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coredump配置、产生、分析以及分析示例

关键词:coredump、core_pattern、coredump_filter等等。

 

应用程序在运行过程中由于各种异常或者bug导致退出,在满足一定条件下产生一个core文件。

通常core文件包含了程序运行时内存、寄存器状态、堆栈指针、内存管理信息以及函数调用堆栈信息。

core就是程序当前工作转改存储生成的一个文件,通过工具分析这个文件,可以定位到程序异常退出的时候对应的堆栈调用等信息,找出问题点并解决。

1. 配置coredump

如果需要使用需要通过ulimit进行设置,可以通过ulimit -c查看当前系统是否支持coredump。如果为0,则表示coredump被关闭。

通过ulimit -c unlimited可以打开coredump。

coredump文件默认存储位置与可执行文件在同一目录下,文件名为core。

可以通过/proc/sys/kernel/core_pattern进行设置。

%p  出Core进程的PID
%u  出Core进程的UID
%s  造成Core的signal号
%t  出Core的时间,从1970-01-0100:00:00开始的秒数
%e  出Core进程对应的可执行文件名

通过echo "core-%e-%p-%s-%t" > /proc/sys/kernel/core_pattern。

在每个进程下都有coredump_filter节点/proc/<pid>/coredump_filter

通过配置coredump_filter可以选择需在coredump的时候,将哪些内容dump到core文件中。

  - (bit 0) anonymous private memory
  - (bit 1) anonymous shared memory
  - (bit 2) file-backed private memory
  - (bit 3) file-backed shared memory
  - (bit 4) ELF header pages in file-backed private memory areas (it is effective only if the bit 2 is cleared)
  - (bit 5) hugetlb private memory
  - (bit 6) hugetlb shared memory
  - (bit 7) DAX private memory
  - (bit 8) DAX shared memory

coredump_filter的默认值是0x33,也即发生coredump时会将所有anonymous内存、ELF头页面、hugetlb private memory内容保存。

coredump_filter可以被子进程继承,可以echo 0xXX > /proc/self/coredump_filter设置当前进程的coredump_filter。

static ssize_t proc_coredump_filter_write(struct file *file,
                      const char __user *buf,
                      size_t count,
                      loff_t *ppos)
{
...
    ret = kstrtouint_from_user(buf, count, 0, &val);-------------------------将buf转换成val值。
    if (ret < 0)
        return ret;
...
    for (i = 0, mask = 1; i < MMF_DUMP_FILTER_BITS; i++, mask <<= 1) {
        if (val & mask)
            set_bit(i + MMF_DUMP_FILTER_SHIFT, &mm->flags);------------------将coredump_filter的值映射到mm->flags上,后续coredump时使用。
        else
            clear_bit(i + MMF_DUMP_FILTER_SHIFT, &mm->flags);
    }
...
}

其中MMF_DUMP_FILTER_SHIFT为2,所以flags和coredump_filter存在如下对应关系。

#define MMF_DUMP_ANON_PRIVATE    2
#define MMF_DUMP_ANON_SHARED    3
#define MMF_DUMP_MAPPED_PRIVATE    4
#define MMF_DUMP_MAPPED_SHARED    5
#define MMF_DUMP_ELF_HEADERS    6
#define MMF_DUMP_HUGETLB_PRIVATE 7
#define MMF_DUMP_HUGETLB_SHARED  8
#define MMF_DUMP_DAX_PRIVATE    9
#define MMF_DUMP_DAX_SHARED    10

2. coredump原理

在do_signal()中根据信号判断是否触发coredump,当然还跟coredump limit、mm->flags等等相关。

满足coredump条件后,由do_coredump()进行coredump文件生成,核心是由binfmt->core_dump()进行的。

2.1 触发coredump的条件?

