the user continues execution of
> program in GDB with "cont" or "si" command.
>
> The issue is that if user changed PC at this point or uses "call
> <func_name>" GDB command, the both changes to PC
> are added (as kernel uses PC relative change i.e. PC - 4), and the
> program continues execution at absolutely wrong place.
>
> The issue may be gracefully observed if breakpoint is set just before
> <func_name> and then PC is changed to the <func_name> address.
> In such case the breakpoint is hit while it must not be.
>
> #include <stdio.h>
> #include <stdlib.h>
>
> void qqq();
> void www();
> void eee();
>
> void * xrealloc(void*, int);
>
> int main(void)
> {
>         eee();
>     return EXIT_SUCCESS;
>
> }
>
> void qqq() {
>     void *a[256];
>     size_t i, n;
>
>     for (i = 0; i < 256; i++)
>     {
>         n = (size_t) ((rand() * 256.0) / (RAND_MAX + 1.0)) + 1;
>         a[i] = malloc(n);
>     }
>     for (i = 256; i > 0; i--)
>     {
>         n = (size_t) ((rand() * 256.0) / (RAND_MAX + 1.0)) + 1;
>         a[i - 1] = xrealloc(a[i - 1], n);
>     }
>     sleep(1);
>     for (i = 0; i < 256; i += 2)
>         free(a[i]);
>     for (i = 256; i > 0; i -= 2)
>         free(a[i - 1]);
>     sleep(1);
>
> }
>
> void www() {
>     void *a[256];
>     size_t i, n;
>
>     for (i = 0; i < 256; i++)
>     {
>         n = (size_t) ((rand() * 256.0) / (RAND_MAX + 1.0)) + 1;
>         a[i] = malloc(n);
>     }
>     for (i = 256; i > 0; i--)
>     {
>         n = (size_t) ((rand() * 256.0) / (RAND_MAX + 1.0)) + 1;
>         a[i - 1] = realloc(a[i - 1], n);
>     }
>     sleep(1);
>     for (i = 0; i < 256; i += 2)
>         free(a[i]);
>     for (i = 256; i > 0; i -= 2)
>         free(a[i - 1]);
>     sleep(1);
>
> }
>
> void eee() {
>
>         while (1) {
>                 qqq();
>
>                 www();
>
>         }
>
> }
>
> void * xrealloc(void* addr, int n) {
>         return realloc(addr, n);
>
> }