如何让ovs-vswitchd产生core文件

        ulimit -c unlimited
        echo "/tmp/core-%e-%p" > /proc/sys/kernel/core_pattern
        echo 2 > /proc/sys/fs/suid_dumpable

因为fedora不用init，而改用systemd，所以他下面的daemon默认不生成core。所以看似简单的问题，而不得不去研究源代码。用systemtap发现do_coredump确实被调到了，但就是不产生core。最后发现是__get_dumpable返回0。找到了几个关键字后（systemd prctl core dump）再在google里搜，就找到了答案。

crash> ps | grep systemd
      1      0  13  ffff881038ed8000  IN   0.0  234456  13612  systemd
    725      1  11  ffff88102e5c9d00  IN   0.1  131072  40944  systemd-journal
    758      1   7  ffff8810317bab80  IN   0.0   87924  10792  systemd-udevd
   1788      1   2  ffff880fd597c880  IN   0.0   71380   8468  systemd-logind
   6161      1   8  ffff88101cfde580  IN   0.0   81948   8144  systemd
crash> task_struct.mm ffff881038ed8000
    mm = 0xffff88103128bc00
crash> mm_struct.flags 0xffff88103128bc00
  flags = 0x200cd
crash> ps |grep ovs-vswitchd
  24934      1   8  ffff881014cf8e80  IN   0.2 1283740 151284  ovs-vswitchd
crash> task_struct.mm ffff881014cf8e80
    mm = 0xffff880fe2431c00
crash> mm_struct.flags 0xffff880fe2431c00
  flags = 0x200ce

ovs-vswitchd的父进程是systemd，mm.flags也继承自父进程。不过设置/proc/sys/fs/suid_dumpable为2后，flags就不一样了。

 0xffffffff812ae300 : do_coredump+0x0/0xeb0 [kernel]
 0xffffffff8109b95a : get_signal+0x29a/0x610 [kernel]
 0xffffffff8102dd77 : do_signal+0x37/0x680 [kernel]
 0xffffffff810035fe : exit_to_usermode_loop+0x6e/0xb0 [kernel]
 0xffffffff81003a1b : prepare_exit_to_usermode+0x8b/0x90 [kernel]
 0xffffffff818009b4 : swapgs_restore_regs_and_return_to_usermode+0x0/0x87 [kernel]

void do_coredump(const siginfo_t *siginfo)
{
        struct core_state core_state;
        struct core_name cn;
        struct mm_struct *mm = current->mm;
        struct linux_binfmt * binfmt;
        const struct cred *old_cred;
        struct cred *cred;
        int retval = 0;
        int ispipe;
        struct files_struct *displaced;
        /* require nonrelative corefile path and be extra careful */
        bool need_suid_safe = false;
        bool core_dumped = false;
        static atomic_t core_dump_count = ATOMIC_INIT(0);
        struct coredump_params cprm = {
                .siginfo = siginfo,
                .regs = signal_pt_regs(),
                .limit = rlimit(RLIMIT_CORE),
                /*
                 * We must use the same mm->flags while dumping core to avoid
                 * inconsistency of bit flags, since this flag is not protected
                 * by any locks.
                 */
                .mm_flags = mm->flags,
        };

        audit_core_dumps(siginfo->si_signo);

        binfmt = mm->binfmt;
        if (!binfmt || !binfmt->core_dump)
                goto fail;
        if (!__get_dumpable(cprm.mm_flags))
                goto fail;
...

#define SUID_DUMP_DISABLE       0       /* No setuid dumping */
#define SUID_DUMP_USER          1       /* Dump as user of process */
#define SUID_DUMP_ROOT          2       /* Dump as root */

除了上述方法还可以在应用程序里面直接调用prctl：

+    prctl(PR_SET_DUMPABLE, 1);