在内核返回用户空间的时候,会调用do_signal()处理信号。

static void do_signal(struct pt_regs *regs, int syscall)
{
    unsigned int retval = 0, continue_addr = 0, restart_addr = 0;
    struct ksignal ksig;
...
    if (get_signal(&ksig)) {
...
    }
...
}

int get_signal(struct ksignal *ksig)
{
...
    for (;;) {
        struct k_sigaction *ka;
...
        signr = dequeue_signal(current, &current->blocked, &ksig->info);
...
        /* Trace actually delivered signals. */
        trace_signal_deliver(signr, &ksig->info, ka);
...
        if (sig_kernel_coredump(signr)) {
            if (print_fatal_signals)------------------------------可以通过kernel.print-fatal-signals = 1进行设置,对应的节点是/proc/sys/kernel/print-fatal-signals。
                print_fatal_signal(ksig->info.si_signo);----------打印当前信号及当前场景的栈信息。
            proc_coredump_connector(current);
            do_coredump(&ksig->info);
        }
...
    }
    spin_unlock_irq(&sighand->siglock);

    ksig->sig = signr;
    return ksig->sig > 0;
}

#define sig_kernel_coredump(sig)    siginmask(sig, SIG_KERNEL_COREDUMP_MASK)

  #define SIG_KERNEL_COREDUMP_MASK (\
    rt_sigmask(SIGQUIT) | rt_sigmask(SIGILL) | \
    rt_sigmask(SIGTRAP) | rt_sigmask(SIGABRT) | \
    rt_sigmask(SIGFPE) | rt_sigmask(SIGSEGV) | \
    rt_sigmask(SIGBUS) | rt_sigmask(SIGSYS) | \
    rt_sigmask(SIGXCPU) | rt_sigmask(SIGXFSZ) | \
    SIGEMT_MASK )

在get_signal()中,判断信号是否会导致coredump。这些信号包括SIGQUIT、SIGILL、SIGTRAP、SIGABRT、SIGFPE、SIGSEGV、SIGBUS、SIGSYS、SIGXCPU、SIGXFSZ

“终止w/core”表示在进程当前工作目录的core文件中复制了该进程的存储图像(该文件名为core,由此可以看出这种功能很久之前就是UNIX功能的一部分)。

void proc_coredump_connector(struct task_struct *task)
{
    struct cn_msg *msg;
    struct proc_event *ev;
    __u8 buffer[CN_PROC_MSG_SIZE] __aligned(8);

    if (atomic_read(&proc_event_num_listeners) < 1)
        return;

    msg = buffer_to_cn_msg(buffer);
    ev = (struct proc_event *)msg->data;
    memset(&ev->event_data, 0, sizeof(ev->event_data));
    ev->timestamp_ns = ktime_get_ns();
    ev->what = PROC_EVENT_COREDUMP;
    ev->event_data.coredump.process_pid = task->pid;
    ev->event_data.coredump.process_tgid = task->tgid;

    memcpy(&msg->id, &cn_proc_event_id, sizeof(msg->id));
    msg->ack = 0; /* not used */
    msg->len = sizeof(*ev);
    msg->flags = 0; /* not used */
    send_msg(msg);
} 

2.2 coredump如何生成?

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),-----------------------------------获取系统对于coredump的限制。
        /*
         * 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;------------------------------------------------获取当前进程所使用的程序加载器。
    if (!binfmt || !binfmt->core_dump)
        goto fail;
    if (!__get_dumpable(cprm.mm_flags))---------------------------------从当前进程的mm->flags中取低两位判断是否可以coredump,SUID_DUMP_DISABLE(0)不可以,其他情况都可以。
        goto fail;

    cred = prepare_creds();
    if (!cred)
        goto fail;
    /*
     * We cannot trust fsuid as being the "true" uid of the process
     * nor do we know its entire history. We only know it was tainted
     * so we dump it as root in mode 2, and only into a controlled
     * environment (pipe handler or fully qualified path).
     */
    if (__get_dumpable(cprm.mm_flags) == SUID_DUMP_ROOT) {--------------区分SUID_DUMP_USER和SUID_DUMP_ROOT。
        /* Setuid core dump mode */
        cred->fsuid = GLOBAL_ROOT_UID;    /* Dump root private */
        need_suid_safe = true;
    }

