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()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 ((&ksig)) {... }...}int get_signal(struct ksignal *ksig){... for (;;) { struct k_sigaction *ka;... signr = dequeue_signal(current, ¤t->blocked, &ksig->info);... /* Trace actually delivered signals. */ trace_signal_deliver(signr, &ksig->info, ka);... if ((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。 (current);(&ksig->info); }... } spin_unlock_irq(&sighand->siglock); ksig->sig = signr; return ksig->sig > 0;}#define sig_kernel_coredump(sig) siginmask(sig, ) #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), /* * 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); if (ispipe) { 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); 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); if (sub_info) retval = call_usermodehelper_exec(sub_info, UMH_WAIT_EXEC); 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 = (&cprm);---------------------------调用对应程序加载器的core_dump进行处理。 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;} 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()。
static struct linux_binfmt elf_format = { .module = THIS_MODULE, .load_binary = load_elf_binary, .load_shlib = load_elf_library, .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《》 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); 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 (!(cprm, elf, sizeof(*elf))) goto end_coredump; if (!dump_emit(cprm, phdr4note, sizeof(*phdr4note))) 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：《》,GDB解析coredump文件参考《》。
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#include #include #include 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: 0x4007d4data addr: 0x7ffff28fdc30mmap addr: 0x10bb010Segmentation 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:77 *(int*)(NULL) = i; /* line 7 */
关于core+gdb更详细的分析方法可以参考《》，在分析过程中需要加载动态库可以参考《》。
4. 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()？coredump生成core文件流程(())？gdb如何识别core文件(《》)？如何通过gdb分析core文件发现问题(gdb->backtrace)？