    retval = coredump_wait(siginfo->si_signo, &core_state);
    if (retval < 0)
        goto fail_creds;

    old_cred = override_creds(cred);

    ispipe = format_corename(&cn, &cprm);-------------------------------根据core_pattern判断是否是ispipe,然后根据core_pattern的设置生成coredump文件名称。

    if (ispipe) {-------------------------------------------------------通过管道处理coredump信息。
        int dump_count;
        char **helper_argv;
        struct subprocess_info *sub_info;

        if (ispipe < 0) {
            printk(KERN_WARNING "format_corename failed\n");
            printk(KERN_WARNING "Aborting core\n");
            goto fail_unlock;
        }

        if (cprm.limit == 1) {
            printk(KERN_WARNING
                "Process %d(%s) has RLIMIT_CORE set to 1\n",
                task_tgid_vnr(current), current->comm);
            printk(KERN_WARNING "Aborting core\n");
            goto fail_unlock;
        }
        cprm.limit = RLIM_INFINITY;

        dump_count = atomic_inc_return(&core_dump_count);
        if (core_pipe_limit && (core_pipe_limit < dump_count)) {
            printk(KERN_WARNING "Pid %d(%s) over core_pipe_limit\n",
                   task_tgid_vnr(current), current->comm);
            printk(KERN_WARNING "Skipping core dump\n");
            goto fail_dropcount;
        }

        helper_argv = argv_split(GFP_KERNEL, cn.corename, NULL);----------将cn.corename参数进行拆分。
        if (!helper_argv) {
            printk(KERN_WARNING "%s failed to allocate memory\n",
                   __func__);
            goto fail_dropcount;
        }

        retval = -ENOMEM;
        sub_info = call_usermodehelper_setup(helper_argv[0],
                        helper_argv, NULL, GFP_KERNEL,
                        umh_pipe_setup, NULL, &cprm);---------------------通过usermodehelper调用用户空间的helper_argv[0]程序进行core_pattern。
        if (sub_info)
            retval = call_usermodehelper_exec(sub_info,
                              UMH_WAIT_EXEC);-----------------------------UMH_WAIT_EXEC表示在内核exec用户空间程序之后就退出,此时用户空间程序就通过pipe等待接收数据。

        argv_free(helper_argv);
        if (retval) {
            printk(KERN_INFO "Core dump to |%s pipe failed\n",
                   cn.corename);
            goto close_fail;
        }
    } else {
        struct inode *inode;
        int open_flags = O_CREAT | O_RDWR | O_NOFOLLOW |
                 O_LARGEFILE | O_EXCL;

        if (cprm.limit < binfmt->min_coredump)
            goto fail_unlock;

        if (need_suid_safe && cn.corename[0] != '/') {
            printk(KERN_WARNING "Pid %d(%s) can only dump core "\
                "to fully qualified path!\n",
                task_tgid_vnr(current), current->comm);
            printk(KERN_WARNING "Skipping core dump\n");
            goto fail_unlock;
        }

        if (!need_suid_safe) {
            mm_segment_t old_fs;

            old_fs = get_fs();
            set_fs(KERNEL_DS);
            /*
             * If it doesn't exist, that's fine. If there's some
             * other problem, we'll catch it at the filp_open().
             */
            (void) sys_unlink((const char __user *)cn.corename);
            set_fs(old_fs);
        }

        if (need_suid_safe) {---------------------------------------------创建coredump文件。
            struct path root;

            task_lock(&init_task);
            get_fs_root(init_task.fs, &root);
            task_unlock(&init_task);
            cprm.file = file_open_root(root.dentry, root.mnt,
                cn.corename, open_flags, 0600);
            path_put(&root);
        } else {
            cprm.file = filp_open(cn.corename, open_flags, 0600);
        }
        if (IS_ERR(cprm.file))
            goto fail_unlock;

        inode = file_inode(cprm.file);
        if (inode->i_nlink > 1)------------------------------------------coredummp文件不能有多个硬链接。
            goto close_fail;
        if (d_unhashed(cprm.file->f_path.dentry))
            goto close_fail;

        if (!S_ISREG(inode->i_mode))--------------------------------------coredump文件必须为普通文件。
            goto close_fail;

        if (!uid_eq(inode->i_uid, current_fsuid()))
            goto close_fail;
        if ((inode->i_mode & 0677) != 0600)
            goto close_fail;
        if (!(cprm.file->f_mode & FMODE_CAN_WRITE))-----------------------coredump文件必须可写。
            goto close_fail;
        if (do_truncate(cprm.file->f_path.dentry, 0, 0, cprm.file))
            goto close_fail;
    }

    /* get us an unshared descriptor table; almost always a no-op */
    retval = unshare_files(&displaced);
    if (retval)
        goto close_fail;
    if (displaced)
        put_files_struct(displaced);
    if (!dump_interrupted()) {
        file_start_write(cprm.file);
        core_dumped = binfmt->core_dump(&cprm);---------------------------调用对应程序加载器的core_dump进行处理,将数据写入到cprm.file中。
        file_end_write(cprm.file);
    }
    if (ispipe && core_pipe_limit)
        wait_for_dump_helpers(cprm.file);
close_fail:
    if (cprm.file)
        filp_close(cprm.file, NULL);
fail_dropcount:
    if (ispipe)
        atomic_dec(&core_dump_count);
fail_unlock:
    kfree(cn.corename);
    coredump_finish(mm, core_dumped);
    revert_creds(old_cred);
fail_creds:
    put_cred(cred);
fail:
    return;
}

format_corename()根据core_pattern中的设置,生成coredump文件名。并且判断coredump文件生成方式,ispipe为真则通过管道传输给其他应用处理;否则直接保存成文件。

static int format_corename(struct core_name *cn, struct coredump_params *cprm)
{
    const struct cred *cred = current_cred();
    const char *pat_ptr = core_pattern;
    int ispipe = (*pat_ptr == '|');------------------------------------------|表示通过pipe处理coredump文件。
    int pid_in_pattern = 0;
    int err = 0;

    cn->used = 0;
    cn->corename = NULL;
    if (expand_corename(cn, core_name_size))
        return -ENOMEM;
    cn->corename[0] = '\0';

    if (ispipe)
        ++pat_ptr;

    /* Repeat as long as we have more pattern to process and more output
       space */
    while (*pat_ptr) {
        if (*pat_ptr != '%') {
            err = cn_printf(cn, "%c", *pat_ptr++);
        } else {
            switch (*++pat_ptr) {
            /* single % at the end, drop that */
            case 0:
                goto out;
            /* Double percent, output one percent */
            case '%':
                err = cn_printf(cn, "%c", '%');
                break;
            /* pid */
            case 'p':
                pid_in_pattern = 1;
                err = cn_printf(cn, "%d",
                          task_tgid_vnr(current));-------------------------%p表示记录当前进程组的pid。
                break;
            /* global pid */
            case 'P':-------------------------------------------------------%P表示记录当前进程组的pid。
                err = cn_printf(cn, "%d",
                          task_tgid_nr(current));
                break;
            case 'i':
                err = cn_printf(cn, "%d",
                          task_pid_vnr(current));--------------------------%i表示记录当前线程的pid。
                break;
            case 'I':------------------------------------------------------%I表示记录当前线程的pid。
                err = cn_printf(cn, "%d",
                          task_pid_nr(current));
                break;
            /* uid */
            case 'u':-------------------------------------------------------%u表示当前用户id。
                err = cn_printf(cn, "%u",
                        from_kuid(&init_user_ns,
                              cred->uid));
                break;
            /* gid */
            case 'g':-------------------------------------------------------%g表示group id。
                err = cn_printf(cn, "%u",
                        from_kgid(&init_user_ns,
                              cred->gid));
                break;
            case 'd':
                err = cn_printf(cn, "%d",
                    __get_dumpable(cprm->mm_flags));------------------------%d表示dump的用户类型:SUID_DUMP_DISABLE/SUID_DUMP_USER/SUID_DUMP_ROOT。
                break;
            /* signal that caused the coredump */
            case 's':
                err = cn_printf(cn, "%d",
                        cprm->siginfo->si_signo);----------------------------%s记录产生coredump的信号。
                break;
            /* UNIX time of coredump */
            case 't': {
                time64_t time;

                time = ktime_get_real_seconds();
                err = cn_printf(cn, "%lld", time);---------------------------%t记录产生coredump的时间。
                break;
            }
            /* hostname */
            case 'h':--------------------------------------------------------%h记录主机名。
                down_read(&uts_sem);
                err = cn_esc_printf(cn, "%s",
                          utsname()->nodename);
                up_read(&uts_sem);
                break;
            /* executable */
            case 'e':
                err = cn_esc_printf(cn, "%s", current->comm);----------------%e记录进程中comm名称。
                break;
            case 'E':
                err = cn_print_exe_file(cn);---------------------------------%E记录可执行文件名称。
                break;
            /* core limit size */
            case 'c':
                err = cn_printf(cn, "%lu",
                          rlimit(RLIMIT_CORE));------------------------------%c记录coredump的limit值。
                break;
            default:
                break;
            }
            ++pat_ptr;
        }

        if (err)
            return err;
    }

out:
    if (!ispipe && !pid_in_pattern && core_uses_pid) {
        err = cn_printf(cn, ".%d", task_tgid_vnr(current));
        if (err)
            return err;
    }
    return ispipe;
}

所以core_%e(%I)_%E(%p)_sig(%s)_time(%t)写入到core_pattern表示core_线程名(线程pid)_进程名(进程pid)_sig(信号值)_time(异常时间点)

umh_pipe_setup()创建了一个管道,这个管道给内核coredump和用户空间程序搭建了一个桥梁。

内核coredump的数据写入管道,用户空间程序在管道另一端接收进行处理。

static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
{
    struct file *files[2];
    struct coredump_params *cp = (struct coredump_params *)info->data;
    int err = create_pipe_files(files, 0);----------------------------创建一个pipe管道,files[0]是管道的读端;files[1]是管道的写端。
    if (err)
        return err;

    cp->file = files[1];----------------------------------------------cp->file指向管道的写端,后面coredump写入这里。

    err = replace_fd(0, files[0], 0);---------------------------------这里将files[0]作为usermodehelper执行程序的输入,coredump的数据通过管道给用户空间程序接收。
    fput(files[0]);
    /* and disallow core files too */
    current->signal->rlim[RLIMIT_CORE] = (struct rlimit){1, 1};

    return err;
}

int create_pipe_files(struct file **res, int flags)
{
    int err;
    struct inode *inode = get_pipe_inode();
    struct file *f;
    struct path path;
    static struct qstr name = { .name = "" };

    if (!inode)
        return -ENFILE;

    err = -ENOMEM;
    path.dentry = d_alloc_pseudo(pipe_mnt->mnt_sb, &name);
    if (!path.dentry)
        goto err_inode;
    path.mnt = mntget(pipe_mnt);

    d_instantiate(path.dentry, inode);

    f = alloc_file(&path, FMODE_WRITE, &pipefifo_fops);------------------------创建管道的写一端。
    if (IS_ERR(f)) {
        err = PTR_ERR(f);
        goto err_dentry;
    }

    f->f_flags = O_WRONLY | (flags & (O_NONBLOCK | O_DIRECT));
    f->private_data = inode->i_pipe;

    res[0] = alloc_file(&path, FMODE_READ, &pipefifo_fops);--------------------创建管道的读一端。
    if (IS_ERR(res[0])) {
        err = PTR_ERR(res[0]);
        goto err_file;
    }

    path_get(&path);
    res[0]->private_data = inode->i_pipe;
    res[0]->f_flags = O_RDONLY | (flags & O_NONBLOCK);
    res[1] = f;
    return 0;

err_file:
    put_filp(f);
err_dentry:
    free_pipe_info(inode->i_pipe);
    path_put(&path);
    return err;

err_inode:
    free_pipe_info(inode->i_pipe);
    iput(inode);
    return err;
}

int replace_fd(unsigned fd, struct file *file, unsigned flags)
{
    int err;
    struct files_struct *files = current->files;

    if (!file)
        return __close_fd(files, fd);

    if (fd >= rlimit(RLIMIT_NOFILE))
        return -EBADF;

    spin_lock(&files->file_lock);
    err = expand_files(files, fd);
    if (unlikely(err < 0))
        goto out_unlock;
    return do_dup2(files, file, fd, flags);

out_unlock:
    spin_unlock(&files->file_lock);
    return err;
}

linux内核支持多种linux_binfmt,这里最常用的是ELF。 

所以do_coredump()中的binfmt即为elf_format,binfmt->core_dump()即为elf_coredump()。

elf_core_dump()将当前进程的vma区域进行dummp,附加相关的头信息等。保存成文件。

static struct linux_binfmt elf_format = {
    .module        = THIS_MODULE,
    .load_binary    = load_elf_binary,
    .load_shlib    = load_elf_library,
    .core_dump    = elf_core_dump,
    .min_coredump    = ELF_EXEC_PAGESIZE,
};

static int elf_core_dump(struct coredump_params *cprm)
{
    int has_dumped = 0;
    mm_segment_t fs;
    int segs, i;
    size_t vma_data_size = 0;
    struct vm_area_struct *vma, *gate_vma;
    struct elfhdr *elf = NULL;
    loff_t offset = 0, dataoff;
    struct elf_note_info info = { };
    struct elf_phdr *phdr4note = NULL;
    struct elf_shdr *shdr4extnum = NULL;
    Elf_Half e_phnum;
    elf_addr_t e_shoff;
    elf_addr_t *vma_filesz = NULL;

    elf = kmalloc(sizeof(*elf), GFP_KERNEL);-----------------------申请存放elfhdr空间。
    if (!elf)
        goto out;

    segs = current->mm->map_count;---------------------------------通过current->mm->map_count得到当前进程已映射的内存段数量。
    segs += elf_core_extra_phdrs();--------------------------------增加附加段数量。

    gate_vma = get_gate_vma(current->mm);--------------------------增加一个segment给vma使用。
    if (gate_vma != NULL)
        segs++;

    /* for notes section */
    segs++;--------------------------------------------------------保留一个segment给PT_NOTE使用。

    /* If segs > PN_XNUM(0xffff), then e_phnum overflows. To avoid
     * this, kernel supports extended numbering. Have a look at
     * include/linux/elf.h for further information. */
    e_phnum = segs > PN_XNUM ? PN_XNUM : segs;

    /*
     * Collect all the non-memory information about the process for the
     * notes.  This also sets up the file header.
     */
    if (!fill_note_info(elf, e_phnum, &info, cprm->siginfo, cprm->regs))-----fill_note_info()填充info信息。
        goto cleanup;

    has_dumped = 1;

    fs = get_fs();
    set_fs(KERNEL_DS);------------------------------------------------------在内核中操作用户空间文件,需要将地址方位扩大。具体参见《Linux内核访问用户空间文件:get_fs()/set_fs()的使用

    offset += sizeof(*elf);                /* Elf header */
    offset += segs * sizeof(struct elf_phdr);    /* Program headers */

    /* Write notes phdr entry */
    {
        size_t sz = get_note_info_size(&info);

        sz += elf_coredump_extra_notes_size();

        phdr4note = kmalloc(sizeof(*phdr4note), GFP_KERNEL);
        if (!phdr4note)
            goto end_coredump;

        fill_elf_note_phdr(phdr4note, sz, offset);
        offset += sz;
    }

    dataoff = offset = roundup(offset, ELF_EXEC_PAGESIZE);

    vma_filesz = kmalloc_array(segs - 1, sizeof(*vma_filesz), GFP_KERNEL);
    if (!vma_filesz)
        goto end_coredump;

    for (i = 0, vma = first_vma(current, gate_vma); vma != NULL;
            vma = next_vma(vma, gate_vma)) {
        unsigned long dump_size;

        dump_size = vma_dump_size(vma, cprm->mm_flags);----------------------mm_flags对应coredump_filter,用于确定哪些vma需要dump,哪些忽略掉。
        vma_filesz[i++] = dump_size;
        vma_data_size += dump_size;
    }

    offset += vma_data_size;
    offset += elf_core_extra_data_size();
    e_shoff = offset;

    if (e_phnum == PN_XNUM) {
        shdr4extnum = kmalloc(sizeof(*shdr4extnum), GFP_KERNEL);
        if (!shdr4extnum)
            goto end_coredump;
        fill_extnum_info(elf, shdr4extnum, e_shoff, segs);
    }

    offset = dataoff;

    if (!dump_emit(cprm, elf, sizeof(*elf)))---------------------------写入elf头到cprm->file文件,在使用pipe的情况下,这些数据都交给usermodehelper启动的用户空间程序进行处理。
        goto end_coredump;

    if (!dump_emit(cprm, phdr4note, sizeof(*phdr4note)))---------------写入phdr4node到cprm->file文件。
        goto end_coredump;

    /* Write program headers for segments dump */
    for (i = 0, vma = first_vma(current, gate_vma); vma != NULL;
            vma = next_vma(vma, gate_vma)) {
        struct elf_phdr phdr;

        phdr.p_type = PT_LOAD;
        phdr.p_offset = offset;
        phdr.p_vaddr = vma->vm_start;
        phdr.p_paddr = 0;
        phdr.p_filesz = vma_filesz[i++];
        phdr.p_memsz = vma->vm_end - vma->vm_start;
        offset += phdr.p_filesz;
        phdr.p_flags = vma->vm_flags & VM_READ ? PF_R : 0;
        if (vma->vm_flags & VM_WRITE)
            phdr.p_flags |= PF_W;
        if (vma->vm_flags & VM_EXEC)
            phdr.p_flags |= PF_X;
        phdr.p_align = ELF_EXEC_PAGESIZE;

        if (!dump_emit(cprm, &phdr, sizeof(phdr)))
            goto end_coredump;
    }

    if (!elf_core_write_extra_phdrs(cprm, offset))
        goto end_coredump;

     /* write out the notes section */
    if (!write_note_info(&info, cprm))
        goto end_coredump;

    if (elf_coredump_extra_notes_write(cprm))
        goto end_coredump;

    /* Align to page */
    if (!dump_skip(cprm, dataoff - cprm->pos))
        goto end_coredump;

    for (i = 0, vma = first_vma(current, gate_vma); vma != NULL;
            vma = next_vma(vma, gate_vma)) {
        unsigned long addr;
        unsigned long end;

        end = vma->vm_start + vma_filesz[i++];

        for (addr = vma->vm_start; addr < end; addr += PAGE_SIZE) {
            struct page *page;
            int stop;

            page = get_dump_page(addr);
            if (page) {
                void *kaddr = kmap(page);
                stop = !dump_emit(cprm, kaddr, PAGE_SIZE);
                kunmap(page);
                put_page(page);
            } else
                stop = !dump_skip(cprm, PAGE_SIZE);
            if (stop)
                goto end_coredump;
        }
    }
    dump_truncate(cprm);

    if (!elf_core_write_extra_data(cprm))
        goto end_coredump;

    if (e_phnum == PN_XNUM) {
        if (!dump_emit(cprm, shdr4extnum, sizeof(*shdr4extnum)))
            goto end_coredump;
    }

end_coredump:
    set_fs(fs);

cleanup:
    free_note_info(&info);
    kfree(shdr4extnum);
    kfree(vma_filesz);
    kfree(phdr4note);
    kfree(elf);
out:
    return has_dumped;
}

int dump_emit(struct coredump_params *cprm, const void *addr, int nr)
{
    struct file *file = cprm->file;
    loff_t pos = file->f_pos;
    ssize_t n;
    if (cprm->written + nr > cprm->limit)
        return 0;
    while (nr) {
        if (dump_interrupted())
            return 0;
        n = __kernel_write(file, addr, nr, &pos);
        if (n <= 0)
            return 0;
        file->f_pos = pos;
        cprm->written += n;
        cprm->pos += n;
        nr -= n;
    }
    return 1;
}

判断一个文件是否是coredump文件,可以通过readelf命令,如果类型是CORE(Core file)。

或者通过file命令进行判断。

参考文档:《Core file 文件格式(Linux Coredump文件结构)》,GDB解析coredump文件参考《GDB如何从Coredump文件恢复动态库信息》。

3. coredump案例

下面创建一个简单产生coredump的示例,然后通过gdb进行分析。

3.1 coredump示例

#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int myfunc(int i) {
    *(int*)(NULL) = i; /* line 7 */
    return i - 1;
}

int main(int argc, char **argv) {
    /* Setup some memory. */
    char data_ptr[] = "string in data segment";
    char *mmap_ptr;
    char *text_ptr = "string in text segment";
    (void)argv;
    mmap_ptr = (char *)malloc(sizeof(data_ptr) + 1);
    strcpy(mmap_ptr, data_ptr);
    mmap_ptr[10] = 'm';
    mmap_ptr[11] = 'm';
    mmap_ptr[12] = 'a';
    mmap_ptr[13] = 'p';
    printf("text addr: %p\n", text_ptr);
    printf("data addr: %p\n", data_ptr);
    printf("mmap addr: %p\n", mmap_ptr);

    /* Call a function to prepare a stack trace. */
    return myfunc(argc);
}

使用如下命令编译,-ggdb3表示产生更多适合GDB的调试信息,3是最高等级。

gcc -ggdb3 -std=c99 -Wall -Wextra -pedantic -o main.out main.c 

3.2 coredump+gdb分析

通过ulimit -c unlimited打开coredump功能,执行./main.out产生core文件。

text addr: 0x4007d4
data addr: 0x7ffff28fdc30
mmap addr: 0x10bb010
Segmentation fault (core dumped)

通过gdb ./main.out core,显示了进程由于什么信号导致的coredump(SIGSEGV)?在哪个文件(main.cc)?在哪个函数(myfunc())?具体位置的代码?等等信息。

GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1...
Reading symbols from ./main.out...done.
[New LWP 8651]
Core was generated by `./main.out'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x0000000000400635 in myfunc (i=1) at main.c:7
7        *(int*)(NULL) = i; /* line 7 */

关于core+gdb更详细的分析方法可以参考《通过core+gdb离线分析》,在分析过程中需要加载动态库可以参考《GDB动态库搜索路径》。

4. coredump使用优化(适用嵌入式)

在/etc/profile中,打开对coredump的配置以及对core_pattern进行配置:

sysctl -p -q -e
ulimit -c unlimited

配置/etc/sysctl.conf文件:

kernel.core_pattern=|/usr/bin/coredump_helper.sh core_%e_%I_%p_sig_%s_time_%t.gz
kernel.core_uses_pid=1

增加处理coredump文件的脚本:

#!/bin/sh

if [ ! -d "/var/coredump" ];then
    mkdir -p /var/coredump
fi
gzip > "/var/coredump/$1"

最终在/var/coredump目录下生成core_<线程名>_<线程ID>_<进程ID>_sig_<信号值>_time_<coredump时间>.gz文件。

5. 小结

至此大概总结了,对coredump的设置(ulimit/core_pattern/coredump_filter)?触发coredump的条件(SIG_KERNEL_COREDUMP_MASK )?coredump生成core文件流程(do_coredump())?gdb如何识别core文件(GDB如何从Coredump文件恢复动态库信息)?如何通过gdb分析core文件发现问题(gdb->backtrace)?

posted on 2019-07-12 00:00  ArnoldLu  阅读(38479)  评论(1编辑  收藏  举报

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